Green flashes from the setting Moon. Credit: ESO/G.Hüdepohl
[/caption]
Green flashes from the Sun at sunset are a rare phenomenon, but even rarer are green flashes from a setting Moon. With the unique atmospheric conditions at Cerro Paranal in Chile, a photographer from ESO’s Very Large Telescope managed to what are likely the best images ever taken of the Moon’s green flash. ESO Photo Ambassador Gerhard Hüdepohl took a series of images of the setting full Moon crossing the horizon, taken on a clear early morning from the Paranal Residencia.
What happens that makes the green flashes appear?
The Earth’s atmosphere bends, or refracts, light, like a giant prism. The effect is greater in the lower denser layers of the atmosphere, so rays of light from the Sun or Moon are curved slightly downwards. Shorter wavelengths of light are bent more than longer wavelengths, so that the green light from the Sun or Moon appears to be coming from a slightly higher position than the orange and red light, from the point of view of an observer. When the conditions are just right, with an additional mirage effect due to the temperature gradient in the atmosphere, the elusive green flash is briefly visible at the upper edge of the solar or lunar disc when it is close to the horizon.
Hüdepohl works as an Electronics Engineer at ESO’s Very Large Telescope. He said he was surprised and delighted to catch the stunning green flash from the Moon.
You can read an article we did about green flashes from the setting Sun here.
An oblique look at the Moon from the Lunar Reconnaissance Orbiter. Credit: Moon Zoo, NASA/GSFC/Arizona State University
[/caption]
The Zooites working at the Moon Zoo citizen science project have uncovered some very unique oblique views of the Moon taken by the Lunar Reconnaissance Orbiter. Occasionally, LRO takes “sideways glances” at the Moon instead of looking straight down like the spacecraft normally does. The Moon doesn’t really look like this close up, because these images aren’t scaled correctly (the width and height pixel scales are different by five times, the Zooites say in the Moon Zoo Forum), but they provide a distinctive look at the lunar surface, and things like craters on the side of a hill, — or perhaps an entrance to a cave — show up better than in normal images. Have fun looking at some more of these images below, or on the Moon Zoo Forum.
Another oblique look at the Moon from the Lunar Reconnaissance Orbiter. Credit: Moon Zoo, NASA/GSFC/Arizona State UniversityLRO image M144564740RC. Credit: Moon Zoo, NASA/GSFC/Arizona State University.LRO image M144653115RC. Credit: Moon Zoo, NASA/GSFC/Arizona State University.
Titan holds yet more secrets, far beneath its haze...
[/caption]
Saturn’s moon Titan just keeps throwing surprises at us. A multi-layered atmosphere thicker than our own? Check. A hydrologic cycle that relies on methane as the operating liquid? Check. Rivers, streams and lakes filled with this same liquid? Check, check and check. And now, scientists are suspecting that Titan may have yet another surprise: a subsurface ocean.
Observations of Titan’s rotation and orbit, carried out by researchers at the Royal Observatory of Belgium using Cassini data, point at an unusual rotational inertia; that is, its resistance to changes in its motion, also known as moment of inertia or angular mass. Basically Titan moves in a way that is not indicative of a solid body of its previously assumed density and mass. Rather, its motion – both around its own axis and in its tidally-locked orbit around Saturn – are more in line with an object that isn’t uniformly solid.
Titan's thick clouds hide its surface well. NASA / JPL / SSI / J.Major
According to the math, Titan may very well be filled with liquid!
Or, at least, have a liquid layer of considerable depth beneath its surface. How far below the surface, how deep and exactly what kind of liquid are all speculative at this point…it’s suggested that it may be a subsurface ocean of yet more methane. This would help answer the question of where Titan gets all of its methane in the first place; methane, – a.k.a. natural gas – is a compound that breaks down quickly in sunlight. In fact, the high-level haze that surrounds the moon like a wispy blue shell is made up of this broken-down methane. So if this stuff is raining down onto the surface in giant, frigid drops and filling streams and lakes, but is still being broken down by ultraviolet light from the Sun to enshroud the entire moon (Titan is BIG, remember…at 5,150 km – 3,200 miles – wide, it’s over a third the size of Earth!) then there has to be somewhere that this methane is coming from.
If these calculations are right, it may be coming from underground.
We propose a new Cassini state model for Titan in which we assume the presence of a liquid water ocean beneath an ice shell… with the new model, we find a closer agreement between the moment of inertia and the rotation state than for the solid case, strengthening the possibility that Titan has a subsurface ocean.
