An Apollo 17 astronaut digs in the lunar regolith to study the mechanical behavior of moon dust. Credit: NASA
That video above is perhaps the ultimate off-roading adventure: taking a rover out for a spin on the moon. Look past the cool factor for a minute, though, and observe the dust falling down around that astronaut.
The crew aboard Apollo 16 (as well as other Apollo missions) had a lot of problems with regolith. It got into everything. It was so abrasive that it wore away some equipment in days. It smelled funny and probably wasn’t all that good to breathe in, either. Many have said that when we return to the moon, dust must be dealt with for long-term survival.
Things could get worse at sunrise and sunset. One new study (not peer-reviewed yet) finds a “serious risk” that rovers “could be engulfed in dust.” That’s because lunar dust appears to have electrostatic properties that, somehow, is triggered by changes in sunlight. (NASA is already doing some serious investigation into this matter using its orbiting missions.)
What the researchers did, in conjunction with ONERA (The French Center of Aerospace Research) was conduct simulations for two types of lunar regions — the terminator (the day/night boundary) and an area experiencing full sunlight.
“Dust particles were introduced into the simulation over a period of time, when both the surface and the rover were in electrical equilibrium,” the Royal Astronomical Society stated.
“In both the test cases, dust particles travel upwards above the height of the rover, but results suggest that they move in different directions. On the day side, the particles are pushed outwards and on the terminator the dust travels upwards and inwards above the rover, regrouping in the vacuum above it. The terminator simulation began with a region void of dust which was later filled by lunar dust particles.”
The bottom line? A lunar rover could accumulate a significant amount of dust on the moon, especially if it’s sitting at or near the terminator. This could be addressed by using dome-shaped rovers that would see the dust fall off, added lead author Farideh Honary, a physicist at the University of Lancaster, in a statement.
The work was presented at the RAS National Astronomy Meeting today (July 3). A paper has been submitted to the Journal for Geophysical Research, so more details should be forthcoming if and when it is published.
An artist's conception of a potentially-habitable exomoon. It seems reasonable that exoplanets have exomoons, and now we're going to look for them. Credit: NASA
The latest exciting undertaking in exoplanet research is the search for exomoons. A team led by Dr. David Kipping at the Harvard-Smithsonian Center for Astrophysics has jumped at this challenge. After having theoretically proven that detecting an Earth-sized exomoon is possible, the team carried out the first detailed search for an exomoon.
Are you leaning forward on the edge of your seat awaiting the results? Well here you go: the data show no evidence of a moon. That’s simply the luck of the draw. We didn’t discover an exoplanet on our first try either. I believe that this non-detection shows that we’re on the verge of our next greatest discovery.
The reasons for searching for exomoons are abundant. “Exomoons may be frequent, habitable abodes for life and so far we know next to nothing about the underlying frequency of such objects in the cosmos,” Dr. Kipping told Universe Today. “They also play an important role in the habitability of those planets which they orbit, for example the Moon is thought to stabilize the axial tilt of the Earth and so too the climate.”
The project titled “The Hunt of Exomoons with Kepler,” more commonly known as HEK, was formed with these reasons in mind. As such, the HEK project will search for exomoons that are likely to be habitable.
The first target is Kepler-22b – the first transiting exoplanet to have been detected in the habitable zone of its host star. At 2.4 Earth radii, it is too large to be considered an Earth-analog, but it could easily have an Earth-sized moon
There are currently two methods in which we may detect exomoons.
1.) Dynamic effects – the exomoon tugs the planet, which causes deviations in the times and durations of the host planet’s transits. This is similar to the radial velocity technique for detecting exoplanets.
2.) Transit effects – the exomoon may transit the star immediately before or just after the planet does. This will cause an added dip in the observed light. See this video for a great demonstration. This is similar to the light curve technique for detecting exoplanets.
