Safe aurora tours: Thinking of a visit to Norway? Marianne's Heaven on Earth Aurora Tours has a 7-seater minivan for families who don't require social distancing. See the Northern Lights or take a scenic day tour. Book here

SUNSET PLANETS: When the sun goes down tonight, step outside and look south. Jupiter and Saturn pop out of the twilight even before the sky fades to black. On Tuesday and Wednesday, the crescent Moon will pass by the two planets, forming a shape-shifting triangle in the evening sky. Sky & Telescope: sky map.

THE SODIUM TAIL OF MERCURY: The biggest comet in the Solar System is actually a planet. It's Mercury. Researchers have known for years that Mercury has an enormous tail. Last week, Dr. Sebastian Voltmer photographed it from his backyard in Spicheren, France:

"This stacked image was exposed through a custom-made sodium filter," explains Voltmer. "The trees are from an initial exposure, which I blended into the final stack."

First predicted in the 1980s, Mercury's tail was discovered in 2001. Its source is Mercury's super-thin atmosphere. Mercury is so close to the sun, pressure from sunlight itself can push atoms out of the atmosphere and into space. The escaping gas forms a tail more than 24 million km long.

The tail is rich in sodium--a substance sputtered from Mercury's surface by solar wind and micrometeorite impacts. That's why Voltmer's yellow sodium filter did such a good job revealing the gaseous stream.