As discussed previously, NASA's DSCOVR (Deep Space Climate Observatory) spacecraft has been observing the Sun and Earth from the Earth-Sun L1 point since early June. Orbiting the Sun at this special gravitational balancing point, the probe can stay directly between the Earth and Sun throughout Earth's orbit, giving it a constant view of the entire daylit side of Earth. So far this has produced a few stunning "blue marble" images of the full Earth, with many more to come; starting next month NASA will set up a public website to post all of the Earth images that DSCOVR sends back each day.

But today NASA unveiled an animation composed of still images taken by the spacecraft's EPIC (Earth Polychromatic Imaging Camera) instrument that shows something never before seen with such a wide field of view -- the daylit far side of the Moon passing in front of the Earth. Click here for full size:

The Moon is tidally locked to the Earth. That is, it takes the same amount of time to rotate once around its axis as it does to complete one orbit around the Earth (29.5 days). This is the result of many millenia of gravitational interactions raising tides on the Earth and Moon, slowing the Moon's rotation and sending it into further and further orbits of Earth. So we only ever see one face of the moon -- what we call the near side. The other side, the far side (or, inaccurately as you can see above, the "dark side"), was totally hidden to humanity for thousands of years; we had no idea what it looked like. The Soviet Union spacecraft Luna 3 changed this in 1959, when it recorded and sent back images of the far side.

First ever image of the Moon's far side, taken by the Luna 3 spacecraft in 1959. Credit: Roscosmos

Since then, modern spacecraft from many countries have imaged the far side, revealing a much more rough and cratered terrain than the near side, for reasons we don't yet fully understand. DSCOVR, though, is the first spacecraft to record the Moon transiting the Earth from the wide field of view offered by the Earth-Sun L1 point. The Earth's axis is tilted 21 degrees relative to its orbit around the Sun, and the Moon orbits slightly out of alignment with that tilt; the Moon's orbit is tilted about 5 degrees relative to the Earth's equator. Both of these facts have been understood for centuries, but to actually see it in real images, not to mention the gorgeous faces of the two bodies, is breathtaking.

Filtering Light To Create Images

Like most spacecraft, DSCOVR has a series of filters through which it collects incoming light to generate images. These are selected to accept only light from certain regions of the electromagnetic spectrum that are scientifically interesting to the mission. DSCOVR's 905 nanometer filter, for example, only accepts light from the infrared wavelength in which water vapor in Earth's atmosphere is prominently visible, since climate monitoring is one of its main goals. Similarly but on the other end of the spectrum, its 317 and 325 nanometer filters only accept light from the ultraviolet wavelengths associated with ozone (O3) molecules in our atmosphere. Three of the filters correspond to light that our human eyes can see -- red, blue, and green visible light.

Each of the 10 filters is arranged on a wheel device so that they can be rotated in front of the Charge Coupled Device -- the imager's "eye" (for more on how CCDs collect light to make images, check out this post) When EPIC makes an observation, it rotates each of the 10 filters in front of the CCD and records an image. These 10 images can later be combined in various ways to show the team what's happening in Earth's atmosphere and on its surface.

By combining just the red, blue, and green visible light filter images, the team can generate images that show the natural color that our naked eye would see, like the moon transit animation above. But because the moon moves slightly between when the red filter image is recorded and when the green filter image is recorded, a careful look will show a slight green offset on the right edge of the moon, and a slight red offset on the left edge.