The team modeled the initial transit light curves of Kepler-22b. They then injected an Earth-sized moon into the system in order to analyze the effects. While this caused clear variations in the light curve, such variations had to be above the level of noise.
As such, they also injected noise in the light curves, which mirrors that of the Kepler data. In the end, the variations in a star’s light curve due to the presence of an exomoon are much higher than the noise. The team is able to recover the correct answer with extremely high confidence.
Here Kipping et al. presents injected moon fits. As an example, the upper left-hand figure shows an exoplanet transit, with a moon transiting as well. Here the moon transits first, causing the light to be blocked, then the planet follows, causing more of the light to be blocked. Source: Kipping et al. 2013
The real data does not show deviations like the previous figure does. This non-detection implies that there is no moon with a mass greater than 0.54 times the mass of the Earth. While there is no Earth-analog in this system, there may be a smaller undetectable moon.
I asked Dr. Kipping about our chances of success in other systems. His answer: “That depends upon nature herself!” We have no idea how regularly nature produces moons in other solar systems. “There is nothing more exciting than working on a project where the answer is wholly unknown.”
But remember: two decades ago we were unsure if nature regularly produced planets. We have since observed them in abundance. I have to believe that with 168 moons in our solar system alone, we’re likely to find them in other systems. We’re on the verge of the next greatest discovery. So stay tuned because I promise I’ll be writing about it when it happens.
The short answer is, the average distance to the Moon is 384,403 km (238,857 miles). But before you go thinking that this is the final answer, you need to consider a few things. For starters, note the use of the word “average”. This refers to the fact that the Moon orbits around the Earth in an elliptical pattern, which means that at certain times, it will be father away; while at others, it will be closer.
Hence, the number 384,403 km, is an average distance that astronomers call the semi-major axis. At its closest point (known as perigee) the Moon is only 363,104 km (225,622 miles) away. And at its most distant point (called apogee) the Moon gets to a distance of 406,696 km (252,088 miles).
This means that distance from the Earth to the Moon can vary by 43,592 km. That’s a pretty big difference, and it can make the Moon appear dramatically different in size depending on where it is in its orbit. For instance, the size of the Moon can vary by more than 15% from when it’s at its closest to when it’s at the most distant point.
It can also have a dramatic effect on how bright the moon appears when it is in its Full phase. As one might expect, the brightest full Moons occur when the Moon is at the closest, which are typically 30% brighter than when it’s fathest away. When it’s a Full Moon, and it’s a close Moon, it’s known as a Supermoon; which is also known by it technical name – perigee-syzygy.
To get an idea of what this all looks like, check out the animation above that was released by the Goddard Space Flight Center Scientific Visualization Studio in 2011. The animation shows the geocentric phase, libration, position angle of the axis, and apparent diameter of the Moon throughout the year, at hourly intervals.
At this point, a good question to ask would be: how do we know how far away the Moon is? Well, that depends on when we’re talking. In the days of ancient Greece, astronomers relied on simple geometry, the diameter of the Earth – which they had already calculated to be the equivalent of 12,875 km (or 8000 miles) – and the measurements of shadows to make the first (relatively) accurate estimates.
Having observed and recorded how shadows work over a long period of history, the ancient Greeks had determined that when an object is placed in front of the Sun, the length of a shadow this generates will always be 108 times the diameter of the object itself. So a ball measuring 2.5 cm (1 inch) across and placed on a stick between the Sun and the ground will create a triangular shadow that extends for 270 cm (108 inches).
This reasoning was then applied to the phenomena of Lunar and Solar Eclipses.
In the former, they found that the Moon was imperfectly blocked by the shadow of the Earth, and that the shadow was roughly 2.5 times the width of the Moon. In the latter, they noted that the Moon was of sufficient size and distance to block out the Sun. What’s more, the shadow it would create terminated at Earth, and would end in the same angle that the shadow of the Earth does – making them different-sized versions of the same triangle.