– Rose-Marie Baland et al.
Of course in order for this hypothesis to be proven many more numbers are going to have to be crunched and more data reviewed. And more possibilities considered, too; Titan’s orbital irregularities may in fact be the result of external forces, such as a close pass by a comet or other large body. Still, there’s something to be investigated here and you can bet there’ll be no shortage of attention on a problem as intriguing as this!
Titan may soon be joining the short list of moons speculated to possess subsurface oceans, alongside Jupiter’s Europa and Ganymede and sister Saturnian satellite Enceladus…and who knows how many others?
Brightening the shadowed area reveals details of the crater floor...and even more boulders!
[/caption]
The crater shown above is located in the lunar highlands and is filled with and surrounded by boulders of all sizes and shapes. It is approximately 550 meters (1800 feet) wide yet is still considered a small crater, and could have been caused by either a direct impact by a meteorite or by an ejected bit of material from another impact. Scientists studying the Moon attempt to figure out how small craters like this were formed by their shapes and the material seen around them…although sometimes the same results can be achieved by different events.
For example, when an object from space strikes the Moon, it is typically traveling around 20 km per second (12 miles/sec). If the impact site happens to have a very hard subsurface, it can make a crater with scattered bouldery chunks composed of the hard material around it. But, if a large piece of ejected material from another impact were to strike the lunar surface at a much slower speed, as ejecta typically do (since they travel slower than incoming space debris and the Moon’s escape velocity is fairly low, meaning any ejecta that does fall back to the surface must be traveling slower than 2.38 km/s,) then the ejected chunk could break apart on impact and scatter boulders of itself around the crater…regardless of subsurface composition.
Really the only way to tell for sure which scenario has taken place around a given crater – such as the one above – is to collect and return samples from the site so they can be tested. (Of course that’s much easier said than done!)
And as an added treat, take a look deep into the shadows of the crater’s interior below…I tweaked the image curves in Photoshop to wrestle some of the details out of there!
Brightening the shadowed area reveals details of the crater floor...and even more boulders!
Image credit: NASA/GSFC/Arizona State University. (Edited by J. Major.)
P.S.: Want to see both image versions combined? Click here. (Thanks to Mike C. for the suggestion!)
2011 GP59 imaged remotely from the GRAS Observatory. Credit: Ernesto Guido & Giovanni Sostero
[/caption]
A newly discovered asteroid could provide one of the best recent viewing opportunities for amateur astronomers, according to the British Astronomical Association. “This is the best NEO close approach these past few years and is bright enough to be observed visually in large (>20cm., or 8-inch) aperture telescopes when on the night of Thursday 14th it will appear as a faint slow-moving star,” writes Richard Miles, the director of the BAA’s Asteroids and Remote Planets Section.
UPDATE: See a new picture of asteroid 2011 GP59 from Ernesto Guido & Giovanni Sostero taken on April 14, 2011, below.
2011 GP59 imaged remotely from the GRAS Observatory. Credit: Ernesto Guido & Giovanni Sostero
Guido & Sostero sent us a note that they imaged 2011 GP59 early on April 14, remotely from the GRAS Observatory (near Mayhill, New Mexico USA) through a 0.51-m, f/6.9 reflector + CCD.
“It’s a single unfiltered exposure of 600 seconds, showing 2011 GP59 as trail with brightness fluctuations clearly evident,” they said.
(end of 4/14 update)
2011 GP59 was discovered just a few days ago and will make its closest approach to the Earth on April 15 at 19h UT at 1.39 lunar-distances. But it will be brightest at an average magnitude of 13.2 around 00h UT on the night of April 14/15 when Miles says it will be very favorably placed in the sky for observers worldwide.
The asteroid is approximately 60 meters in diameter and appears to be rotating very quickly, about once every 7.35 minutes. Its oblong in shape and rotation will vary the object’s brightness every 4 minutes or so.
Miles reported that David Briggs observing with the Hampshire Astronomy Group’s 0.4-m instrument on the evening of April 11 commented, “This is probably the fastest rotator I’ve seen so far in that it completely disappears from view every 3 to 4 images.”
This object was discovered on the night of April 8/9 by the Observatorio Astronomico de Mallorca (OAM) using a 0.45-m f/2.8 reflector at their La Sagra facilities (J75) in Andalusia, Spain (see http://www.minorplanets.org/OLS/ ). The observers involved were S. Sanchez, J. Nomen, R. Stoss, M. Hurtado, J. A. Jaume and W. K. Y. Yeung.
