Guide to Space Archives

June 14, 2013October 18, 2016 by Fraser Cain

Closest Star to the Sun

This is a classic trick question. Ask a friend, “what is the closest star?” and then watch as they try to recall some nearby stars. Sirius maybe? Alpha something or other? Betelgeuse?

The answer, obviously, is the Sun; that massive ball of plasma located a mere 150 million km from Earth.

Let’s be more precise; what’s the closest star to the Sun?

Closest Star

You might have heard that it’s Alpha Centauri, the third brightest star in the sky, just 4.37 light-years from Earth.

But Alpha Centauri isn’t one star, it’s a system of three stars. First, there’s a binary pair, orbiting a common center of gravity every 80 years. Alpha Centauri A is just a little more massive and brighter than the Sun, and Alpha Centauri B is slightly less massive than the Sun. Then there’s a third member of this system, the faint red dwarf star, Proxima Centauri.

It’s the closest star to our Sun, located just a short 4.24 light-years away.

Closest Star, Proxima Centauri — Proxima Centauri

Alpha Centauri is located in the Centaurus constellation, which is only visible in the Southern Hemisphere. Unfortunately, even if you can see the system, you can’t see Proxima Centauri. It’s so dim, you need a need a reasonably powerful telescope to resolve it.

Let’s get sense of scale for just how far away Proxima Centauri really is. Think about the distance from the Earth to Pluto. NASA’s New Horizons spacecraft travels at nearly 60,000 km/h, the fastest a spacecraft has ever traveled in the Solar System. It will have taken more than nine years to make this journey when it arrives in 2015. Travelling at this speed, to get to Proxima Centauri, it would take New Horizons 78,000 years.

Proxima Centauri has been the nearest star for about 32,000 years, and it will hold this record for another 33,000 years. It will make its closest approach to the Sun in about 26,700 years, getting to within 3.11 light-years of Earth. After 33,000 years from now, the nearest star will be Ross 248.

What About the Northern Hemisphere?

Bernard's Star, one of the closest stars to the Sun — Bernard’s Star

For those of us in the Northern Hemisphere, the closest visible star is Barnard’s Star, another red dwarf in the constellation Ophiuchus. Unfortunately, just like Proxima Centauri, it’s too dim to see with the unaided eye.

The closest star that you can see with the naked eye in the Northern Hemisphere is Sirius, the Dog Star. Sirius, has twice the mass and is almost twice the size of the Sun, and it’s the brightest star in the sky. Located 8.6 light-years away in the constellation Canis Major – it’s very familiar as the bright star chasing Orion across the night sky in Winter.

How do Astronomers Measure the Distance to Stars?

They use a technique called parallax. Do a little experiment here. Hold one of your arms out at length and put your thumb up so that it’s beside some distant reference object. Now take turns opening and closing each eye. Notice how your thumb seems to jump back and forth as you switch eyes? That’s the parallax method.

To measure the distance to stars, you measure the angle to a star when the Earth is one side of its orbit; say in the summer. Then you wait 6 month, until the Earth has moved to the opposite side of its orbit, and then measure the angle to the star compared to some distant reference object. If the star is close, the angle will be measurable, and the distance can be calculated.

You can only really measure the distance to the nearest stars this way, since it only works to about 100 light-years.

The 20 Closest Stars

Here is a list of the 20 closest star systems and their distance in light-years. Some of these have multiple stars, but they’re part of the same system.

Alpha Centauri – 4.2
Barnard’s Star – 5.9
Wolf 359 – 7.8
Lalande 21185 – 8.3
Sirius – 8.6
Luyten 726-8 – 8.7
Ross 154 – 9.7
Ross 248 – 10.3
Epsilon Eridani – 10.5
Lacaille 9352 – 10.7
Ross 128 – 10.9
EZ Aquarii – 11.3
Procyon – 11.4
61 Cygni – 11.4
Struve 2398 – 11.5
Groombridge 34 – 11.6
Epison Indi – 11.8
Dx Carncri – 11.8
Tau Ceti – 11.9
GJ 106 – 11.9

According to NASA data, there are 45 stars within 17 light years of the Sun. There are thought to be as many as 200 billion stars in our galaxy. Some are so faint that they are nearly impossible to detect. Maybe, with technological improvements, scientists will find even closer stars.

June 12, 2013December 23, 2015 by David Dickinson

What is a Super Moon?

It’s a bird, it’s a plane it’s…

OK, it’s a bad gag, I know. But the movie Man of Steel isn’t the only thing that’s “super” about June this year. The closest full Moon of 2013 occurs on June 23, when it will be 356,991 kilometres from Earth, within 600 kilometres of its closest possible approach. When the Moon is closest to Earth in its orbit, it also appears just a bit larger in the sky. But that’s if you’re really paying attention, however!

Some claims circulating on the Internet tend to exaggerate how large the Moon will actually appear. And as for the assertions that the Moon will look bright purple or blue on June 23, that’s just not true. As seems to happen every year, the term “supermoon” has once again reared its (ugly?) head across ye ole Internet. Hey, it’s a teachable moment, a good time to look at where the term came from, and examine the wonderful and wacky motion of our Moon.

I’ll let you in on a small secret. Most astronomers, both of the professional and backyard variety, dislike the informal term “supermoon”. It arose in astrology circles over the past few decades, and like the term “Blue Moon” seems to have found new life on the Internet. A better term from the annuals of astronomy for the near-coincidence of the closest approach of the Full Moon would be Perigee Full Moon. And if you really want to be archaic, Proxigean Moon is also acceptable.

On June 23, 2013, the Moon will be full at 7:32 AM EDT/ 11:32 UT, only 20 minutes after it reaches perigee, or its closest point to Earth in its orbit.

