Very Large Moonset... but not why you think

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June 7 2010 11:45 AM

Very Large Moonset... but not why you think

The European Southern Observatory just posted this lovely picture of the moon setting behind the Very Large Telescope observatory in Chile:

eso_vlt_moonset

Phil Plait 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!  

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[Click to embiggen.]

Photographer Gordon Gillet was 14 km (8.5 miles) away from the observatory when he caught the full Moon behind it. The sky is pink because behind him, the Sun was rising -- as it must be when the full Moon is setting.

But I had to chuckle when I read the description:

Contrary to what one may think, this picture is no montage. The Moon appears large because it is seen close to the horizon and our perception is deceived by the proximity of references on the ground. In order to get this spectacular close view, a 500-mm lens was necessary. The very long focal length reduces the depth of field making the objects in focus appear as if they were at the same distance. This effect, combined with the extraordinary quality of this picture, gives the impression that the Moon lies on the VLT platform, just behind the telescopes, even though it is in fact about 30 000 times further away.

The part about the long lens collapsing the perspective is absolutely correct. That effect has been used for decades in the film industry to make far away things look as close as things much nearer (like, say, people running along train tracks to escape an oncoming train).

When I first read the description, though, I thought they were referencing the Moon Illusion, where the Moon looks huge on the horizon. Reading it again, I see they aren't (though it would be easy to think they are). And that's good, because the Moon Illusion isn't playing into this at all. I explain how it works in more detail elsewhere; it's an illusion caused by the shape of the sky and the way our brains perceive it. Also, the Moon Illusion is an effect that only happens when we can see large parts of the sky and get a sense of perspective on it; so it can't be photographed (or at least I've never seen it done, and don't see how it could be). In the case of this picture, it's all due to the magnification of the lens used.

And it's a beautiful shot. If you're on Twitter, you can follow the ESO and get notified when they post another gorgeous image like this one. This is part of their Image of the Week series, which is updated every Monday.

Image credit: G.Gillet/ESO