Using the calculations on the diameter of the Earth, the Greeks reasoned that the larger triangle would measure one Earth diameter at its base (12,875 km/8000 miles) and be 1,390,000 km (864,000 miles) long. The other triangle would be the equivalent of 2.5 Moon diameters wide and, since the triangles are proportionate, 2.5 Moon orbits tall.
Adding the two triangles together would yield the equivalent of 3.5 Moon orbits, which would create the largest triangle and gave the (again, relatively) accurate measurement of the distance between the Earth and the Moon. In other words, the distance is 1.39 million km (864,000 miles) divided by 3.5, which works out to around 397,500 km (247,000 miles). Not exactly bang on, but not bad for ancient peoples!
Lunar Laser Ranging Experiment from the Apollo 11 mission. Credit: NASA
Today, millimeter-precision measurements of the lunar distance are made by measuring the time it takes for light to travel between LIDAR stations here on the Earth and retroreflectors placed on the Moon. This process is known as the Lunar Laser Ranging experiment, a process that was made possible thanks to the efforts of the Apollo missions.
When astronauts visited the Moon more than forty years ago, they left a series of retroreflecting mirrors on the lunar surface. When scientists here on Earth shoot a laser at the Moon, the light from the laser is reflected right back at them from one of these devices. For every 100 quadrillion photons shot at the Moon, only a handful come back, but that’s enough to get an accurate appraisal.
Since light is moving at almost 300,000 kilometers (186,411 miles) per second, it takes a little more than a second to make the journey. And then it takes another second or so to return. By calculating the exact amount of time it takes for light to make the journey, astronomers are able to know exactly how far the Moon is at any time, down to millimeter accuracy.
From this technique, astronomers have also discovered that the Moon is slowly drifting away from us, at a glacial rate of 3.8 cm (1.5 inches) a year. Millions of years in the future, the Moon will appear smaller in the sky than it does today. And within a billion years or so, the Moon will be visually smaller than the Sun and we will no longer experience total solar eclipses.
37 separate images showing the movement of the Moon as it rises behind the buildings in downtown Houston, Texas on June 22, 2013. Credit and copyright: Sergio Garcia Rill/SGR Photo.
While we recently posted a huge batch of images from the recent “Super Moon,” this new image from Sergio Garcia Rill in Houston is something special. It’s a composite photo of the Moonrise on June 22nd, and is a mosaic made from 37 separate images that show the Moon rising over the course of three hours, with the buildings of downtown Houston in the foreground.
“I stayed in place for over three hours,” Sergio explained on Flickr. “The hardest part was selecting which shots showed a sequential movement of the Moon, since I was altering shutter speeds between shots to compensate for changing light conditions.”
The full Moon of June 2013 was at perigee — or at its closest point in its orbit to Earth, and appeared up to 14% bigger and 30% brighter than other full Moons of 2013.
Want to get your astrophoto featured on Universe Today? Join our Flickr group or send us your images by email (this means you’re giving us permission to post them). Please explain what’s in the picture, when you took it, the equipment you used, etc.
The perigee 'Super Moon' of June 23, 2013 (21:43 PHT) over Marikina City, Philippines compared to the apogee Moon of November, 2012. Credit and copyright: Raven Yu.
The full Moon of June 23, 2013 was the largest Moon of the year. This so-called “Super Moon” was at perigee — or at its closest point in its orbit to Earth, and was 14% bigger and 30% brighter than other full Moons of 2013.
But, if you looked up at the Moon last night and didn’t know about this, you may not have noticed! Some claims circulating on the internet tended to exaggerate how large the Moon would actually appear. However, that doesn’t mean the Moon wasn’t photogenic last night! The Moon is always a great target for photography or just gazing with your own eyes, and these images from Universe Today readers attest to the beauty of our closest companion in the night sky.
This lead image from Raven Yu from the Philippines shows the difference in size between last night’s perigee Moon and the apogee Moon (when it was farthest from Earth during its orbit) last November.
