That's no moon

Again this is a little off topic but I think that there are some nice cool elements here and a couple of points to be made about common misconceptions.

I just came across this tweet with this really amazing animated gif,

The moon transiting earth, from brand-new satellite "DSCOVR"
HD resolution:https://t.co/NLPmPt5uYz pic.twitter.com/HBHFwolITX

— SecuriTay (@SwiftOnSecurity) February 1, 2016

One can find the gif itself at this link. The pictures are taken by the NASA satellite DSCOVR (Deep Space Climate Observatory), which is observing the Earth from the L1 point, one of the Lagrange equilibrium points of the circular restricted three body problem of the Sun-Earth system. This 3 body problem consists of two massive main bodies circling around their common centre of mass, with the one of the two being more massive, and an additional third much smaller body that doesn't affect the motion of the two larger ones. One can define an effective potential for the motion of the smaller body in the frame that rotates with the two larger bodies, and from that effective potential identify equilibrium points. The following figure shows contours of that potential for different values of the energy of the third body.

The three crossing points along the horizontal axis in the
middle are from left to right the L3, L1, and L2 points.

In the case of the Sun-Earth system, the Sun would be at the centre of the left set of (larger) circles, while the Earth would be at the centre of the right (smaller) set of circles. You could also imagine one of the inner circles on the right to represent the orbit of the Moon (of course the picture is nowhere near the right scale). What we see therefore in the gif is what the satellite sees from that cross point in the middle between the Sun and the Earth.

There are some really interesting things about this sequence of photographs. One is pointed out and answered in the comments of the original tweet.

.@josiahgrinnell Based on my experience as photographer, it looks fake because moon has no atmosphere and the lens' depth of field is huge

— SecuriTay (@SwiftOnSecurity) February 1, 2016

It is crazy, but it really looks sort of fake. And it is true that we are not used to images without an atmosphere and the sharpness seems unreal (the edge of the Earth looks much more fuzzy). I wonder if this is really the reason why it looks so unusual (I am no expert so I can't tell).

The other thing is that the moon that we are seeing in the pictures is not the Moon that we are used to, as someone else points out (probably trolling)

@SwiftOnSecurity Fake. The moon doesn't look like that. This is what it looks like: pic.twitter.com/up7GzPuKSZ

— Rob Graham ❄️ (@ErrataRob) February 1, 2016

This is the familiar, to everyone on Earth, view of the Moon. Of course, someone from the far side would never see that, since the Moon is tidally locked in its orbit around the Earth and rotates around its axis with the same frequency as it rotates around the Earth, therefore presenting to us only one face. Which means that the DSCOVR satellite from its vantage point will always show us in such pictures the side of the Moon we never see from here. And it will always be lit like this, since the Sun is right behind the satellite providing this full illumination (there is no dark side of the Moon, except for this).

Finally, let's end with a meme

"thats no moon taylor"@SwiftOnSecurity pic.twitter.com/nplIl56aq3

— darth™ (@darth) February 1, 2016

Which of course is a Star Wars reference...



Cheers