Saturday, June 9, 2012

Size of the Sun

Someone asked me about this photo:

(via NASA)

The question was: would the Sun really appear so large if you were standing on Venus?  I knew the answer to that is no, but I have been guilty of wondering if it would appear that large if you were standing on Mercury.  But again: the answer is no.  So how big would the Sun be if you were on either planet?  Below are some photos showing how the Sun would look from each one:

From Venus, the Sun would be a third larger (which really isn't that big a difference).  Mercury follows are relatively more elliptical orbit, with the Sun appearing twice as big at its furthest and three times as big at its nearest.  From Mercury the Sun is certainly much larger in the sky, but probably not nearly as big as you might have thought it would be.

So just how is it that the Sun is so big on those photos?  I mean, the camera can't be really close to the Sun... because otherwise it wouldn't be able to also have Venus in that picture at the top of this post.  So what if I was standing on a spaceship positioned at exactly where that photo was taken, in orbit above Venus looking toward the Sun?  Why wouldn't the Sun appear exactly as big as it looks in that image?

And to that I answer: the Dolly Zoom-

The short of it is that as you zoom in on something: further objects grow larger at a faster rate than nearer objects, distorting the perspective.  As you zoom in: your near object (Venus) only gets a little bigger, but your far object (the Sun) gets immensely bigger.

You've probably seen this before in the movies.  One of the most prominent uses of the Dolly Zoom is in Hitchcock's Vertigo, but more recently there's a memorable effect in the first Lord of the Rings, where Frodo senses the Ringwraiths approaching along the forest path.

Since telescopes are just massive zoom lenses: the ever-distant sun can appear to loom massively about the planet.  If you were to physically travel from Earth to Venus: you'd see Venus growing ever larger as you get close to it, but the Sun would pretty much stay the same size.