What is so super about tomorrow's supermoon? Tomorrow, a full moon will occur that appears slightly larger and brighter than usual. The reason is that the Moon's fully illuminated phase occurs within a short time from perigee - when the Moon is its closest to the Earth in its elliptical orbit. Although the precise conditions that define a supermoon vary, given one definition, tomorrow's will be the third supermoon of the year -- and the third consecutive month that a supermoon occurs. One reason supermoons are popular is because they are so easy to see -- just go outside and sunset and watch an impressive full moon rise! Since perigee actually occurs today, tonight's sunset moonrise should also be impressive. Pictured above, a supermoon from 2012 is compared to a micromoon -- when a full Moon occurs near the furthest part of the Moon's orbit -- so that it appears smaller and dimmer than usual. Given many definitions, at least one supermoon occurs each year, with the next being 2015 August 30.

Like a rainbow at night, a beautiful moonbow shines above the western horizon in this deserted beach scene from Molokai Island, Hawaii, USA, planet Earth. Captured last June 17 in early morning hours, the lights along the horizon are from Honolulu and cities on the island of Oahu some 30 miles away. So where was the Moon? A rainbow is produced by sunlight internally reflected in rain drops from the direction opposite the Sun back toward the observer. As the light passes from air to water and back to air again, longer wavelengths are refracted (bent) less than shorter ones resulting in the separation of colors. And so the moonbow is produced as raindrops reflect moonlight from the direction opposite the Moon. That puts the Moon directly behind the photographer, still low and rising over the eastern horizon, a few days past its full phase.

This rich starscape spans nearly 7 degrees on the sky, toward the Sagittarius spiral arm and the center of our Milky Way galaxy. A telescopic mosaic, it features well-known bright nebulae and star clusters cataloged by 18th century cosmic tourist Charles Messier. Still popular stops for skygazers M16, the Eagle (far right), and M17, the Swan (near center) nebulae are the brightest star-forming emission regions. With wingspans of 100 light-years or so, they shine with the telltale reddish glow of hydrogen atoms from over 5,000 light-years away. Colorful open star cluster M25 near the upper left edge of the scene is closer, a mere 2,000 light-years distant and about 20 light-years across. M24, also known as the Sagittarius Star Cloud, crowds in just left of center along the bottom of the frame, fainter and more distant Milky Way stars seen through a narrow window in obscuring fields of interstellar dust.

On October 19th, a good place to watch Comet Siding Spring will be from Mars. Then, this inbound visitor (C/2013 A1) to the inner solar system, discovered in January 2013 by Robert McNaught at Australia's Siding Spring Observatory, will pass within 132,000 kilometers of the Red Planet. That's a near miss, equivalent to just over 1/3 the Earth-Moon distance. Great views of the comet for denizens of planet Earth's southern hemisphere are possible now, though. This telescopic snapshot from August 29 captured the comet's whitish coma and arcing dust tail sweeping through southern skies. The fabulous field of view includes, the Small Magellanic Cloud and globular star clusters 47 Tucanae (right) and NGC 362 (upper left). Worried about all those spacecraft in Martian orbit? Streaking dust particles from the comet could pose a danger and controllers plan to position Mars orbiters on the opposite side of the planet during the comet's close flyby.

To some, the outline of the open cluster of stars M6 resembles a butterfly. M6, also known as NGC 6405, spans about 20 light-years and lies about 2,000 light years distant. M6, pictured above, can best be seen in a dark sky with binoculars towards the constellation of the Scorpion (Scorpius), coving about as much of the sky as the full moon. Like other open clusters, M6 is composed predominantly of young blue stars, although the brightest star is nearly orange. M6 is estimated to be about 100 million years old. Determining the distance to clusters like M6 helps astronomers calibrate the distance scale of the universe. Now Available: APOD 2015 Wall Calendars

How different are space and time at very small scales? To explore the unfamiliar domain of the miniscule Planck scale -- where normally unnoticeable quantum effects might become dominant -- a newly developed instrument called the Fermilab Holometer has begun operating at the Fermi National Accelerator Laboratory (Fermilab) near Chicago, Illinois, USA. The instrument seeks to determine if slight but simultaneous jiggles of a mirror in two directions expose a fundamental type of holographic noise that always exceeds a minimum amount. Pictured above is one of the end mirrors of a Holometer prototype. Although the discovery of holographic noise would surely be groundbreaking, the dependence of such noise on a specific laboratory length scale would surprise some spacetime enthusiasts. One reason for this is the Lorentz Invariance postulate of Einstein's special relativity, which states that all length scales should appear contracted to a relatively moving observer -- even the diminutive Planck length. Still, the experiment is unique and many are curious what the results will show. Astrophysicists: Browse 900+ codes in the Astrophysics Source Code Library

