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Comment by gurtwo | Hacker News Reader

Why don't we see the Moon's shadow projected over the Earth in the pic? I guess it has to do with the relative distances of the 2 bodies to the Sun and the Moon's penumbra or something. Can someone make a digital image simulation of this to verify?

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One rarely sees earth and moon to scale at the correct distance. I think this leads many of us to often have a broken intuition about these things. And while these new images are spectacular, the distance between moon and earth is not obvious here. Look at e.g. [1]. It becomes immediately obvious why the shadow of the moon will only hit the earth rarely (during an eclipse). If it's not obvious, try holding two real objects at the correct scale / distance in your hands and make one throw a shadow on the other.

At scale, if the earth was the size of a basketball and the moon the size of a baseball, they would be about 7.5m apart.

[1] http://i.imgur.com/LbFGl96.jpg

When the moon casts a shadow on the earth we (on the surface) experience it as a solar eclipse.

For these images, the moon is not directly between the sun and the earth, so it cannot block sunlight from hitting the earth. So, no shadow.

But the satellite is at the L1 Lagrange point which is between the earth and the sun. So if you see the moon pass in front of the earth from L1 shouldn't the moon then also be directly between the earth and the sun? And as someone in another post noted: you can see the suns reflection in the ocean. And the moon passes directly over this spot.

All you can take away from the moon passing over the sun's reflection is this. The moon crossed over the sunlight reflected from the earth. To create a shadow, the moon must cross through the light emitted from the sun.

This image might better help explain. Since the earth's orbital plane is in a different xy plane than the moon's, the moon crossing over the reflection guarantees nothing about whether it crossed the sun's emitted light.

http://www.cnyo.org/wp-content/uploads/2014/04/2014april10_e...

Ah, yes of course. And as cyanoacry noted in another comment, the satellite isn't actually in the Lagrange point - it is orbiting it. Thanks for clearing that up.

Oh nice, ya, I didn't address the Lagrange point because I didn't have any knowledge of it.

The lunar orbit is inclined to Earth's ecliptic plane by 5.1°. That is enough to throw the shadow of the Moon outside the Earth most of the time (expect in cases of solar eclipses, which are rare)

DSCOVR actually is in a "halo orbit" around L1, which means it's not sitting in the plane of the Earth-Sun orbit. From the DSCOVR site[1]: "The spacecraft will be orbiting [L1] in a six-month orbit with a spacecraft-Earth-sun angle varying between 4 and 15 degrees."

As some other folks have mentioned, the Moon's orbit is tilted a total of 5.2 degrees[2] relative to the Earth-Sun plane, so DSCOVR will see quite a number of DSCOVR-Moon-Earth alignments, but never a Sun-DSCOVR-Moon-Earth alignment.

[1] http://www.nesdis.noaa.gov/DSCOVR/

[2] http://hyperphysics.phy-astr.gsu.edu/hbase/solar/solecl.html

I thought the dark edge at the top-left of the moon was a shadow, but there was no solar eclipse in the time-frame reported for these pictures.

So, even though a perfect alignment of DSCOVR (at L1), the moon, and Earth would mean a shadow and thus eclipse, the moon must be a little off that alignment on this pass, throwing its shadow into dark space, even though it looks very nearly aligned in the photos.

That dark edge is an artefact of the delay in capturing the three monochrome pictures through the different RGB filters.

The article says : "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 the 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."

It also says "A thin sliver of shadowed area of moon is visible on its right side." which is a better explanation. It's not a shadow on the Earth, it's the dark part of the moon.

What shadow? When is the last time you walked out the door in the daytime and it was dark because the the moon was casting a shadow? That only happens during eclipses.

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