You can see the change in apparent size of the Moon (along with a rocking motion of the Moon known as nutation and libration) in this video from the Goddard Space Flight Center’s Scientific Visualization Studio. You can also see full animations for Moon phases and libration for 2013 from the northern hemisphere and southern hemisphere.

And all perigees are not created equal, either. Remember, a Full Moon is an instant in time when the Moon’s longitude along the ecliptic is equal to 180 degrees. Thus, the Full Moon rises (unless you’re reading this from high polar latitudes!) opposite as the Sun sets. Perigee also oscillates over a value of just over 2 Earth radii (14,000 km) from 356,400 to 370,400 km. And while that seems like a lot, remember that the average distance to the Moon is about 60 earth radii, or 385,000 km distant.

Astronomers yearn for kryptonite for the supermoon. The Moon passes nearly as close every 27.55 days, which is the time that it takes to go from one perigee to another, known as an anomalistic month. This is not quite two days shorter than the more familiar synodic month of 29.53 days, the amount of time it takes the Moon to return to similar phase (i.e. New to New, Full to Full, etc).

This offset may not sound like much, but 2 days can add up. Thus, in six months time, we’ll have perigee near New phase and the smallest apogee Full Moon of the year, falling in 2013 on December 19^th. Think of the synodic and anomalistic periods like a set of interlocking waves, cycling and syncing every 6-7 months.

You can even see this effect looking a table of supermoons for the next decade;

Super Moons for the Remainder of the Decade 2013-2020.
Year	Date	Perigee Time	Perigee Distance	Time from Full	Notes
2013	June 23	11:11UT	356,989km	< 1 hour
2013	July 21	20:28UT	358,401km	-21 hours
2014	July 13	8:28UT	358,285km	+21 hours
2014	August 10	17:44UT	356,896km	< 1 hour
2014	September 8	3:30UT	358,387km	-22 hours
2015	August 30	15:25UT	358,288km	+20 hours
2015	September 28	1:47UT	356,876km	-1 hour	Eclipse
2015	October 26	13:00UT	358,463km	-23 hours
2016	October 16	23:37UT	357,859km	+19 hours	Farthest
2016	November 14	11:24UT	356,511km	-2 hours	Closest
2017	December 4	8:43UT	357,495km	+16 hours
2018	January 1	21:56UT	356,565km	-4 hours
2019	January 21	19:59UT	357,344km	+14 hours	Eclipse
2019	February 19	9:07UT	356,761km	-6 hours
2020	March 10	6:34UT	357,122km	+12 hours
2020	April 7	18:10UT	356,908km	-8 hours
Sources: The fourmilab Lunar Perigee & Apogee Calculator & NASA’s Eclipse Website 2011-2020.Note: For the sake of this discussion, a supermoon is defined here as a Full Moon occurring within 24 hours of perigee. Other (often arbitrary) definitions exist!

Note that the supermoon slowly slides through our modern Gregorian calendar by roughly a month a year.

In fact, the line of apsides (an imaginary line drawn bisecting the Moon’s orbit from perigee to apogee) completes one revolution every 8.85 years. Thus, in 2022, the supermoon will once again occur in the June-July timeframe.

To understand why this is, we have to look at another unique feature of the Moon’s orbit. Unlike most satellites, the Moon’s orbit isn’t fixed in relation to its primaries’ (in this case the Earth’s) equator. Earth rotational pole is tilted 23.4 degrees in relation to the plane of its orbit (known as the ecliptic), and the Moon’s orbit is set at an inclination of 5.1 degrees relative to the ecliptic. In this sense, the Earth-Moon system behaves like a binary planet, revolving around a fixed barycenter.

The two points where the Moon’s path intersects the ecliptic are known as the ascending and descending nodes. These move around the ecliptic as well, lining up (known as a syzygy) during two seasons a year to cause lunar and solar eclipses.

The complex motion of the Moon, depicting the precession of the nodes versus the average movement of the line of apsides. (Credit: Geologician, Homunculus 2. Wikimedia Commons graphic under a Creative Common Attribution 3.0 Unported license). — The complex motion of the Moon, depicting the movement of the nodes versus the average movement of the line of apsides. (Credit: Geologician, Homunculus 2. Wikimedia Commons graphic under a Creative Common Attribution 3.0 Unported license).

But our friend the line of apsides is being dragged backwards relative to the motion of the nodes, largely by the influence of our Sun. Not only does this cause the supermoons to shift through the calendar, but the Moon can also ride ‘high’ with a declination of around +/-28 degrees relative to the celestial equator once every 19 years, as happened in 2006 and will occur again in 2025.

Falling only two days after the solstice, this month’s supermoon is also near where the Sun will be in December and thus will also be the most southerly Full Moon of 2013. Visually, the Full Moon only varies 14% in apparent diameter from 34.1’ (perigee) to 29.3’ (apogee).

Can you see the difference? A side by side comparison of the perigee and apogee Moon. (Credit: Inconstant Moon).

A fun experiment is to photograph the perigee Moon this month and then take an image with the same setup six months later when the Full Moon is near apogee. Another feat of visual athletics would be to attempt to visually judge the Full Moons throughout a given year. Which one do you think is largest & smallest? Can you discern the difference with the naked eye? Of course, you’d also have to somehow manage to insulate yourself from all the supermoon hype!

A comparison of the rising Moon (left) & the Full Moon high in the sky... as you can see, atmospheric refraction actually tends to "shrink" the apparent size of a rising Moon! (Credit: — A comparison of the rising Moon (left) & the Full Moon high in the sky… as you can see, atmospheric refraction actually tends to “shrink” the apparent size of a rising Moon! (Credit & Copyright: Richard Fleet (@dewbow) The Moon Illusion).

