Both the Earth and Moon are 99% illuminated .Remo wrote:Almost, but not quite. Indeed, the Moon's right edge is green and the left edge is purple because of the time delay in between camera exposures; however, the Earth's right edge is blue because we are looking at the edge of the atmosphere ( http://www.spaceflight.nasa.gov/gallery ... 20073.html ). The Earth's right side edge would be blue too, except for the fact that it is in darkness.
The thing is that the Moon is moving with respect to the camera, but the Earth is fixed (it does rotate slightly, however). This is because the Deep Space Climate Observatory satellite orbits at the L1 Lagrange Point: Basically a stable orbit directly between the Earth at the sun. Because the camera is fixed with respect to the Earth, you get the same big ball in the same place each time. Thank you (almost) President Al Gore.
As an aside, I find it interesting that the satellite's position on July 16th was not quite in perfect alignment. This is evidence by both the Moon and the Earth having a dark edge on the right side. Also interesting is that only this slight misalignment/dark edge allowed the green edge artifact to show up. The green filter was shot first. By the time the blue and red filters were shot, the Moon had moved towards the right and those images just recorded black thus allowing the green tint image of the earth below to be superimposed on top of the black Moon edge creating the green edge effect.
DSCOVR is not at the exactly at L1 - it is "orbiting" nominally about it in a halo / Lissajou orbit. If we define the L1 point to be exactly on a line connecting the Earth and Sun, then the DSCOVER-Earth line is off by ≈ 11.5°. This puts the craft about 200,000 miles (325,000km) from an ideal single-point L1 location. Also, in the time-lapse video, the Moon was directly between DSCOVR and Earth at about 22:00UT, Jul 16. New Moon was 1:24UT the same day, so the Moon was already 20½ hours old. These related factors are what's behind the 99% illumination fraction, thus the darker limb edge(s).
In fact the red filter was used first, and the green filter last. The green edge logically follows from the direction the moon is traveling (to the right). This is also described here.
I recreated this event using Stellarium. It took a little time because I had to create DSCOVR's orbital location (but not the actual orbit as there are no elements available and the halo orbit is complicated). Anyway, from this simulation I determined where the craft was wrt the L1 point location.
- Stellarium Simulation - DSCOVR, Jul 16, 19:50UT + 5 Hrs
- DSCOVR Moon-Earth View from L1_2.gif (3.11 MiB) Viewed 12063 times
