Two Moons: Photograph of the International Space Station near the Moon.

Two of Earth’s Moons Seen in One Picture

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Jan. 25 2013 9:32 AM

Two of Earth’s Moons in One Picture

Photographer Lauren Hartnett took a fantastic picture: Two Moons literally passing in the night!

Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!

Hartnett was in Houston, Texas—at NASA’s Johnson Space Center, appropriately enough—on Jan. 4, 2012 when she snapped this. It shows the Moon, just a few days past half full, with the International Space Station sailing past it to the upper left. The station is in low-Earth orbit, so it's much closer than the Moon; it's just a little perspective and orbital mechanics that made them look so close together.

Most people aren’t aware the ISS is easily visible to the naked eye when it passes overhead. In fact, if the geometry is right it can outshine Venus and be the third brightest object in the sky (after the Sun and Moon). I’ve seen it many, many times myself, and a few times I’ve also seen incredibly bright flashes of light off the solar panels as they catch the Sun just right. It’s amazing.

What fascinates me about this picture are the relative sizes of the two moons. I can do some math with this!

The Moon was about 404,000 kilometers (250,000 miles) away when this picture was taken, and is about 3470 km (2170 miles) in diameter.

The ISS orbits the Earth at a height of 400 km (250 miles). That’s how far away it would be if it passed straight overhead, but for Hartnett it was at a 30° angle from the zenith (given the Moon's altitude above the horizon at the time). If you remember your trig, that multiplies the distance by a factor of 1.15 (the distance roughly scales as the secant of the angle away from the zenith, if you’re playing along at home), so the ISS was actually about 460 km (290 miles) away at the moment this shot was snapped.