Many folks also fall prey to the rising “Moon Illusion.” The Moon isn’t visually any bigger on the horizon than overhead. In fact, you’re about one Earth radii closer to the Moon when it’s at the zenith than on the horizon. This phenomenon is a psychological variant of the Ponzo illusion.

The supermoon of March 19, 2011 (right), compared to an average moon of December 20, 2010 (left). Note the size difference. Image Credit: Marco Langbroek, the Netherlands, via Wikimedia Commons.

Here are some of the things that even a supermoon can’t do, but we’ve actually heard claims for:

– Be physically larger. You’re just seeing the regular-sized Moon, a tiny bit closer.

– Cause Earthquakes. Yes, we can expect higher-than-normal Proxigean ocean tides, and there are measurable land tides that are influenced by the Moon, but no discernible link between the Moon and earthquakes exists. And yes, we know of the 2003 Taiwanese study that suggested a weak statistical correlation. And predicting an Earthquake after it has occurred, (as happened after the 2011 New Zealand quake) isn’t really forecasting, but a skeptical fallacy known as retrofitting.

– Influence human behavior. Well, maybe the 2013 Full Moon will make some deep sky imagers pack it in on Sunday night. Lunar lore is full of such anecdotes as more babies are born on Full Moon nights, crime increases, etc. This is an example the gambler’s fallacy, a matter of counting the hits but not the misses. There’s even an old wives tale that pregnancy can be induced by sleeping in the light of a Full Moon. Yes, we too can think of more likely explanations…

– Spark a zombie apocalypse. Any would-be zombies sighted (Rob Zombie included) during the supermoon are merely coincidental.

Do get out and enjoy the extra illumination provided by this and any other Full Moon, super or otherwise. Also, be thankful that we’ve got a large nearby satellite to give our species a great lesson in celestial mechanics 101!

June 10, 2013June 5, 2017 by Fraser Cain

Ten Interesting Facts About Saturn

This article comes from our archive, but we updated it with this video.

Saturn is my absolute favorite object in the night sky. When I was a child, I had a dog-eared book on the Solar System, which I read over and over, stopping and staring with wonder at the section on Saturn. How could a planet have rings of ice? What would it be like to fly out and visit the planet, to see the rings with your own eyes. How did it get all those strange moons?

When I was 14, I purchased my first telescope, a 4-inch Newtonian from a local company in Vancouver. It was summer, and one of the first planets, appearing just after sunset was Saturn. And my telescope had just enough power and magnification to resolve the planet and its famous rings. In fact, when I first looked at Saturn through the eyepiece, I couldn’t believe that I was now seeing the planet with my own eyes. It didn’t look quite like the photographs, but my imagination could fill in the gaps.

From those first observations, my fascination with astronomy and Saturn only grew, leading me to a career in science journalism. It’s funny to think how far I’ve come, and how I can trace everything back to those warm summer nights, looking at Saturn.

Think you know everything about Saturn? Think again. Here are 10 facts about Saturn, some you may know, and some you probably didn’t know.

1. Saturn is the least dense planet in the Solar System

Saturn has a density of 0.687 grams/cubic centimeter. Just for comparison, water is 1 g/cm³ and the Earth is 5.52. Since Saturn is less dense than water, it would actually float like an apple if you could find a pool large enough. Of course, why you’d want to ruin a pool with all that hydrogen, helium and ices…

2. Saturn is a flattened ball

Saturn spins so quickly on its axis that the planet flattens itself out into an oblate spheroid. Seriously, you see this by eye when you look at a picture of Saturn; it looks like someone squished the planet a little. Of course, it’s the rapid spinning that’s squishing it, causing the equator to bulge out.

While the distance from the center to the poles is 54,000 km, the distance from the center to the equator is 60,300 km. In other words, locations on the equator are approximately 6,300 km more distant from the center than the poles.

We have a similar phenomenon here on Earth, where points on the equator are more distant from the center of the Earth, but on Saturn, it’s much more extreme.

Cassini's drawings of Saturn — Cassini’s drawings of Saturn

3. The first astronomers thought the rings were moons.

When Galileo first turned his rudimentary telescope on Saturn in 1610, he could see Saturn and its rings, but he didn’t know what he was looking at. He though that the rings might actually be two large moons stuck to either side of Saturn – ears maybe?

It wasn’t until 1655 that the Dutch astronomer Christian Huygens used a better telescope to observe Saturn. He had the resolution to realize that the moons on either side of Saturn were actually rings: “a thin, flat ring, nowhere touching, and inclined to the ecliptic.” Huygens was also the first person to discover Saturn’s largest moon, Titan.

4. Saturn has only been visited 4 times by spacecraft

Only 4 spacecraft sent from Earth have ever visited Saturn, and three of these were just brief flybys. The first was Pioneer 11, in 1979, which flew within 20,000 km of Saturn. Next came Voyager 1 in 1980, and then Voyager 2 in 1981. It wasn’t until Cassini’s arrival in 2004 that a spacecraft actually went into orbit around Saturn and captured photographs of the planet and its rings and moons.

Unfortunately, there are no future plans to send any more spacecraft to Saturn. A few missions have been proposed, including such radical concepts as a sailboat that could traverse the liquid methane lakes on Titan.

5. Saturn has 62 moons

Jupiter has 67 discovered moons, but Saturn is a close second with 62. Some of these are large, like Titan, the second largest moon in the Solar System. But most are tiny – just a few km across, and they have no official names. In fact, the last few were discovered by NASA’s Cassini orbiter just a few years ago. More will probably be discovered in the coming years.

6. The length of a day on Saturn was a mystery until recently

Determining the rotation speed of Saturn was actually very difficult to do, because the planet doesn’t have a solid surface. Unlike Mercury, you can’t just watch to see how long it takes for a specific crater to rotate back into view; astronomers needed to come up with a clever solution: the magnetic field.