The British Astronomical Association is also seeking observations of the Moon on Friday, April 15, between 19:00 and 21:00 UT, when the Aristarchus and Herodotus area of the Moon will match the same illumination, to within +/- 0.5 degrees, as that observed during the famous Transient Lunar Phenomena (TLP) seen by Greenacre and Barr from Flagstaff observatory back on Oct. 30, 1963.
TLPs are very short changes in the brightness of patches on the face of the Moon, which can last anywhere from a few seconds to a few hours and can grow from less than a few to a hundred kilometers in size. This phenomenon has been observed by hundreds of amateur and professional astronomers, but how and why this occurs is not understood. Some astronomers believe that they are the outcome of lunar outgassing, where gas is being released from the surface of the Moon, but most commonly astronomers think it could be an effect from Earth’s own atmosphere.
If you want to help understand TLPs and perhaps observe an event like this for yourself, the BAA Lunar Section is looking for high resolution monochrome, or especially color, images of this area during this time period,, which favors observers in Europe.
A lithograph published in the New York Sun newspaper in 1835 showing life on the Moon. Credit: Wikipedia
[/caption]
Say the words “Moon Hoax” these days, and everyone thinks you are talking about the people who don’t believe the Apollo astronauts ever went to the Moon. But back in 1835 there was the original Moon hoax that thousands of people fell for, despite the tall tale being complete fiction. A series of articles were published in the New York Sun newspaper reporting incredible new astronomical observations of the Moon supposedly made by astronomer Sir John Herschel during an observing run at the Cape of Good Hope with his powerful new telescope. Detailed descriptions of winged beings, plants, animals and a sapphire temple increased sales and subscriptions to the fledgling newspaper.
Here’s a selection from one of the articles:
“We counted three parties of these creatures, of twelve, nine and fifteen in each, walking erect towards a small wood… Certainly they were like human beings, for their wings had now disappeared and their attitude in walking was both erect and dignified… About half of the first party had passed beyond our canvas; but of all the others we had perfectly distinct and deliberate view. They averaged four feet in height, were covered, except on the face, with short and glossy copper-colored hair, and had wings composed of a thin membrane, without hair, lying snugly upon their backs from the top of the shoulders to the calves of their legs.”
The descriptions were allegedly reprinted from the nonexistent Edinburgh Journal of Science, and only several weeks after the articles were published did questions arise about the truth of these tales. The newspaper did not issue a retraction back then, and now, even over 175 years later has not issued a full retraction of it, either.
It is said that Herschel was initially amused by the hoax, noting that his own real observations could never be as exciting. But he became annoyed later when he had to answer questions from people who believed the hoax was serious.
To be honest, I had not heard of this hoax until it was discussed by professor Rob Knop in today’s 365 Days of Astronomy podcast. He does a great job telling the story, so it is definitely worth a listen.
Lockheed Martin’s Space Operations Simulation Center in Littleton, Colorado, simulates on-orbit docking maneuvers with full-scale Orion and International Space Station mockups. The spacious center includes an 18,000 square-foot high bay area used to validate Orion’s new relative navigation system (STORRM), which will be tested on orbit during the STS-134 mission set to blast off on April19, 2011. Credit: Lockheed Martin
Lockheed Martin is aiming for a first unmanned orbital test flight of Orion as soon as 2013, said John Karas, vice president and general manager for Lockheed Martin’s Human Space Flight programs in an interview with Universe Today . The first operational flight with humans on board is now set for 2016 as stipulated in the NASA Authorization Act of 2010.
Orion manned capsule could launch in 2016 atop proposed NASA heavy lift booster from the Kennedy Space Center This Orion prototype capsule was assembled at NASA’s Michoud Assembly Facility (MAF) in New Orleans, LA and shipped by truck to Denver. At Denver, the capsule will be put through a rigorous testing program to simulate all aspects of a space mission from launch to landing and examine whether the vehicle can withstand the harsh and unforgiving environment of deep space.
Orion was originally designed to be launched by the Ares 1 booster rocket, as part of NASA’s Project Constellation Return to the Moon program, now cancelled by President Obama. The initial Orion test flight will likely be atop a Delta IV Heavy rocket, Karas told me. The first manned flight is planned for the new heavy lift rocket ordered by the US Congress to replace the Project Constellation architecture.
The goal is to produce a new, US-built manned capsule capable of launching American astronauts into space following the looming forced retirement of NASA’s Space Shuttle orbiters later this year. Thus there will be a gap of at least three years until US astronauts again can launch from US soil.
