The Moon's rarely-seen far side just photobombed NASA again

Monday, July 11, 2016, 7:56 PM - Rarely seen outside of Apollo mission footage or computer simulations, the far side of the Moon just photobombed NASA for the second time in a year!

NASA's EPIC camera, on board the joint NASA/NOAA Deep Space Climate Observatory (DSCOVR), has been aimed at Earth from around 1.6 million kilometres closer to the Sun, snapping multiple pictures of our planet nearly every day since July 6, 2015.

Less than a year after EPIC's "first light" - on July 4 to July 5, 2016 - the camera has caught the far side of the Moon making a seldom-seen appearance as it passes in front of the Earth.

NASA EPIC's 15-picture view of the latest lunar transit, from 11:50 p.m. EDT July 4 to 3:18 a.m. EDT July 5, 2016. Look closely, and you can see the cloud patterns change throughout the day. Image credits: NASA/NOAA. Animation by S. Sutherland

The apparent difference in "texture" or "lighting" between the Moon and Earth in the animation above is not due to any image processing. It's actually due to Earth's atmosphere.

Since the Moon has only the thinnest wisp of argon around it, sunlight strikes its surface and is reflected unfiltered. Also, due to the texture of the lunar surface, the light is reflected more or less directly back in the direction of the Sun. Light passing through Earth's atmosphere, on the other hand - both inbound towards the surface and outbound into space - is scattered by the much thicker air, and the direction of the reflected light is highly dependent on the surfaces of the water and the weathered continents. All of this adds up to give Earth a "softer" look.

There are also some small colour imbalances, especially with the Moon, however those are due to how these images are captured.

According to NASA:

EPIC’s “natural color” images of Earth are generated by combining three separate monochrome exposures taken by the camera in quick succession. EPIC takes a series of 10 images using different narrowband spectral filters -- from ultraviolet to near infrared -- to produce a variety of science products. The red, green and blue channel images are used in these color images.
Combining three images taken about 30 seconds apart as the moon moves produces a slight but noticeable camera artifact on the right side of the moon. Because the moon has moved in relation to Earth between the time the first (red) and last (green) exposures were made, a thin green offset appears on the right side of the moon when the three exposures are combined. This natural lunar movement also produces a slight red and blue offset on the left side of the moon in these unaltered images.

This is the second time EPIC has spied the Moon's far side, and the third time it has caught the Moon crossing its field of view.

The first time was on July 16, 2015, just 10 days after it began taking pictures from its position at Lagrange Point 1. On that day, the Moon passed between the camera and Earth, although this did not coincide with a solar eclipse, since DSCOVR was not perfectly aligned with the Sun and Earth. Instead, it caught the Moon transiting across the Earth, while the Moon's shadow slipped past the planet. The same thing has happened in this second transit.

The second time EPIC spied the Moon, though, it captured the much more familiar near-side, on September 27, 2015, as the Moon slipped behind the Earth, from EPIC's perspective. On this date, there was an eclipse, however - the rare "supermoon" lunar eclipse, as the Moon passed directly through the darkest part of Earth's shadow, while also being the closest perigee Full Moon of the entire year.

The far side of the Moon is seldom seen by us due to the Moon being tidally-locked to Earth. That is, the Moon's rotation - the length of its day, from sunrise to sunrise for anyone standing on its surface - takes the same amount of time as it does for the Moon to travel once around the Earth and come back to the same lunar phase.

The Moon doesn't always show up in EPIC photographs because the spacecraft's oscillating "orbit" around Lagrange Point 1 changes the angle of the camera, so often the Moon passes by either above or below the camera's field of view.

Source: NASA