To determine the rotational speed of Saturn, astronomers had to measure the rotation of the planet’s magnetic field. By one measurement, Saturn takes 10 hours and 14 minutes to turn on its orbit, but when Cassini approached Saturn, it clocked the rotation at 10 hours and 45 minutes. Astronomers now agree on an average day of 10 hours, 32 minutes and 35 seconds.

7. Saturn’s rings could be old, or they could be young.

It’s possible that Saturn’s rings have been around since the beginning of the Solar System – around 4.54 billion years ago. Or maybe they’re relatively brand new compared to the age of Saturn. Astronomers still don’t fully understand the origin of Saturn’s rings.

They might have formed recently, when a 300-km ice moon was torn apart by Saturn’s gravity, forming a ring around the planet.

It’s also possible that they’re the left over material when Saturn formed in the solar nebula. The material in the rings might have gotten jostled by Saturn’s gravity, and never could pull together into a cohesive Moon.

But astronomers have also found that the ring material looks just too clean to have formed so long ago, and could be as young as 100 million years old. It’s all just a big mystery.

Credit: NASA/JPL-Caltech/Space Science Institute

8. Sometimes the rings disappear

Well, they don’t actually disappear, but they look like they’re going away. Saturn’s axis is tilted, just like Earth. From our point of view, we see Saturn’s changing position as it takes its 30 year journey around the Sun. Sometimes, the rings are fully open, and we see them in all their glory, but other times we see the rings edge on – it looks like they’ve disappeared. This happened in 2008-2009, and will happen again in 2024-2025.

9. You can see Saturn with your own eyes

Saturn appears as one of the 5 planets visible with the unaided eye. If Saturn is in the sky at night, you can head outside and see it. To see the rings and the ball of the planet itself, you’ll want to peer through a telescope. But you can amaze your friends and family by pointing out that bright star in the sky, and let them know they’re looking at Saturn.

10. There could be life near Saturn

Not life on Saturn; the planet is way too hostile to support life. But there could be life on one of Saturn’s moons: Enceladus.

Water vapour geysers on Enceladus. Credit: NASA/JPL

NASA’s Cassini spacecraft recently discovered ice geysers blasting out of Enceladus’ southern pole. This means that some process is keep the moon warm enough that water can remain a liquid underneath the surface. And wherever we find liquid water on Earth, we find life.

Want to learn more about Saturn?

We’ve recorded two episodes of Astronomy Cast just about Saturn. The first is Episode 59: Saturn, and the second is Episode 61: Saturn’s Moons.

June 6, 2013October 16, 2016 by Fraser Cain

How Can You See a Satellite View of Your House?

This is an article from our archive, but we’ve updated it with this spiffy video.

Every now and then I go looking for a free aerial view of my home. It’s amazing what’s available through the internet now, totally free. Thanks to commercial Earth observation satellites, and internet tools that make these photos accessible through the internet, it’s easy to see your house from space.

In our modern space age, there are more than 8,000 satellites currently orbiting the Earth. The vast majority of these are relaying data to and from the Earth, and many are equipped with high power cameras. Just look up into the sky any night, and you’re sure to see satellite after satellite passing overhead. But what are some ways you can get access to these satellite and aerial images of your house?

Satellite Images of the Whole Earth
If you want to go way out and just see a satellite image of the entire planet, there are some solutions for you: weather satellites. For example, NOAA’s Geostationary Operational Environmental Satellites (GOES) release images of an entire hemisphere of planet Earth every 3 hours. From these images you can see major weather patterns affecting parts of the Earth. But you really can’t see any specific spot on Earth with any detail.

What is really cool about these satellite views is that they’re live. The weather systems you’re seeing in those images are happening on the planet right now.

If you don’t want a live view, and really just want to see a beautiful view of the Earth’s hemisphere, check out these images produced by NASA. Here’s a composite photograph that shows the Earth’s Western Hemisphere, and here’s a view of the Earth’s Eastern Hemisphere.

There were also some amazing new satellite images of the Earth released from the European Space Agency’s 3rd generation Meteosat spacecraft.

Zoom in. Let’s see Satellite Pictures of Houses
If those whole Earth pictures don’t give you enough detail, let’s zoom in, and see some pictures of houses from space. The best tool on the market, in my opinion, is the service from Google Maps. All you need is a web browser and a connection to the internet. When you first start up, Google Maps displays a satellite view of North America. You can then zoom in, or pan the camera around to see any location on Earth. You can also type in the address of the location that you want to see. Once you do that, you’ll get a free satellite view of your house. You can save the image or print it off.

View Larger Map

Another cool tool from Google is Google Earth. You can access by going to http://earth.google.com. The main difference between Google Maps and Google Earth is that you have to download and install Earth on your local computer (they have a version for PCs, Mac, Linux, and even the iPhone). Once you’ve downloaded and installed Google Earth, you can see a 3-dimensional view of Earth that you can zoom in and out and spin around. You can type in your address and get a view of your house from above. I actually like the printing function of Google Earth better, since it’s using your printer directly, and not going through the web browser.

And if you really hate using products from Google, no problem. There are similar services from Yahoo and Microsoft. Microsoft’s mapping service used to be called MSN Maps, and now it’s been changed to Bing Maps with their new identity. The Yahoo service is called Yahoo Maps, and it’s very similar to Google Maps. The two services do have some big differences, though, and there’s a cool application that lets you see the two of them side-by-side. I used it for my home and found that Google Maps has better resolution maps for my city.

Where Do All these Pictures Come From?
Google Maps and the other internet mapping services are really just customers for the satellite services that actually take these photographs from space. There are a few major services on the market, including GeoEye. GeoEye’s main competitors include DigitalGlobe and Spot Image.

