Category: Guide to Space

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Hey, here’s a question: does the Moon orbit the Sun? Of course not, it’s a silly question. Of course the Moon orbits the Earth.

But wait a second and think. The Moon follows the Earth around the Sun in its orbit, and if you didn’t have the Earth, the Moon would really be orbiting the Sun. So can we say that the Moon is really orbiting the Sun?

No. But the case is pretty compelling.

First, take a look at the orbital velocity of the Moon. The Moon’s velocity around the Moon is a mere 1 km/second. But the Moon’s velocity around the Sun is 30 km/sec; same as the Earth.

And here’s something stranger. the Moon doesn’t follow a spiral pattern around the Sun, as you would think, but it always follows a convex path compared to the Sun. It’s not exactly a circle, but it looks like a 12-sided object with rounded corners. Check out this page to see what the Moon’s path around the Sun actually looks like.

Furthermore, the Moon experiences twice the pull of gravity from the Sun than it does from the Earth. So, does the Moon really orbit the Sun?

To figure out what orbits what, you need to know a term called the “Hill sphere” (named after the American astronomer George William Hill). This is the volume of space around an object where its gravity dominates the influence of gravity from a more distant object. If an object is orbiting within this Hill sphere, it’s a moon of the larger object.

You have to calculate the gravity from the primary body (the Earth), the gravity from the secondary body (the Moon) as well as the centrifugal force experienced by a particle moving around the Sun at the same orbit as the Earth. If the sum of those three forces is pointed towards Earth, the Moon is held in orbit. If they didn’t point towards Earth, the Moon would drift away from the Earth and orbit the Sun directly instead.

In other words, because the Moon actually orbits around the Earth; it’s orbiting the Earth. I know that sounds like circular logic, but stay with me. If the Moon wasn’t orbiting Earth, it would go into a smooth orbit around the Sun, and have no regular gravitational interaction with the Earth.

Of course, my good friend Phil Plait over at Bad Astronomy has a great explainer for this puzzler.

You can listen to a very interesting podcast about the formation of the Moon from Astronomy Cast, Episode 17: Where Did the Moon Come From?

Reference:
http://www.math.nus.edu.sg/aslaksen/teaching/convex.html

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A new moon occurs when the Earth, Sun and Moon are lined up so that the Moon is directly in between the Earth and the Sun. From this perspective, the side of the Moon covered in sunlight is turned away from the Earth, and the face we see is in shadow. The Moon takes approximately 29 days to complete the cycle from new moon to full moon and then back to new moon again. During a new moon, the Moon is in conjunction with the Sun, as seen from Earth.

Astronomers call it a “new moon”, because shortly after the Moon is completely shadowed, a think crescent peeks out from one side. This is the beginning of the Moon’s rise to brightness over the next 2 weeks. The exact time this occurs depends on your geography on Earth, but it will always occur in the Western horizon between sunset and moonset.

The orbit of the Moon isn’t exactly lined up the the orbital plane of the Sun and the planets, so it doesn’t actually get exactly between the Earth and the Sun. When this exact line does happen, the new moon is seen as a solar eclipse, darkening the Sun by various amounts depending on your position on the Earth.

We’ve got a list of all the new moons for the next few years. Here’s a list for 2008, 2009, 2010, 2011 and 2012.

Want to know every Moon phase for the next 6000 years? Here’s a handy chart from NASA, and here’s a cool image of a new crescent moon from Astronomy Picture of the Day.

You can listen to a very interesting podcast about the formation of the Moon from Astronomy Cast, Episode 17: Where Did the Moon Come From?

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The full moon occurs when the Sun and Moon are located on opposite sides of the Earth. In this situation, the face of the Moon visible from the Earth is completely illuminated by the Sun. More specifically, the full moon occurs when the geocentric apparent longitudes of the Sun and the Moon are 180 degrees apart. This is a fancy way of saying that the Sun and the Moon are on opposite sides of the sky.

The Moon takes 27.3 days to orbit the Earth. But because the Moon is orbiting around the Earth in the same direction that the Earth is orbiting the Sun, the Moon takes an additional 2.2 days to return to the same position in the sky, where it’s perfectly lined up with the Sun. That’s why the amount of time it takes to go from a full moon to a full moon is 29.5 days long. Astronomers call this length of time a lunar month.