“Our nation’s next bold step in exploration could begin by 2016,” said Karas in a statement. “Orion was designed from inception to fly multiple, deep-space missions. The spacecraft is an incredibly robust, technically advanced vehicle capable of safely transporting humans to asteroids, Lagrange Points and other deep space destinations that will put us on an affordable and sustainable path to Mars.”
Jim Bray, Director, Orion Crew & Service Module, unveils the first Orion crew module to guests and media at the Lockheed Martin Space Systems Company Waterton Facility in Denver, CO. The vehicle is temporarily positioned in the composite heat shield before installation begins. Following installation of the heat shield and thermal backshell panels, the spacecraft will undergo rigorous testing to validate Orion’s ability to endure the harsh environments of deep space. Credit: Lockheed Martin
Lockheed Martin is the prime contractor for Orion under a multiyear contract awarded by NASA worth some $3.9 Billion US Dollars.
The SOSC was built at a cost of several million dollars. The 41,000 square foot facility will be used to test and validate vehicles, equipment and software for future human spaceflight programs to ensure safe, affordable and sustainable space exploration.
Mission scenarios include docking to the International Space Station, exploring the Moon, visiting an Asteroid and even journeying to Mars. Lockheed has independently proposed the exploration of several challenging deep space targets by astronauts with Orion crew vehicles which I’ll report on in upcoming features.
Orion capsule and Abort rocket mockups on display at Kennedy Space Center.
Full scale mockups of the Orion capsule and emergency abort rocket are on public display at the Kennedy Space Center Visitor Complex in Florida. Orion crew capsule mockup (at left) and Launch Abort System (LAS) at right. The emergency rocket will be bolted atop an Orion spaceship for the initial orbital test flight currently slated for 2013 launch. The LAS mockup was used in launch pad exercises at the New Mexico launch site of the LAS rocket blast-off in May 2010. Credit: Ken Kremer
The SOSC facility provides the capability for NASA and Lockheed Martin engineers to conduct full-scale motion simulations of many types of manned and robotic space missions. Demonstrations are run using laser and optically guided robotic navigation systems.
Inside the SOSC, engineers can test the performance of a vehicles ranging, rendezvous, docking, proximity operations, imaging, descent and landing systems for Earth orbiting mission as well as those to other bodies in our solar system.
“The Orion spacecraft is a state-of-the-art deep space vehicle that incorporates the technological advances in human life support systems that have accrued over the last 35 years since the Space Shuttle was designed.” says Karas. “In addition, the Orion program has recently been streamlined for additional affordability, setting new standards for reduced NASA oversight. Orion is compatible with all the potential HLLVs that are under consideration by NASA, including the use of a Delta IV heavy for early test flights.”
The Orion flight schedule starting in 2013 is however fully dependent on the level of funding which NASA receives from the Federal Government.
This past year the, Orion work was significantly slowed by large budget cuts and the future outlook is murky. Project Orion is receiving about half the funding originally planned by NASA.
And more deep cuts are in store for NASA’s budget – including both manned and unmanned projects – as both political parties wrangle about priorities as they try to pass a federal budget for this fiscal year. Until then, NASA and the entire US government are currently operating under a series of continuing resolutions passed by Congress – and the future is anything but certain.
Orion prototype crew cabin with crew hatch and windows
built at NASA Michoud Assembly Facility, New Orleans, LA. Credit: Ken KremerLockheed Martin team of aerospace engineers and technicians poses with first Orion crew cabin after welding into one piece at NASA Michoud Assembly Facility, New Orleans, LA. Credit: Ken KremerOrion and ISS simulated docking
Russia's Elektro-L spacecraft captured this view of the Moon over the Red Sea region of the Earth. Credit: NPO Lavochkin
[/caption]
This stunning picture of the Moon and Earth was taken by Russia’s new Elektro-L spacecraft, a weather-forecasting satellite that launched in January 2011. This is the first major spacecraft developed in post-Soviet Russia, and it is designed to give Russian meteorologists the ability to watch the entire disk of the planet, thanks to the satellite’s position in the geostationary orbit 36,000 kilometers above the equator. The clarity of the images is fantastic, as you can see in another image of just the Earth, below. The Elektro-L is designed to last at least a decade, and will enable local and global weather forecasting, analysis of oceanic conditions, as well as space weather monitoring, such as measurements of solar radiation, properties of Earth’s ionosphere and magnetic field.
