Distance Learning

Bad Astronomy
The entire universe in blog form
June 26 2005 9:52 PM

Distance Learning

Note: I wrote this on Sunday afternoon, fully expecting the normal California weather to allow me to go outside and get some photographs of the planets in the western sky. But the best-laid schemes o' mice an' men gang aft agley, as they say ("they" being Robert Burns, in this case). It's cloudy out. So pretend there is a picture here of three planets really close together. Thanks.

In affairs of the heart, it is said, distance adds perspective. "Give yourself time to distance yourself from the situation," people will say, "and things will seem clearer."

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!  


But in astronomy, the exact opposite is the case. Distance does not add perspective. It robs us of it.

Consider a nearby object, such as your thumb held at arm's length, and a distant one, such as a tree across the street. If you hold one eye closed and look at the tree past your thumb, you may note (say) that your thumb appears to be on the left of the tree. Now switch eyes. Suddenly, you may see your thumb jump to the other side of the tree. That's because the angle made from your eye to your thumb and again to the tree has changed. This effect is called parallax. If you want more details on this, I have page describing it more thoroughly here.

But when an object is too far away, this method fails us. The distance to the object is much larger than the separation of our eyes, and parallax using just our eyes is useless (and so we must employ other methods in that case). Such is the situation with astronomy. Astronomical objects are so far away that it's impossible for us to directly perceive their distance. We literally lack perspective.

A case in point is hovering in the sky above me even as I write this. Three planets are making a relatively rare neighborly pass-- Venus, Mercury, and Saturn are all in a part of the sky small enough to cover with your thumb, if you still have it hanging on the end of your outstretched arm.

If you're reading this on Monday, then look to the west just after sunset. Venus is the brightest of the three, and the most obvious. Just next to it is Mercury, and below the pair and to the right is Saturn (if you are in the southern hemisphere, then reverse left and right). Saturn is faintest, and you may not see it until it starts to get darker outside (Sky and Telescope magazine has an animation of this event online which should help you visualize the event) .

Since we don't know their distances just by looking, we might assume Venus is the closest, since it's brightest (it's about 33 times brighter than Mercury right now). Mercury and Saturn are roughly the same brightness, so maybe, you'd think, they are equally distant from us.

BZZZZT! Nope. Venus is actually about 1.5 times farther away from us than Mercury, even though (overly) simple math based on its brightness would imply that Venus is 1/6 the distance of Mercury. And Saturn is actually a whopping 10 times farther away!

The reason Venus is brighter is because it's much bigger than Mercury, and reflects light better. Saturn, of course, is way bigger than Mercury (25 times the diameter, meaning over 600 times the surface area!), so even though it's really far away, it rivals Mercury for brightness.

So, without knowing the distances to these objects we'd have no clue about their real properties. Appearances can be deceiving.

Distance may be just the solution for some emotional turmoil, but for a more cosmic perspective it's absolutely crucial.



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