One interesting side note, the month of February only has 28 days. Since that’s less than the 29 day lunar month, there are some years where February doesn’t have a single full moon. The last time this happened was in 1999, and it’s expected to happen again in 2018.

When the Moon is full, it’s at its brightest. Astronomers measure the brightness of an object using a term called apparent magnitude. The apparent magnitude of the full Moon is -12.7. When the Moon is only at its first quarter, its brightness is -10.0, which is a reduction of 12x. Ancient peoples carefully recorded the times from full moon to full moon since those were some of the few times that they could actually see and get work done in the night – before we had artificial illumination.

A blue moon occurs when a single month has two full moons. The second full moon in a calendar month is known as a blue moon. Blue moons tend to occur every 2.7 years.

We have written several stories on Universe Today about the full moon. Here’s one about interesting things that might happen during a full moon. And here’s one about blue moons.

Want to know when the next full moon is going to happen? Here’s a calculator from the US Navy’s Observatory.

You can listen to a very interesting podcast about the formation of the Moon from Astronomy Cast, Episode 17: Where Did the Moon Come From?

References:
http://lunar.arc.nasa.gov/science/phases.htm
http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question3.html

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A lunar month is the amount of time it takes for the Moon to pass through each of its phases (new moon, half, full moon), and then return back to its original position. It takes 29 days, 12 hours, 44 minutes and 3 seconds for the Moon to complete one lunar month.

You might have heard that the Moon only takes 27.3 days to complete one orbit around the Earth. So why is a lunar month more than 2 days longer than the orbit of the Moon?

A lunar month is the amount of time it takes for the Moon to get from a specific phase, like a new moon, back to the same phase. In other words, the Moon has to get back to the point in its orbit where the Sun is in the same position from our point of view. Since the Moon is going around the Sun with the Earth as part of its orbit, the Moon has to catch up a little bit on each orbit. It takes 2.2 additional days each orbit of the Moon to catch up.

This method of measuring a lunar month, from new moon to new moon, is known as a synodic month. A new moon is defined as when the Moon has the same ecliptic longitude as the Sun, as seen from the center of the Earth; when the Sun, Moon and Earth are perfectly lined up.

Were you interested in learning about a lunar day? Here’s an article from Universe Today about some strange things that can happen during the full moon.

Here’s a cool moon phase calculator from stardate.org.

You can listen to a very interesting podcast about the formation of the Moon from Astronomy Cast, Episode 17: Where Did the Moon Come From?

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The Moon takes about 27.3 days to complete one orbit around the Earth. That’s the simple answer to the orbit of the Moon, but there’s a lot more going on, so let’s take a look.

The first think that you have to understand is that that Earth and the Moon actually orbit a common center of gravity. This place is about 4,700 km from the center of the Earth. In other words, the Earth wobbles back and forth because of the orbit of the Moon. Some scientists have even proposed that the Earth and the Moon are actually a double planet because of their relationship, but this would only be the case if the common center of gravity was outside the surface of the Earth.

The orbit of the Moon is about 385,000 km from the Earth on average. Like the planets in the Solar System, the orbit of the Moon isn’t circular; it actually follows an elliptical path around the Earth. At its closest point, called perihelion, the Moon is 364 397 km from the Earth. And then at its most distant point, called aphelion, the Moon is 406 731 km.

You might have heard that the Moon is slowly drifting away from the Earth. Although the Moon is tidally locked to the Earth, presenting the same face to our planet, the Earth isn’t tidally locked. But in about 50 billion years from now, the Moon will complete an orbit once every 47 days, and it will remain in exactly the same place in the sky. One half of the Earth will be able to see the Moon, and it will be hidden from the other half. Of course, the Sun is expected to become a red giant in about 5 billion years and potentially destroy the Earth and Moon, so this time may never come.

Want to learn more about orbits? Here’s an article about the orbit of the Earth, and here’s one about the orbit of Mars.

Here’s more information about the orbit of the Moon, and here’s a cool article from Windows on the Universe.

You can listen to a very interesting podcast about the formation of the Moon from Astronomy Cast, Episode 17: Where Did the Moon Come From?

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The albedo of the Moon is 0.12. In other words, the Moon reflects back 12% of all the radiation that falls upon it.