Each company has a fleet of Earth observation satellites, with a capability of resolving features on the surface of the Earth as small as about 45 cm (18 inches). In other words, an object 45 centimeters across would appear as a single pixel in their photographs.

Private earth observation satellites:

GeoEye – 5 satellites: IKONOS, OrbView-2, OrbView-3, GeoEye-1, GeoEye-2 (in 2013).
DigitalGlobe – 4 satellites: Early Bird 1, Quickbird, WorldView-1, Worldview-2
Spot Image – 2 satellites: Spot 4, Spot 5

Each of these services allow customers to purchase satellite imagery directly, but I’ll warn you, the prices are extremely high: hundreds or even thousands of dollars for satellite imagery. You typically can’t buy directly from the satellite company itself, but through their international partners.It’s better to stick to the free sources.

Are These Live Satellite Views?
When you look at these amazing views of your house from space, you might wonder if this is live. If you walk outside and look up, will you be able to see yourself from space? Unfortunately, no. All of the free satellite images you’re accessing were captured by various spacecraft over the last couple of years.

You can actually get a pretty good sense of when the picture was taken by the image of your property. For example, in the photo of my house from space, I can see a car that I sold a couple of years ago. Obviously, this image isn’t live, it’s at least a couple of years old – even a decade old in some cases. A live satellite view of your house, is still a few years off.

There are some services which will give you a live view of Earth from space. For example, you can access a live broadcast from NASA’s International Space Station. About 40% of the time, if you follow this link you can see a live view of Earth from the space station. Another service called Urthecast will be attaching a high definition camera to the International Space Station in 2013 to broadcast a live view of Earth from space.

Streaming video by Ustream

June 5, 2013May 7, 2020 by Nancy Atkinson

Can You See the Great Wall of China from Space?

This picture, apparently the first verifiable photo of the Great Wall of China shot from low Earth orbit, was taken by International Space Station Commander Leroy Chiao on Nov. 24, 2004. Credit: NASA

One popular myth about space exploration is that the Great Wall of China is the only human-built structure that can be seen from space. But this is not true. The reality is that you can’t easily see the Great Wall with the unaided eye, even from low Earth orbit. And certainly, the Apollo astronauts couldn’t see it from the Moon, even though that urban legend has been widely circulated.

Canadian astronaut Chris Hadfield, who spent five months aboard the International Space Station in 2012-2013, reiterated the facts about the Great Wall’s visibility from space.

“The Great Wall of China is not visible from orbit with the naked eye,” Hadfield said via Twitter. “It’s too narrow, and it follows the natural contours and colours [of the landscape].”

Additionally, when China’s first astronaut, Yang Liwei, went into space in 2003, he said that he couldn’t see the structure of the Great Wall from out his capsule window.

NASA has confirmed that US astronaut Leroy Chiao took what is thought to be the first verifiable image of the Great Wall of China from out his window on the International Space Station in 2004, using a zoom lens. He photographed a region of Inner Mongolia, about 200 miles north of Beijing, but said Chiao himself said he didn’t see the wall with his unaided eyes, and wasn’t sure if the picture showed it.

The image above was taken with a 180mm zoom lens. If you can’t make out the Great Wall in the image above, here’s a cropped version of the image with annotation to help make out the feature:

This photo of central Inner Mongolia, about 200 miles north of Beijing, was taken on Nov. 24, 2004, from the International Space Station. The yellow arrow points to an estimated location of 42.5N 117.4E where the wall is visible. The red arrows point to other visible sections of the wall. Credit: NASA.

What human-made structures are visible from space? Space Station astronauts have said the ancient pyramids at Giza are relatively easy to see out the window, but most visible are roads or long bridges across straits. Those features stand out as straight lines on the landscape, such as this image shared by Chris Hadfield:

'One straight human line drawn onto incredibly rough terrain,' said astronaut Chris Hadfield about this image. Credit: NASA/CSA/Chris Hadfield. — ‘One straight human line drawn onto incredibly rough terrain,’ said astronaut Chris Hadfield about this image. Credit: NASA/CSA/Chris Hadfield.

And, of course, at night cities are visible from space because the light they produce. You can see some stunning images here that NASA released in 2012 from the Suomi NPP satellite of city lights from space.

The Apollo astronauts confirmed that you can’t see the Great Wall of China from the Moon. In fact, all you can see from the Moon is the white and blue marble of our home planet.

With all of the human construction, many buildings and other structures can be seen from space. But you can’t see the Great Wall of China from space.

The Blue Marble from Apollo 17 — The Blue Marble image of Earth from Apollo 17. Credit: NASA

June 3, 2013October 16, 2016 by Fraser Cain

How Many Stars are There in the Universe?

When we look at the night sky, filled with stars, it’s hard to resist counting. Just with the unaided eye, in dark skies, you can see a few thousand.

How many stars are there in the entire Universe? Before we get to that massive number, let’s consider what you can count with the tools available to you.

Perfect vision in dark skies allows us to see stars down to about magnitude 6. But to really make an accurate census of the total number of stars, you’d need to travel to both the Northern and Southern Hemispheres, since only part of the sky is visible from each portion of the Earth. Furthermore, you’d need to make your count over several months, since a portion of the sky is obscured by the Sun. If you had perfect eyesight and traveled to completely dark skies in both the Northern and Southern Hemispheres, and there was no Moon, you might be able to get to count up almost 9,000 stars.

With a good pair of binoculars, that number jumps to about 200,000, since you can observe stars down to magnitude 9. A small telescope, capable of resolving magnitude 13 stars will let you count up to 15 million stars. Large observatories could resolve billions of stars.

But how many stars are out there? How many stars are there in the Milky Way?