Three different views of the Moon over Italy during the night of June 23, 2013 helps debunk the optical illusion of the Moon looking bigger when it’s low on the horizon. Credit and copyright: Giuseppe Petricca.
Three different pictures of the Moon from June 23, shared by Guiseppe Petricca from Italy, detailing not only the perigee Super Moon, but the ‘Moon Illusion” — of how the Moon looks bigger when it is close to the horizon.
“The middle one is the Moon at culmination in the local sky and the other two are taken as low as possible my local horizon permitted,” Guiseppe said via email. “Doing this, I managed to obtain two results: the first one is observing the different colours that due to the Rayleigh Scattering, ‘paint’ our satellite, when it’s low on its elevation. The second one is that, keeping a fixed magnification (24x – 110mm) one can easily debunk the optical illusion of the ‘bigger moon when it’s low on the horizon’. Since, if you observe carefully, the lower two ‘Moons’ are smaller than the higher one. However, the total personal experience is surely wonderful!! And the ‘horizon illusion’ makes you really think that the Moon is way bigger that the reality.”
The Supermoon rising on June 23rd, 22:40 pm above the forest canopy top in Puerto Rico. This is a 6 panel mosaic. Credit and copyright: Efrain Morales, Jaicoa Observatory.The Super Moon on June 23, 2013 as seen over Malta. Credit and copyright: Leonard E. Mercer.The perigee ‘Super Moon’ of June 23, 2013 as seen over Sesimbra, Portugal and the church Nossa Senhora do Castelo. Credit and copyright: Miguel Claro.
Miguel Claro captured this beautiful image of the huge full Moon rising above a Moorish castle in Sesimbra, Portugal. “The church Nossa Senhora do Castelo stands on the spot where king Sancho I built a Romanesque chapel in the early 13th century,” Miguel said via email. “This image was captured 2 km away from the subject.” Miguel used a Canon 50D – ISO640; 1/80 sec. + ED80 APO refractor Astro Professional 560mm at f/7 taken on 23/06/2013 at 21h22.
The perigee Super Moon of June 23, 2013 as seen over São Paulo, Brazil. Time: 01:40 UTC, using a Maksutov Cassegrain Vixen 110 mm – F = 1035 mm – F/9.4 – Plano Focal – Nikon D3100 – 1/80 – ISO 200. Credit and copyright: Ednilson Oliveira.An early image of the perigee Super Moon — the Moon setting the morning of June 23, 2013 just after 5 a.m. EDT over Toronto, Canada. ‘As I understand it perigee occurred between 7:11 – 7:13 a.m. EDT, so this was my ‘launch window’ to ‘Shoot the Perigee Moon’. The atmosphere was thick with haze which dimmed the Moon substantially and allowed the surface maria to be photographed.’ Credit and copyright: Rick Ellis.The perigee Super Moon rising over the floodwaters of the Bow River during a record flood that inundated many parts of southern Alberta around rivers, including here on the Siksika First Nations reserve. Credit and copyright: Alan Dyer/Astronomy Calgary/Amazing Sky Photography.A 3-photo HDR image of the supermoon rising over downtown Tucson, Arizona. During the longer exposure the Moon gave out its own flare due to its intensity. Credit and copyright: Sean Parker/Sean Parker Photography.The full perigee Moon rising on June 23, 2013. Credit and copyright: Sculptor Lil on Flickr.Full Moon Rising Over Northwest Georgia on June 22nd, 2013. Credit and copyright: Stephen Rahn.The perigee Super Moon on June 23, 2013, taken with a Skywatcher ED80 Refractor and a Canon 600D at prime focus. Best 20 of 40 images stacked in Registax 6. False colour removed as the Moon appeared dull red as it was so low in sky. Credit and copyright: James Lennie.The view of June 23rd 2013 Supermoon from Trinidad and Tobago, West Indies (Caribbean). Credit and copyright: Apple Lilly. (This image is the right size to fit a Facebook cover image, the photographer says).
