DSCOVR - Earth/Moon sizes

[Paul]

Hi all (first post - eek!),

I've been defending the DSCOVR "Moon passing in front of Earth" images http://www.nasa.gov/feature/goddard/from-a-million-miles-away-nasa-camera-shows-moon-crossing-face-of-earth from people crying 'fake!' and in particular someone had questioned the relative sizes of the Moon and Earth in the images, saying the Moon wasn't big enough.

I'm sure that everything is correct, and indeed have seen a Starry Night Pro simulation posted that matched field of view and Earth/Moon sizes.

The trouble is I can't find a way to prove it. I tried a bit of maths to compare angles etc but my maths wasn't up to the job!

Can anyone prove why the relative Earth and Moon sizes are correct please?

[Zakalwe]

Can anyone prove why the relative Earth and Moon sizes are correct please?

Can the person crying fake explain why the sizes are incorrect?

[Paul]

Hi Zakalwe,
The line of reasoning was that the Moon is roughly half the diameter of Earth so if they were side by side viewed from any distance that would appear to be the case.  If you then bring the Moon closer then it will get bigger. So Moon should appear at very least half Earth size.  In the NASA animation the Moon is less than half the size.

I then tried drawing it out on paper and trying to work out angular sizes but these didn't seem to help. I know everything about the sizes will turn out to be fine but at the moment I'm missing something...

[bknight]

Hi Zakalwe,
The line of reasoning was that the Moon is roughly half the diameter of Earth so if they were side by side viewed from any distance that would appear to be the case.  If you then bring the Moon closer then it will get bigger. So Moon should appear at very least half Earth size.  In the NASA animation the Moon is less than half the size.

I then tried drawing it out on paper and trying to work out angular sizes but these didn't seem to help. I know everything about the sizes will turn out to be fine but at the moment I'm missing something...

The moon is closer to .27 the diameter of the earth.  The satellite I believe is in a million miles from earth, so subtracting the roughly 240 K miles then it would appear larger than the .27.  You would have to go back to trig to do the exct calculations, but the size seems appropriate.

[Paul]

Lol, I was using radius for Earth and diameter for Moon.  Sorry everyone... I knew there'd be a simple answer, just didn't think it would be that simple! 

[Zakalwe]

The line of reasoning was that the Moon is roughly half the diameter of Earth

No it's not. The Moon diameter is 3,473km and the Earth's is 12,742.

[onebigmonkey]

I've been doing exactly the same thing. Posting this:



to show the view from the sun ("But why isn't there a shadow of the moon?")

and this:



to show that the clouds are actually moving.

But really it's just being Canute against a tide of "Yeah but it looks kinds funny and boo NASA..."

[Paul]

Yes, it's unfortunate that the animation does look 'a bit unusual' on first look.  It all makes sense when you dig a bit deeper.

[Chew]

Keep in mind DSCOVR is 1.6 million km from the Earth but it was 1.2 million km from the Moon. The Moon should appear 36% as big as the Earth.