As you may or may not know, albedo is a term that astronomers use to measure reflectivity of an object in space; more specifically, it measures how much of the Sun’s radiation an object reflects. An albedo of 0 means that object is dark, while a 1 means that it’s very bright and reflective.

So, how does the Moon albedo compare to other objects in the Solar System? As bright as the Moon looks from our perspective here on Earth, the Moon’s albedo is actually pretty low. The object with the highest albedo in the Solar System is Saturn’s moon Enceladus, which has an albedo of 0.99, which means that it’s covered with very reflective snow and ice. The Moon is much more similar to a very dark object, like an asteroid. The darkest asteroids in the Solar System have an albedo of 0.06. That’s pretty close to 0.

The brightness of the Moon changes as its phases change. During the first and last quarters, the visible Moon is illuminated 50% by the Sun, but it only has about 8% of the brightness of a full Moon. This is because the sunlight is hitting the Moon at an angle and deflecting away from us. When the Moon is full, sunlight is hitting the Moon and then bouncing directly back. This is why the Moon is so much brighter during a full Moon.

We have done lots of articles about the Moon on Universe Today. Here’s an article about explosions on the Moon. And here’s another about building a Moon base.

Here’s a great article that helps explain the Moon’s albedo.

You can listen to a very interesting podcast about the formation of the Moon from Astronomy Cast, Episode 17: Where Did the Moon Come From?

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The light from the Moon is actually the reflected light from the Sun!

Every second, the Sun is converting 600 million tons of hydrogen into helium. This reaction lets out a tremendous amount of energy. Most of this energy heads off into space, but some of it falls on the planets and the moons in the Solar System. This is why we can see them at all. Without the Sun, only Jupiter and Saturn would be readily detectable in the infrared spectrum, as they give off more heat than they absorb from the Sun.

Different objects in the Solar System have a different amount of reflectivity. Astronomers call reflected light from the Sun albedo. The values for an object’s albedo can range between 0 (dark) and 1 (bright). The albedo for the Moon is 0.12. In other words, the Moon reflects 12% of the sunlight that falls onto it.

The highest albedo in the Solar System is Saturn’s moon Enceladus, with an albedo of .99. In other words, it reflects 99% of the light that falls upon it. This is because it’s composed of mostly ice. Dark objects like asteroids can have an albedo down to 5%.

When the Moon is full, astronomers measure its apparent magnitude at -12.6. This is bright enough to easily walk around in otherwise total darkness; almost bright enough to read.

When there’s a new moon, however, there’s no light from the Sun falling on the Moon, and yet we can still see the surface of the Moon. Where’s the light coming from? The Earth. Astronomers call this reflected light “Earthshine”, and it helps them calculate how much sunlight is falling on the Earth.

So now you know that the light of the Moon actually comes from the Sun, unless it’s coming from the Earth.

Here’s an article we’ve done on Universe Today about Earthshine, and here’s an article about gardening on the Moon.

Here’s an article from Extreme Science about the Moon, and an article from NASA about Earthshine.

You can listen to a very interesting podcast about the formation of the Moon from Astronomy Cast, Episode 17: Where Did the Moon Come From?

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Except for the Sun, the Moon and Venus are the two brightest objects in the sky. You just can’t miss them.

Astronomers measure relative brightness of objects in the sky with a term called magnitude. The Sun has the highest magnitude of any object in the sky, at -26.73 apparent magnitude. The brightness of the Moon, for comparison, has a magnitude of -12.6. These numbers are done on a logarithmic scale. So the brightness of the Sun is actually 449,000 times brighter than the full moon.

The maximum brightness of Venus is -4.7 magnitude, which is a fraction of the brightness of the Moon (and the Sun!).

So why is the Moon so much brighter than Venus? It’s closer. The distance to the Moon is about 384,000 km, while the closest distance to Venus is about 38 million km. In other words, the Moon is about 100 times closer to Earth than Venus.

Both the Moon and Venus can cast shadows when they’re in the sky. The Moon can be bright enough to almost read by. It’s easy to navigate when the full moon is in the sky. When the Moon isn’t in the sky, but Venus is very bright, you have to look carefully to detect shadows cast by Venus.

One of the most amazing things in astronomy is when Venus and the Moon are both in the sky; especially when they’re very close to one another. Make sure you head outside to see the Moon and Venus with your own eyes.

We have done several articles informing readers about times when both the Moon and Venus are visible in the sky. Here’s an example, and here’s another.

