### This thing on?

Aug. 5th, 2015 09:45 pmI recently saw the DSCOVR image of the Earth and Moon that's making the rounds. If you haven't seen it, please do. It's inspiring, at least to me.

As a do-I-remember-my-geometry exercise, I found myself wondering if I could figure out how far away DSCOVR was using this picture (well, plus a couple other facts). So I downloaded the image, and measured the Earth and the Moon's sizes in pixels. The Earth is 1590 pixels in diameter, and the Moon's diameter is 582 pixels.

Next I looked up the ratio of the diameters of Moon:Earth (0.273), which meant that if Earth were at the same distance as the Moon, it should be 582/0.273=2132 pixels, or 2132/1590=1.34 times bigger. Similar triangles means that Earth must be 1.34x farther from DSCOVR than the moon. Let's call the distance from DSCOVR to the Moon "D", so that makes the distance to Earth 1.34*D. So Earth-Moon distance is 0.34D.

Looking up the approximate Earth-Moon distance (384000km), and dividing by 0.34, tells us D is 384000/0.34=1129000km; adding back in the 384000km E-M gives us the approximate distance to DSCOVR from Earth. 1,513,000km.

Now to check the answer. (Drumroll as I google "distance to L1"): "about 1.5 million kilometers".

Booyah!