Looking at stars and planets in the sky, we lack the usual distance-indicating clues we rely on daily to unravel the true size of and distance to local objects in our 3-D world. For example, when you hold up your hand to wave at a neighbor across the street, you immediately know that the neighbor is really a full-sized person, not a diminutive copy who is smaller than your hand.
You immediately recognize that the reason your neighbor appears so small is because she is far away — and, by comparing her size to the objects around her and to the size of your waving hand, you can have some idea how far away she actually is.
We can also use brightness as a clue to distance.
If you are crossing a railroad track late at night and see a faint light down the middle of the rails, you know it is safe to cross: the train is obviously not close.
But in astronomy, apparent size and actual size, and apparent brightness and actual brightness, are hopelessly tangled. The four planets knitted close together before sunrise for the next few days serve up a good example of this tangling.
Jupiter and Venus appear about the same brightness, and both are much brighter than Mercury and Mars, yet their distances are all quite different.
A common measure of distance in the solar system is the astronomical unit (AU). One AU equals the average distance between Earth and the sun — about 93 million miles.
Presently, Venus is about 1.5 AU away, while Jupiter is about 5.8 AU away. Jupiter appears so bright compared to much closer Venus because Jupiter is more than 11 times the size of Venus (and Earth, because Venus and Earth are about the same size).
The other pair of planets to compare is Mercury and Mars. As they now stand, Mercury is just under 1 AU away while Mars is nearly 2.4 AU away. Yet, they appear the same brightness, and Mars is only about 30-percent bigger than Mercury (both planets are roughly half the size of Earth).
Mars is now about as bright as Mercury in spite of being more than two-times farther away for a different reason.
As Mars is now situated on its orbit, it is nearly on the other side of the sun from us here on Earth. Seen through a small telescope, Mars would have the shape of a tiny full moon.
Mercury, on the other hand, is mostly between the Earth and the sun. Seen through a small telescope, Mercury would look like a tiny crescent moon.
So in the way they are standing, we can see much more of the illuminated portion of Mars’ surface than we can of Mercury, which helps Mars stay bright in comparison.
If you would ever like to know the exact distance to any planet in real time, there is a nice calculator available online at www.wolframalpha.com (search for “planet distance calculator”), and a visual and interactive planet size comparison at www.sciencenetlinks.com (search for “planet size comparison”).
Doug Furton is a member of the physics faculty at GVSU. Send questions and suggestions to firstname.lastname@example.org. An archive of some of his “What’s up” columns is available online at http://gegenschein.wordpress.com.