On Feb. 26, 2011, at 14:30 Moscow Time, the Elektro-L satellite produced its first breathtaking image of the home planet. Credit: NPO Lavochkin
The full Moon on March 19, 2011, as seen in Ankara, Turkey. Credit: Ra?id Tu?ra
[/caption]
How super was your full Moon on March 19, 2011? I was completely clouded out, but thankfully quite a few people have been kind enough to share their images. Here are a pictures sent in by readers, as well as via Twitter and Facebook. We’ve got images from all around the world, and even though the size of the Moon really wasn’t that much bigger than usual, (read here why not) it is great to see so many people getting out and looking up at the sky! Our lead image comes from Rasid Tugral in Ankara, Turkey.
This view of the March 19, 2011 full Moon was taken on West Kennet Avenue at the Avebury Stone Circle in Wiltshire. Credit: Pete GlastonburyPerigee moonrise from Rothenfels, Germany. Credit: Daniel Fischer.
Jason Major from Lights in the Dark created this image from a combination of two exposures from his Nikon D80 and 200mm telephoto.
The full super moon. Credit: Peter RiesettThe full moon is seen as it rises near the Lincoln Memorial, Saturday, March 19, 2011, in Washington. The full moon tonight is called a "Super Perigee Moon" since it is at it's closest to Earth in 2011. The last full moon so big and close to Earth occurred in March of 1993. Photo Credit: (NASA/Bill Ingalls)
Supermoon through the trees. Credit: Adam Schaefer‘I took a few shots of the moon during last week and collected three of them to the same picture adding color lines to help the viewer to compare the size of the moon when it is nearing to its perigree status. All the shots have been taken in Laukaa, Central Finland with Sony Alpha700 dslr -camera equipped with 300mm minolta telephoto lens and 2x tele converter, hand held, manual focus. Unfortunately, the night 19.3.2011 was here cloudy, so I couldn't take photos then.’ Credit: Jukka Seppala, Teacher, nature photographer, Vihtavuori, Central FinlandFull Moon over Florida, sent in by cmurray6.'I see the Supermoon a rising, I see trouble on the way ....' taken with an iPhone and a 3.5-inch scope: Credit: Bill DillonThe Moon over the San Francisco, CA Bay Area. Credit: Diane GarberThe Moon and an old coal fired power station in Fremantle, Western Australia. Credit: Donna Oliver Rockingham, Western Australia
This gorgeous shot, was sent in by Donna Oliver from western Australia, take a bit of creative license. She says: “The goal was not to shoot the moon as such but to take advantage of the additional light. Obviously on a long exposure, the moon would not look this good, so I shot the moon, then added it. You can see star movement if you look carefully. I made the moon extra large as my interpretation of the Super Moon.”
'The Moon rising behind a couple of palm trees with cows grazing in the foreground. As you can see in the image, the bottom half of the moon has a different tint due to the earths atmosphere.' Credit: Tom Connor, Parrish, FL SuperMoon taken from Alpha Ridge, March 19, 2011. Credit: James WillinghanMoon over New Orleans. Credit: Peter JansenMoon over Cape Town, South Africa. Equipment: Canon 400D, Sigma 170-500 lens 'The Moon was definitely at its best. I did not try any new tricks as I wanted to compare the "supermoon" with my previous attempts. Phocussing was definitely much easier. My exposure was just right to show up the ejecta rays of the impact craters, Tycho and Copernicus as well.' Credit: Carol BothaThe Moon over Gulf Islands National Seashore near Navarre Beach, Florida. Credit: Mindi Meeks. Click the image to see more in a series taken by Mindi.A 'side by side' comparison of 4 different shots taken over the period of 30 hours before 'SuperMoon'. It shows the progression of Moon in it's orbit until the closest point. Credit: Ramiz Qureshi, from Karachi, Pakistan.This one is pretty creative: Saturdays "Supermoon" compared to the size of an apogee moon (2008). The 'big one' was taken yesterday (March 19, 2011). It is compared to the full moon fotographed at 20.4.2008. The same camera and optics was used (Canon EOS 40 D and Canon 100-400L IS @400mm). In 2008 moon distance was 406,000km, Saturday only 357,000km. Credit: Hans Schremmer Niederkrüchten GermanyThe Moon over Teneriffa, Canary Islands. Camera: Atik 314 E, Astrotrac and 70/420 tube. Credit: Vesa K.'I took this in my garden this evening about 9pm using my Tokina 500mm mirror lens. More detail than I was expecting to be honest,' said photographer Dave Green. Click the image to see his Flickr page. 'Supermoon was scared to shows its face to me.' Credit: Euan McIntoshFull moon over Bassett, Virginia, 03/19/2011. Credit: Essie HollandswortImage of the Full Moon at perigee, taken from Tabuk, Kingdom of Saudi Arabia, on March 19th., 2011 at 20.05UT using a Canon 30D camera set at 1/800sec and 1000ASA. The camera was attached to an 80mm refractor of 500mm focal length and a x3 teleconverter giving an effective focal length of 1,500mm. Credit: Colin Henshaw.The full Moon over England. Credit: Jerry T Krzyzanowski. Click the image to see his gallery.This Super Moon image was taken in Pointe-Claire,Canada. The Super Moon is right behind Mercier bridge, one of the key bridge that ties the Montreal island to the south shore. Credit: Jean-Guy Corbeil, Beaconsfield, QuébecFull Moon over Lake Ontario, beside Hamilton, Ontario (Canada). Credit: Nona Clark
Check out these two from Tavi Greiner on her blog, A Sky Full of Stars: In this one, the Moon rises over a boat on the Shallotte River, just a few hundred yards from the Atlantic Ocean.