You can see more great images of this perigee Super Moon — and lots more great astrophotography at our Flickr group page.
Want to get your astrophoto featured on Universe Today? Join our Flickr group or send us your images by email (this means you’re giving us permission to post them). Please explain what’s in the picture, when you took it, the equipment you used, etc.
Fireworks and the quarter Moon seen over the skies of Pisa, Italy on June 16, 2013. Credit and copyright: Giuseppe Petricca.
This lovely image of the Moon with fireworks exploding nearby in the sky was taken by astrophotgrapher Giuseppe Petricca over the weekend. “In Pisa, it was the Patron Saint’s Day, and I managed to catch fireworks, launched from the middle of the river Arno, exploding near the first quarter Moon!” This is an actual shot — not a mosaic — and Guiseppe said he only used Photoshop to make the Moon’s surface detail more clear and reduced the overall noise in the picture.
The event must have been awe-inspiring in person!
This image taken with a Nikon P90 Bridge Digital Camera on tripod.
Want to get your astrophoto featured on Universe Today? Join our Flickr group or send us your images by email (this means you’re giving us permission to post them). Please explain what’s in the picture, when you took it, the equipment you used, etc.
This is an image of a unique eclipse as viewed by NASA's Solar Dynamics Observatory, with a model of the moon from NASA's Lunar Reconnaissance Orbiter replacing the lunar shadow. Credit: NASA/SDO/LRO/GSFC
You’ve probably never before seen an image like the one above. That’s because it is the first time something like this has ever been created, and it is only possible thanks to two fairly recent NASA missions, the Solar Dynamics Observatory and the Lunar Reconnaissance Orbiter. We’ve shared previously how two or three times a year, SDO goes through “eclipse season” where it observes the Moon traveling across the Sun, blocking its view.
Now, Scott Wiessinger and Ernie Wright from Goddard Space Flight Center’s Scientific Visualization Studio used SDO and LRO data to create a model of the Moon that exactly matches SDO’s perspective of a lunar transit from October 7, 2010. They had to precisely match up data from the correct time and viewpoint for the two separate spacecraft, and the end result is this breathtaking image of the Sun and the Moon.
“The results look pretty neat,” Wiessinger said via email, “and it’s a great example of everything working: SDO image header data, which contains the spacecraft’s position; our information about lunar libration, elevation maps of the lunar surface, etc. It all lines up very nicely.”
‘Nicely’ is an understatement. How about “freaking awesome!”
And of course, they didn’t just stop there.
his is an up close shot of two NASA images: An image rendered from a model of the moon from the Lunar Reconnaissance Orbiter overlaid onto an image of the sun from the Solar Dynamics Observatory, during a lunar transit as seen by SDO on Oct. 7, 2010. The various features of the moon’s horizon are labeled. Credit: NASA/SDO/LRO/GSFC
Since the data from both spacecraft are at such high resolution, if you zoom in to the LRO image, features of the Moon’s topography are visible, such as mountains and craters. This annotated image shows what all is visible on the Moon. And then there’s the wonderful and completely unique view in the background of SDO’s data of the Sun.
So while the imagery is awesome, this exercise also means that both missions are able to accurately provide images of what’s happening at any given moment in time.
The image on the left is a view of the sun captured by NASA’s Solar Dynamics Observatory on Oct. 7, 2010, while partially obscured by the moon. Looking closely at the crisp horizon of the moon against the sun shows the outline of lunar mountains. A model of the moon from NASA’s Lunar Reconnaissance Orbiter has been inserted into a picture on the right, showing how perfectly the moon’s true topography fits into the shadow observed by SDO. Credit: NASA/SDO/LRO/GSFC
The CRaTER instrument aboard NASA's Lunar Reconnaissance Orbiter measures the effect of cosmic rays on "human tissue-equivalent" plastic. (NASA)
It could work, say researchers from the University of New Hampshire and the Southwest Research Institute.