According to astronomers, our Milky Way is an average-sized barred spiral galaxy measuring up to 120,000 light-years across. Our Sun is located about 27,000 light-years from the galactic core in the Orion arm. Astronomers estimate that the Milky Way contains up to 400 billion stars of various sizes and brightness.

A few are supergiants, like Betelgeuse or Rigel. Many more are average-sized stars like our Sun. The vast majority of stars in the Milky Way are red dwarf stars; dim, low mass, with a fraction of the brightness of our Sun.

As we peer through our telescopes, we can see fuzzy patches in the sky which astronomers now know are other galaxies like our Milky Way. These massive structures can contain more or less stars than our own Milky Way.

There are spiral galaxies out there with more than a trillion stars, and giant elliptical galaxies with 100 trillion stars.
And there are tiny dwarf galaxies with a fraction of our number of stars.

So how many galaxies are there?

According to astronomers, there are probably more than 170 billion galaxies in the observable Universe, stretching out into a region of space 13.8 billion light-years away from us in all directions.

And so, if you multiply the number of stars in our galaxy by the number of galaxies in the Universe, you get approximately 10²⁴ stars. That’s a 1 followed by twenty-four zeros.

That’s a septillion stars.

But there could be more than that.

It’s been calculated that the observable Universe is a bubble of space 47 billion years in all directions.

It defines the amount of the Universe that we can see, because that’s how long light has taken to reach us since the Big Bang.

This is a minimum value, the Universe could be much bigger – it’s just that we can’t ever detect those stars because they’re outside the observable Universe. It’s even possible that the Universe is infinite, stretching on forever, with an infinite amount of stars. So add a couple more zeros. Maybe an infinite number of zeroes.

That’s a lot of stars in the Universe.

Additional Resources:
How Many Stars Can you See?
Astronomy Cast: How Big is the Universe?
How Big is Our Observable Universe
Astronomy Cast: The Observable Universe
How Many Galaxies in the Universe?

May 30, 2013October 16, 2016 by Fraser Cain

How Old Is The Earth?

This article comes from the Universe Today archive, but was updated with this spiffy video.

How old is the Earth? Scientists think that the Earth is 4.54 billion years old. Coincidentally, this is the same age as the rest of the planets in the Solar System, as well as the Sun. Of course, it’s not a coincidence; the Sun and the planets all formed together from a diffuse cloud of hydrogen billions of years ago.

In the early Solar System, all of the planets formed in the solar nebula; the remnants left over from the formation of the Sun. Small particles of dust collected together into larger and larger objects – pebbles, rocks, boulders, etc – until there were many planetoids in the Solar System. These planetoids collided together and eventually enough came together to become Earth-sized.

At some point in the early history of Earth, a planetoid the size of Mars crashed into our planet. The resulting collision sent debris into orbit that eventually became the Moon.

How do scientists know Earth is 4.54 billion years old? It’s actually difficult to tell from the surface of the planet alone, since plate tectonics constantly reshape its surface. Older parts of the surface slide under newer plates to be recycled in the Earth’s core. The oldest rocks ever found on Earth are 4.0 – 4.2 billion years old.

Scientists assume that all the material in the Solar System formed at the same time. Various chemicals, and specifically radioactive isotopes were formed together. Since they decay in a very known rate, these isotopes can be measured to determine how long the elements have existed. And by studying different meteorites from different locations in the Solar System, scientists know that the different planets all formed at the same time.

Failed Methods for Calculating the Age of the Earth
Our current, accurate method of measuring the age of the Earth comes at the end of a long series of estimates made through history. Clever scientists discovered features about the Earth and the Sun that change over time, and then calculated how old the planet Earth is from that. Unfortunately, they were all flawed for various reasons.

Declining Sea Levels – Benoit de Maillet, a French anthropologist who lived from 1656-1738 and guessed (incorrectly) that fossils at high elevations meant Earth was once covered by a large ocean. This ocean had taken 2 billion years to evaporate to current sea levels. Scientists abandoned this when they realized that sea levels naturally rise and fall.
Cooling of the Earth – William Thompson, later known as Lord Kelvin, assumed that the Earth was once a molten ball of rock with the same temperature of the Sun, and then has been cooling ever since. Based on these assumptions, Thompson calculated that the Earth took somewhere between 20 and 400 million years to cool to its current temperature. Of course, Thompson made several inaccurate assumptions, about the temperature of the Sun (it’s really 15 million degrees Kelvin at its core), the temperature of the Earth (with its molten core) and how the Sun is made of hydrogen and the Earth is made of rock and metal.
Cooling of the Sun – In 1856, the German physicist Hermann Ludwig Ferdinand von Helmholtz attempted to calculate the age of the Earth by the cooling of the Sun. He calculated that the Sun would have taken 22 million years to condense down to its current diameter and temperature from a diffuse cloud of gas and dust. Although this was inaccurate, Helmholtz correctly identified that the source of the Sun’s heat was driven by gravitational contraction.
Rock Erosion – In his book, The Origin of Species by Means of Natural Selection, Charles Darwin proposed that the erosion of chalk deposits might allow for a calculation of the minimum age of the planet. Darwin estimated that a chalk formation in the Weald region of England might have taken 300 million years to weather to its current form.

Orbit of the Moon – George Darwin, the son of Charles Darwin, guessed that the Moon might have been formed out of the Earth, and drifted out to its current location. The fission theory proposed that the Earth’s rapid rotation caused a chunk of the planet to spin off into space. Darwin calculated that it had taken the Moon at least 56 million years to reach its current distance from Earth. We now know the Moon was probably formed when a Mars-sized object smashed into the Earth billions of years ago.
Salinity of the Ocean – In 1715, the famous astronomer Edmund Halley proposed that the salinity of the oceans could be used to estimate the age of the planet. Halley observed that oceans and lakes fed by streams were constantly receiving more salt, which then stuck around as the water evaporated. Over time, the water would be come saltier and saltier, allowing an estimate of how long this process has been going on. Various geologists used this method to guess that the Earth was between 80 and 150 million years old. This method was flawed because scientists didn’t realize that geologic processes are extracting salt out of the water as well.

