What’s Up: September 2021

What's Up for September? Moving fast in the cosmos with planet Mercury, and stars Arcturus and Altair...

You'll have to be quick to catch a glimpse of Mercury this month, as the innermost and fastest-moving planet – namesake of the fleet-footed, mythological messenger – appears low in the west for a short time following sunset. But those with a clear view toward the horizon will be rewarded with some nice planetary groupings in the first week and a half of September.

You'll find Mercury just a few degrees above the western horizon about half an hour after the Sun sets, with much brighter Venus higher and slightly farther to the south. On the 9th and 10th look for the pair to be joined by the crescent Moon.

In between the two planets is the bright star Spica, which is actually two stars orbiting each other at a distance 3 times closer than Mercury orbits our Sun!

Mercury should be visible for you from the mid-northern latitudes on south. The farther south you are, the longer Mercury will be above the horizon for you before it sets.

In addition to Spica, two other bright stars you can easily spot early in the evening in September are Arcturus and Altair.

Arcturus is the brightest star in the northern sky. It owes this status largely to the fact that it's relatively close by, at about 37 light-years from our solar system.

You can find Arcturus in the west in the first couple of hours after it gets dark.

Look for the Big Dipper and follow its handle over toward the south about the width of an outstretched hand. It's easy to remember with the phrase "arc to Arcturus."

One interesting thing about Arcturus is that, compared to other stars, it's moving extremely fast with respect to our solar system. In fact, the discovery of the star's motion was a huge moment in astronomy. Before that, the positions of stars were thought to be fixed and unchanging. After Edmund Halley's discovery, the understanding that the stars move around as independent objects began to take hold.

After you've found orange-colored Arcturus, spin yourself toward the south-southeast to find Altair. You'll spy it hanging right above Saturn all month – in fact, it's about as high above Saturn as Saturn is from the horizon.

Altair is a bright white-colored star, which makes for a nice color comparison with Arcturus and nearby Antares. At just 17 light-years away, it's definitely one of the closest bright stars to our solar system.

One of the coolest things about Altair is that it rotates so fast that it's flattened into an oval shape. Since it's so close by, astronomers have actually been able to image this fast spinner's flattened shape directly.

So look for Altair and Arcturus in the September sky – two bright, nearby stars that, along with Mercury, each have their own spin on what it means to be fast.

You can catch up on all of NASA's missions to explore the solar system and beyond at nasa.gov. I'm Preston Dyches from NASA's Jet Propulsion Laboratory, and that's What's Up for this month.