One of the inherent dangers of space travel and long-term exploration missions beyond Earth is the constant barrage of radiation, both from our own Sun and in the form of high-energy particles originating from outside the Solar System called cosmic rays. Extended exposure can result in cellular damage and increased risks of cancer at the very least, and in large doses could even result in death. If we want human astronauts to set up permanent outposts on the Moon, explore the dunes and canyons of Mars, or mine asteroids for their valuable resources, we will first need to develop adequate (and reasonably economical) protection from dangerous space radiation… or else such endeavors will be nothing more than glorified suicide missions.
While layers of rock, soil, or water could protect against cosmic rays, we haven’t yet developed the technology to hollow out asteroids for spaceships or build stone spacesuits (and sending large amounts of such heavy materials into space isn’t yet cost-effective.) Luckily, there may be a much easier way to protect astronauts from cosmic rays — using lightweight plastics.
While aluminum has always been the primary material in spacecraft construction, it provides relatively little protection against high-energy cosmic rays and can add so much mass to spacecraft that they become cost-prohibitive to launch.
Using observations made by the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) orbiting the Moon aboard LRO, researchers from UNH and SwRI have found that plastics, adequately designed, can provide better protection than aluminum or other heavier materials.
“This is the first study using observations from space to confirm what has been thought for some time—that plastics and other lightweight materials are pound-for-pound more effective for shielding against cosmic radiation than aluminum,” said Cary Zeitlin of the SwRI Earth, Oceans, and Space Department at UNH. “Shielding can’t entirely solve the radiation exposure problem in deep space, but there are clear differences in effectiveness of different materials.”
Zeitlin is lead author of a paper published online in the American Geophysical Union journal Space Weather.
A block of tissue-equivalent plastic (TEP) Credit: UNH
The plastic-aluminum comparison was made in earlier ground-based tests using beams of heavy particles to simulate cosmic rays. “The shielding effectiveness of the plastic in space is very much in line with what we discovered from the beam experiments, so we’ve gained a lot of confidence in the conclusions we drew from that work,” says Zeitlin. “Anything with high hydrogen content, including water, would work well.”
The space-based results were a product of CRaTER’s ability to accurately gauge the radiation dose of cosmic rays after passing through a material known as “tissue-equivalent plastic,” which simulates human muscle tissue.
(It may not look like human tissue, but it collects energy from cosmic particles in much the same way.)
Prior to CRaTER and recent measurements by the Radiation Assessment Detector (RAD) on the Mars rover Curiosity, the effects of thick shielding on cosmic rays had only been simulated in computer models and in particle accelerators, with little observational data from deep space.
The CRaTER observations have validated the models and the ground-based measurements, meaning that lightweight shielding materials could safely be used for long missions — provided their structural properties can be made adequate to withstand the rigors of spaceflight.
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 19th. 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 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 & 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!
Looking west at sunset from latitude 30 degrees north. The ecliptic and Mercury's orbit along with a 10 degree field of view outlined for reference. All graphics created by the author using Starry Night).
If you’ve never seen Mercury, this week is a great time to try.
Over the past few weeks, observers worldwide have been following the outstanding tight triple conjunction of Mercury, Venus and Jupiter low to the west at dusk.
Jupiter has exited the evening sky, headed for conjunction with the Sun on June 19th. I caught what was probably our last glimpse of Jupiter for the season clinging to the murky horizon through binoculars just last week. If you’re “Jonesin’ for Jove,” you can follow its progress this week through superior conjunction as it transits the Solar Heliospheric Observatory’s LASCO C3 camera.
This leaves the two innermost worlds of our fair solar system visible low to the west at dusk. And tonight, they’re joined by a very slender waxing crescent Moon, just over two days after New phase.