And in this one, the Moon appears captured by the rigging, and even almost appears to have lit the ropes on fire.
You’ve probably all seen it before, a huge Full Moon sitting on the horizon and you wonder why it looks much bigger than at other times? It isn’t, really; it’s an illusion.
And now, if you have heard about the close approach of the moon, or so called “Supermoon” on March 19th and are concerned about the disasters and mayem it may cause, there is no need to worry. And surely, when this so-called “Supermoon” occurs on March 19th — at its closest approach to Earth in two decades — people will indeed report that the Moon looks much bigger than normal. But it won’t really be much bigger in the sky at all. It’s all an illusion, a trick of the eye.
The moon does have an effect on the Earth with its gravity affecting ocean tides and even land to a lesser extent, but the moon on the 19th won’t interact with our planet any differently than any other time it’s been at its closest (also known as perigee).
If anything we may get slightly stronger tides, but nothing out of the ordinary.
The Moon orbits the Earth in an elliptical orbit, meaning that it is not always the same distance from the Earth. The closest the Moon ever gets to Earth (called perigee) is 364,000km, and the furthest is ever gets (Apogee) is around 406,000km (these figures vary, and in fact this Full Moon on March 19, 2011 will see a slightly closer approach of 357,000km).
So the percentage difference in distance between the average perigee and the average apogee is ~10%. That is, if the Full Moon occurs at perigee it can be up to 10% closer (and therefore larger) than if it occurred at apogee.
This is quite a significant difference, and so it is worth pointing out that the Moon does appear to be different sizes at different times throughout the year.
Moon at Perigee and Apogee Credit NASA
But that’s NOT what causes the Moon to look huge on the horizon. Such a measly 10% difference in size cannot account for the fact that people describe the Moon as “huge” when they see it low on the horizon.
What’s really causing the Moon to look huge on such occasions is the circuitry in your brain. It’s an optical illusion, so well known that it has its own name: the Moon Illusion.
If you measure the angular size of the Full Moon in the sky it varies between 36 arc minutes (0.6 degrees) at perigee, and 30 arc minutes (0.5 degrees) at apogee, but this difference will occur within a number of lunar orbits (months), not over the course of the night as the Moon rises. In fact if you measure the angular size of the Full Moon just after it rises, when it’s near the horizon, and then again hours later once it’s high in the sky, these two numbers are identical: it doesn’t change size at all.
So why does your brain think it has? There’s no clear consensus on this, but the two most reasonable explanations are as follows:
When the Moon is low on the horizon there are lots of objects (hills, houses, trees etc) against which you can compare its size. When it’s high in the sky it’s there in isolation. This might create something akin to the Ebbinghaus Illusion, where identically sized objects appear to be different sizes when placed in different surroundings.
Ebbinghaus Illusion – the two orange circles are exactly the same size
When seen against nearer foreground objects which we know to be far away from us, our brain thinks something like this: “wow, that Moon is even further than those trees, and they’re really far away. And despite how far away it is, it still looks pretty big. That must mean the Moon is huge!”.
These two factors combine to fool our brains into “seeing” a larger Moon when its near the horizon compared with when it’s overhead, even when our eyes – and our instruments – see it as exactly the same size.
Source: “Moon Illusion” on Dark Sky Diary Special thanks to Steve Owens
