A study in contrasts is on display in the western sky the next few evenings as two planets slide past each other. Although they’re siblings, everything about them is different — their size, composition, and even their color.

Tonight, Mars and Uranus are separated by about twice the width of your finger held at arm’s length. Over the next few nights, Mars will move past Uranus, so they’ll be even closer.

Mars is the second-smallest planet in the solar system. It’s a bit more than half the diameter of Earth. It’s made of rock, and it has a series of layers like those that make up our own planet. A craft that landed on Mars last year is probing the details of those layers.

Mars is topped by dark rock and rusty orange dirt. They give the planet an overall orange color.

Uranus, by contrast, looks blue-green. Methane in its upper atmosphere absorbs red light, so only bluer wavelengths shine through.

Uranus is the third-largest planet in the solar system — about four times the diameter of Earth. It probably has a dense, rocky core, surrounded by layers of ice and gas — a major contrast with its sibling planet.

Mars is well up in the southwest at nightfall, and looks like an orange star. Tonight, it’s directly above the Moon. Uranus is a little to the upper left of Mars. Mars will move closer to Uranus tomorrow, stand almost side by side with it on Monday, then begin pulling away on Tuesday.

More tomorrow.

Script by Damond Benningfield