The Moon, Venus and Mercury as seen from 30 degrees north tonight at 9PM EDT.
The evening of June 10th finds a 4% illuminated Moon passing just over 5 degrees (about 10 Full Moon diameters) south of Venus and Mercury. Venus will be the first to appear as the sky darkens, shining at magnitude -3.9 and Mercury will shine about 40 times fainter above it at magnitude +0.3.
Ashen light, also known as Earthshine will also be apparent on the darkened limb of the Moon. Another old-time term for this phenomenon is “the Old Moon in the New Moon’s Arms.” Ashen light is caused by sunlight being reflected off of the Earth and illuminating the nighttime Earthward facing portion of the Moon. Just how prominent this effect appears can vary depending on the total amount of cloud cover on the Earth’s Moonward facing side.
….and the orientation of the Moon, Mercury and Venus on the night of June 12th and ~9PM EDT.
This week sets the stage for the best dusk apparition of Mercury for northern hemisphere viewers in 2013. Orbiting the Sun every 88 Earth days, we see Mercury either favorably placed east of the Sun in the dusk sky or west of the Sun in the dawn sky roughly six times a year. Mercury’s orbit is markedly elliptical, and thus not all apparitions are created the same. An elongation near perihelion, when Mercury is 46 million kilometers from the Sun, can mean its only 17.9 degrees away from the Sun as viewed from the Earth. An elongation near aphelion, 69.8 million kilometers distant, has a maximum angular separation of 27.8 degrees.
This week’s greatest elongation of 24.3 degrees occurs on June 12th. It’s not the most extreme value for 2013, but does have another factor going for it; the angle of the ecliptic. As we approach the solstice of June 21st, the plane of the solar system as traced out by the orbit of the Earth is at a favorable angle relative to the horizon. Thus, an observer from 35 degrees north latitude sees Mercury 18.4 degrees above the horizon at sunset, while an observer at a similar latitude in the southern hemisphere only sees it slightly lower at 16.9 degrees.
Venus and the Moon make great guides to locate Mercury over the next few nights. It’s said that Copernicus himself never saw Mercury with his own eyes, though this oft repeated tale is probably apocryphal.
We also get a shot at a skewed “emoticon conjunction” tonight, not quite a “smiley face” (: as occurred between Jupiter, Venus and the Moon in 2008, but more of a “? :” Stick around until February 13th, 2056 and you’ll see a much tighter version of the same thing! A time exposure of a pass of the International Space Station placed near Mercury and Venus could result in a planetary “meh” conjunction akin to a “/:” Hey, just throwing that obscure challenge out there. Sure, there’s no scientific value to such alignments, except as testimony that the universe may just have a skewed sense of humor…
Through the telescope, Venus currently shows a 10” diameter gibbous phase, while Mercury is only slightly smaller at 8” and is just under half illuminated. No detail can be discerned on either world, as a backyard telescope will give you the same blank view of both worlds that vexed astronomers for centuries. These worlds had to await the dawn of the space age to give up their secrets. NASA’s MESSENGER spacecraft entered a permanent orbit around Mercury in 2011, and continues to return some outstanding science.
Both planets are catching up to us from the far side of their orbits. Mercury will pass within 2 degrees of Venus on June 20th, making for a fine wide field view in binoculars.
And now for the wow factor of what you’re seeing tonight. The Moon just passed apogee on June 9th and is currently about 416,500 kilometers or just over one light second distant. Mercury meanwhile, is 0.86 astronomical units (A.U.), or almost 133 million kilometers, or about 7 light minutes away. Finally, Venus is currently farther away from the Earth than the Sun at 1.59 A.U.s, or about 13.7 light minutes distant.
All this makes for a great show in the dusk skies this week. And yes, lunar apogee just after New sets us up for the closest Full Moon of 2013 (aka the internet sensation known as the “Super Moon”) on June 23rd. More to come on that soon!
