The Sun’s closest planets are crossing paths in the southwest in early evening. There’s a limited viewing window because they set by the time twilight fades away.

The brighter planet is Venus. It’s the “evening star,” quite low in the sky at sunset. It’s so bright that you might mistake it for an airplane with its landing lights turned on. But it’s so low that you need a clear horizon to pick it out — any trees or buildings will block it from view.

Mercury stands a little to the lower left of Venus this evening, but it’s moving upward. It’ll be roughly even with Venus tomorrow evening, then will move up and away from it. It’ll stand a little higher in the sky at sunset for several days.

Mercury is the closest planet to the Sun, while Venus is the next one out. Yet Venus is much hotter. That’s because it has a thick atmosphere, made mainly of carbon dioxide. As we know from our changing climate here on Earth, C-O-2 is a potent greenhouse gas — it traps heat. It’s warmed the surface of Venus to more than 850 degrees Fahrenheit.

By comparison, peak temperatures on Mercury — which has no atmosphere to speak of — are about a hundred degrees lower. And most of the time, across most of the planet, it’s not nearly as hot. It can reach hundreds of degrees below zero on the nightside and at the poles — frigid conditions close to the Sun.

Keep an eye on these close, hot worlds as they cross paths in the early evening sky.

Script by Damond Benningfield

Support McDonald Observatory