Why would the sky look like a giant target? Airglow. Following a giant thunderstorm over Bangladesh in late April, giant circular ripples of glowing air appeared over Tibet, China, as pictured above. The unusual pattern is created by atmospheric gravity waves, waves of alternating air pressure that can grow with height as the air thins, in this case about 90 kilometers up. Unlike auroras powered by collisions with energetic charged particles and seen at high latitudes, airglow is due to chemiluminescence, the production of light in a chemical reaction. More typically seen near the horizon, airglow keeps the night sky from ever being completely dark. Now Available: APOD 2015 Wall Calendars

How was this picture taken? Usually, pictures of the shuttle, taken from space, are snapped from the space station. Commonly, pictures of the space station are snapped from the shuttle. How, then, can there be a picture of both the shuttle and the station together, taken from space? The answer is that during the Space Shuttle Endeavour's last trip to the International Space Station in 2011 May, a supply ship departed the station with astronauts that captured a series of rare views. The supply ship was the Russian Soyuz TMA-20 which landed in Kazakhstan later that day. The above spectacular image well captures the relative sizes of the station and docked shuttle. Far below, clouds of Earth are seen above a blue sea. Follow APOD on: Facebook, Google Plus, or Twitter

Look up in New Zealand's Hollow Hill Cave and you might think you see a familiar starry sky. And that's exactly what Arachnocampa luminosa are counting on. Captured in this long exposure, the New Zealand glowworms scattered across the cave ceiling give it the inviting and open appearance of a clear, dark night sky filled with stars. Unsuspecting insects fooled into flying too far upwards get trapped in sticky snares the glowworms create and hang down to catch food. Of course professional astronomers wouldn't be so easily fooled, although that does look a lot like the Coalsack Nebula and Southern Cross at the upper left ...

Open star cluster NGC 7380 is still embedded in its natal cloud of interstellar gas and dust popularly known as the Wizard Nebula. Seen with foreground and background stars along the plane of our Milky Way galaxy it lies some 8,000 light-years distant, toward the constellation Cepheus. A full moon would easily fit inside this telescopic view of the 4 million year young cluster and associated nebula, normally much too faint to be seen by eye. Made with telescope and camera firmly planted on Earth, the image reveals multi light-year sized shapes and structures within the Wizard in a color palette made popular in Hubble Space Telescope images. Recorded with narrowband filters, the visible wavelength light from the nebula's hydrogen, oxygen, and sulfur atoms is transformed into green, blue, and red colors in the final digital composite. But there is still a trick up the Wizard's sleeve. Sliding your cursor over the image (or following this link) will make the stars disappear, leaving only the cosmic gas and dust of the Wizard Nebula.

The beautiful Trifid Nebula, also known as Messier 20, is easy to find with a small telescope in the nebula rich constellation Sagittarius. About 5,000 light-years away, the colorful study in cosmic contrasts shares this well-composed, nearly 1 degree wide field with open star cluster Messier 21 (top right). Trisected by dust lanes the Trifid itself is about 40 light-years across and a mere 300,000 years old. That makes it one of the youngest star forming regions in our sky, with newborn and embryonic stars embedded in its natal dust and gas clouds. Estimates of the distance to open star cluster M21 are similar to M20's, but though they share this gorgeous telescopic skyscape there is no apparent connection between the two. In fact, M21's stars are much older, about 8 million years old.

The Milky Way was not created by an evaporating lake. The colorful pool of water, about 10 meters across, is known as Silex Spring and is located in Yellowstone National Park in Wyoming, USA. Illuminated artificially, the colors are caused by layers of bacteria that grow in the hot spring. Steam rises off the spring, heated by a magma chamber deep underneath known as the Yellowstone hotspot. Unrelated and far in the distance, the central band of our Milky Way Galaxy arches high overhead, a band lit by billions of stars. The above picture is a 16-image panorama taken late last month. If the Yellowstone hotspot causes another supervolcanic eruption as it did 640,000 years ago, a large part of North America would be affected. Follow APOD on: Facebook, Google Plus, or Twitter