Radiometric Dating Provides an Accurate Method to Know the Age of the Earth
In 1896, the French chemist A. Henri Becquerel discovered radioactivity, the process where materials decay into other materials, releasing energy. Geologists realized that the interior of the Earth contained a large amount of radioactive material, and this would be throwing off their calculations for the age of the Earth. Although this discovery revealed flaws in the previous methods of calculating the age of the Earth, it provided a new method: radiometric dating.

Geologists discovered that radioactive materials decay into other elements at a very predictable rate. Some materials decay quickly, while others can take millions or even billions of years to fully decay. Ernest Rutherford and Frederick Soddy, working at McGill University, determined that half of any isotope of a radioactive element decays into another isotope at a set rate. For example, if you have a set amount of Thorium-232, half of it will decay over a billion years, and then half of that amount will decay in another billion years. This is the source of the term “half life”.

By measuring the half lives of radioactive isotopes, geologists were able to build a measurement ladder that let them accurately calculate the age of geologic formations, including the Earth. They used the decay of uranium into various isotopes of lead. By measuring the amount of three different isotopes of lead (Pb-206, Pb-207, and Pb-208 or Pb-204), geologists can calculate how much Uranium was originally in a sample of material.

If the Solar System formed from a common pool of matter, with uniformly distributed Pb isotopes, then all objects from that pool of matter should show similar amounts of the isotopes. Also, over time, the amounts of Pb-206 and Pb-207 will change because as these isotopes are end-products of uranium decay. This makes the amount of lead and uranium change. The higher the uranium-to-lead ratio of a rock, the more the Pb-206/Pb-204 and Pb-207/Pb-204 values will change with time. Now, supposing that the source of the Solar system was also uniformly distributed with uranium isotopes, then you can draw a data line showing a lead-to-uranium plot and, from the slope of the line, the amount of time which has passed since the pool of matter became separated into individual objects can be computed.

Bertram Boltwood applied this method of dating to 26 different samples of rocks, and discovered that they had been formed between 92 and 570 million years old, and further refinements to the technique gave ages between 250 million to 1.3 billion years.

Geologists set about exploring the Earth, seeking the oldest rock formations on the planet. The oldest surface rock is found in Canada, Australia and Africa, with ages ranging from 2.5 to 3.8 billion years. The very oldest rock was discovered in Canada in 1999, and estimated to be just over 4 billion years old.

This set a minimum age for the Earth, but thanks to geologic processes like weathering and plate tectonics, it could still be older.

Meteorites as the Final Answer to the Age of the Earth
The problem with measuring the age of rocks on Earth is that the planet is under constant geological change. Plate tectonics constantly recycle portions of the Earth, blending it up and forever hiding the oldest regions of the planet. But assuming that everything in the Solar System formed at the same time, meteorites in space have been unaffected by weathering and plate tectonics here on Earth.

Geologists used these pristine objects, such as the Canyon Diablo meteorite (the fragments of the asteroid that impacted at Barringer Crater) as a way to get at the true age of the Solar System, and therefore the Earth. By using the radiometric dating system on these meteorites, geologists have been able to determine that the Earth is 4.54 billion years old within a margin of error of about 1%.

Sources:
Understanding Science – Lord Kelvin
USGS Age of the Earth
Lord Kelvin’s Failed Scientific Clock
The Role of Radioactive Decay
Astronomy Cast Episode 51: Earth
Oldest Rock Formations Found

May 27, 2013October 16, 2016 by Fraser Cain

How Many Planets are in the Solar System?

I’m just going to warn you, this is a controversial topic. Some people get pretty grumpy when you ask: how many planets are in the Solar System? Is it eight, ten, or more?

I promise you this, though, we’re never going back to nine planets… ever.

When many of us grew up, there were nine planets in the Solar System. It was like a fixed point in our brains.

As kids, memorizing this list was an early right of passage of nerd pride: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto.

But then in 2005, Mike Brown discovered Eris, an icy object thought to be about the same size as Pluto, out beyond its orbit.

That would bring the total number of planets to ten. Right? There’s no turning back, textbooks would need to be changed.
In order to settle the dispute, the International Astronomical Union met in 2006, and argued for, and against Pluto’s planethood. Some astronomers advocated widening the number of planets to twelve, including Pluto, its moon Charon, the Asteroid Ceres, and the newly discovered Eris.

In the end, they changed the definition of what makes a planet, and sadly, Pluto doesn’t make the cut:

Here are the new requirements of planethood status:

A planet has to orbit the Sun. Okay fine, Pluto does that.
A planet needs enough gravity to pull itself into a sphere. Okay, spherical. Pluto’s fine there too.
A planet needs to have cleared out its orbit of other objects. Uh oh, Pluto hasn’t done that.

For example, planet Earth accounts for a million times the rest of the material in its orbit, while Pluto is just a fraction of the icy objects in its realm.

The final decision was to demote Pluto from planet to dwarf planet.

But don’t despair, Pluto is in good company.

There’s Ceres, the first asteroid ever discovered, and the smallest of the dwarf planets. The surface of Ceres is made of ice and rock, and it might even have a liquid ocean under its surface. NASA’s Dawn mission is flying there right now to give us close up pictures for the first time.

Haumea, named after the Hawaiian goddess of fertility, is about a third the mass of Pluto, and has just enough gravity to pull itself into an ellipsoid, or egg shape. Even though it’s smaller, it’s got moons of its own.