And there are some great articles out there on the Internet. Here’s one from SPACE.com, and another from Astronomy Picture of the Day.

You can listen to a very interesting podcast about the formation of the Moon from Astronomy Cast, Episode 17: Where Did the Moon Come From?

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When the NASA astronauts first landed on the Moon, they left a few items on the surface to commemorate their visit. These items included a plaque, mission badges and an American flag. If you’ve ever seen images or video of the flag on the Moon, you might have a few questions.

Why does the flag stand straight out and not just slump down? Here on Earth, flags are pushed out by the wind. Obviously, there’s no wind on the Moon, so what’s holding the flag up? The answer is pretty easy. There’s a rod, sort of the like a curtain rod running across the top. So the flag on the Moon is being held out by the rod and isn’t blowing in the wind.

What makes the flag flap if there’s no wind? You might have also see a few videos of the flag on the Moon waving back and forth. This happened when the astronauts first planted the flag. There’s no wind to make the flag to flag, but there’s also no wind to stop it from moving back and forth. When the astronauts planted the flag on the Moon, they couldn’t help but give it a sideways push. Without the wind resistance the flag would experience on the Earth, the flag can flap back and forth a few times before finally settling down. That’s why it looks like it’s flapping, even though there’s no wind.

There’s another scene where the flag flaps, as the lunar ascent module is taking off. In this case, the exhaust from the rocket is blasting the flag and causing it to flap back and forth. In the case of Apollo 11, the exhaust blast was so strong that the flag actually fell over. Later missions kept the flag much further away from the ascent rocket.

Can we see the flag on the Moon from Earth with a big telescope, or even Hubble? Even though we have some powerful telescopes, they’re just not powerful enough to spot objects the size of a flag on the surface of the Moon. The flag is only a meter across. In fact, you would need a telescope 200 meters across to spot objects that size from here on Earth. Future space missions will return to the Moon, and they should be able to resolve objects as small as the flags on the Moon.

Does the flag mean that the US claims the Moon? Nope, the Moon can’t be owned by anyone. NASA had the astronauts plant the flag to commemorate the journey made by American astronauts, but to not actually claim the Moon for any single nation.

We’ve done a few articles about this topic. Here’s a review of the Mythbusters episode where they debunk the Moon flag myth.

NASA has answered some more questions about the flag on the Moon. Here’s a link to their article. And here’s another article debunking the conspiracy theory that NASA didn’t even go to the Moon.

You can listen to a very interesting podcast about the formation of the Moon from Astronomy Cast, Episode 17: Where Did the Moon Come From?

References:
NASA Science: The Great Moon Hoax
NASA Apollo 11 Videos

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The 1st man on the moon was the Apollo 11 Commander Neil Armstrong, who made history on July 20, 1969.

The Apollo 11 mission consisted of Command Module Pilot Michael Collins, Lunar Module Pilot Buzz Aldrin and Commander Neil Armstrong. The mission launched atop a Saturn V rocket on July 16, 1969. After a 4 day journey from the Earth to the Moon, the lunar module detached from the command module and landed on the surface of the Moon in the southern Sea of Tranquility.

The crew remained inside the module for 6 and a half hours, preparing to make their exit onto the lunar surface. And then Neil Armstrong descended the ladder from the lunar module and onto the lunar surface. The first words spoken by the first man on the Moon were, “that’s one small step for (a) man, one giant leap for mankind.”

Buzz Aldrin followed Armstrong, and the two remained on the surface of the Moon for 2.5 hours, taking photographs, collecting rocks, drilling samples, and placing scientific experiments. They they gathered up all their samples, stowed them in the lunar module, and left some souvenirs on the surface of the Moon, like an American flag, Apollo 1 mission patch, and commemorative plaque. They launched again and returned to Earth on July 24.

After the 1st man on the Moon, Neil Armstrong, there were a total of 12 astronauts to walk on the surface of the Moon.

Want to experience what it might have been like to be the first man on the Moon? Here’s a movie review of Fly Me to the Moon.

Of course, NASA has a tremendous amount of information about Apollo 11. Here’s the NASA history page about Apollo 11. And here’s a page that was put together for the 30th anniversary of the first man on the Moon.

You can listen to a very interesting podcast about the formation of the Moon from Astronomy Cast, Episode 17: Where Did the Moon Come From?