Makemake, a much larger Kuiper belt object, has a diameter about two-thirds the size of Pluto. It was discovered in 2005 by Mike Brown and his team. So far, Makemake doesn’t seem to have any moons.

Eris is the most massive known dwarf planet, and the one that helped turn our definition of a planet upside-down. It’s 27% more massive than Pluto and the ninth most massive body that orbits the Sun. It even has a moon: Dysnomia.

And of course, Pluto. The founding member of the dwarf family.

Want an easy way to remember the eight planets, in order? Just remember this mnemonic: my very excellent mother just served us noodles.

For all you currently writing angry tweets to Mike Brown, hold on a sec. Changing Pluto’s categorization is an important step that really needed to happen.

The more we discover about our Universe, the more we realize just how strange and wonderful it is. When Pluto was discovered 80 years ago, we never could have expected the variety of objects in the Solar System. Categorizing Pluto as a dwarf planet helps us better describe our celestial home.

So, our Solar System now has eight planets, and five dwarf planets.

May 23, 2013October 16, 2016 by Fraser Cain

How to See the International Space Station

The International Space Station is one of the most complicated machines ever built and the largest object ever assembled in space.

At any time there are up to six astronauts on board, each originating from one of fifteen different nations on Earth. It orbits at an altitude of approximately four-hundred kilometers, and completes an orbit around the Earth every ninety-two minutes and fifty seconds. The station has a mass of four-hundred-and-twenty metric tonnes, and contains a dozen pressurized modules, and many more unpressurized modules, trusses and solar panels.

It truly is a feat of human ingenuity.

But did you know that the International Space Station is one of the brightest objects in the night sky? And it’s easy to see if you know when, and where, to look.

In fact, with your ability to find the station you can amaze your friends and neighbours.

The best place to start is NASA’s Spot the Station website. Enter your Country, Region, City along with an email address or mobile phone number. Then give your preference for notifications in the evening, morning or both and that’s it.

About twelve hours before the station is due to fly overhead, you’ll get a notification from NASA. Depending on your location, you might get notified a couple of times a week, or as rarely as once a month. As soon as you get the notification, create an alarm on your phone for about a minute before the flyover.

When the alarm goes off, take your friends outside and look to the West.

Station's path across the sky — Station’s path across the sky

The station orbits the Earth from West to East, so you’ll see it appear on the Western horizon as a very bright star, moving rapidly across the sky. It will take only few minutes to cross the entire sky.

The station moves so quickly if you’re using a telescope you will have a tough time tracking its movement. A nice pair of binoculars will make it look a lot brighter, and even let you see the H-shape of its solar panels. But even viewing it with the naked eye is a great experience.

NASA’s website is just one of the many ways you can get notifications.

If you use Twitter, follow @twisst. They can figure out your location and then send you a notification when the station is about to fly overhead via Twitter.

There are also dozens of Android and iPhone apps that will perform this function; many of which are free to use.

If you’ve never seen the station, head on over to NASA and set up a notification right away.

Then kick back and let orbital mechanics bring the station to your backyard at a time that’s convenient for you.

Want more details? We’ve got a detailed guide on how to View the International Space Station for Beginners, and How to Photograph the International Space Station.

May 20, 2013October 16, 2016 by Fraser Cain

How Fast Does the Earth Rotate?

This article originally appeared in 2009, but I’ve updated it and added this video.

The ground feels firm and solid beneath your feet. Of course, the Earth is rotating, turning once on its axis every day. Fortunately gravity keeps you firmly attached to the planet, and because of momentum, you don’t feel the movement – the same way you don’t feel the speed of a car going down the highway. But how fast does the Earth rotate?

You might be surprised to know that a spot on the surface of the Earth is moving at 1675 km/h or 465 meters/second. That’s 1,040 miles/hour. Just think, for every second, you’re moving almost half a kilometer through space, and you don’t even feel it.

Want to do the calculation for yourself? The Earth’s circumference at the equator is 40,075 km. And the length of time the Earth takes to complete one full turn on its axis is 23.93 hours.

Wait, 23.93 hours? Isn’t a day 24 hours? Astronomers calculate a day in two ways. There’s the amount of time it takes for the Earth to complete one full rotation on its axis, compared to the background stars. Imagine you were looking down at the Earth from above the North Pole. You’d see the Earth turn once completely in 23 hours and 56 minutes. Astronomers call this a sidereal day.

And then there’s the time it takes for the Sun to return to the same spot in the sky. Since the Earth is orbiting the Sun, we actually need an extra 4 minutes each day to return the Sun to the same spot. Astronomers call this a solar day.

Then we divide the length of a day into the distance a point on the equator travels in that period: 40,075 km/23.93 hours = 1,675 km/hour, 465 meters/second.

The speed of the Earth’s rotation changes as you go North or South away from the equator. Finally, when you reach one of the Earth’s poles, you’re taking a whole day to just turn once in place – that’s not very fast.

Because you’re spinning around and around on the Earth, there’s a force that wants to spin you off into space; like when you spin a weight on a string. But don’t worry, that force isn’t very strong, and it’s totally overwhelmed by the force of gravity holding you down. The force that wants to throw you into space is only 0.3% the force of gravity. In other words, if the Earth wasn’t spinning, you would weigh 0.3% more than you do right now.

Space agencies take advantage of the higher velocities at the Earth’s equator to launch their rockets into space. By launching their rockets from the equator, they can use less fuel, or launch more payload with the same amount of fuel. As it launches, the rocket is already going 1,675 km/hour. That makes it easier to reach the 28,000 km/hour orbital velocity; or even faster to reach geosynchronous orbit.

We have written many articles about the Earth for Universe Today. Here’s an article about why the Earth rotates.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

References:
NASA Space Place
NASA Solar System Exploration: Earth