Sun rises at 6:46am
Sun sets at 5:55pm
Moon phase: fourth quarter, waning
Moon sets at 3:15pm, Saturday
Moon rises at 4:43am, Sunday
This weekend, if the sky is very dark, you may be able to see the celestial river, Eridanus. It begins near the star Rigel in the constellation Orion the Hunter, and wells up in a great loop before ambling back down toward the southern horizon.
Eridanus is one of the longest and faintest constellations. It’s variously said to represent the Nile in Egypt, Euphrates in western Asia, or the River Po in Italy. Eridanus is also sometimes called the River of Orion, or River of Ocean. In Homer’s day in ancient Greece, it was thought that the River of Ocean encircled a flat Earth.
Why search for such a faint constellation? Only because it’s very beautiful – and seeing Eridanus gives us an understanding of the importance of a river in the minds of the early stargazers. With this reflection, you can feel a kinship with those stargazers from centuries ago.
From most of the U.S. and all of Europe, the River Eridanus is not visible; it remains below the southern horizon. But lucky us! We live at a very southerly latitude and so we can see a special sight: the star that represents the end of the River. This star is named Achernar.
Achernar is very bright, and it is located just above the southern horizon at sunset and from there makes a small arc until it sets in the southwest about 9:00pm. The name Achernar derives from an Arabic phrase which means “the end of the river.”
Achernar is about 144 light-years away. It is considered a “normal” star” but it is much hotter and brighter than the sun. In fact, it is nearly 1,100 times as bright, visually, as our sun. Brighter, hotter (and bluer) than the sun, Achernar produces more energy in the non-visible ultraviolet (UV) wavelengths. When you take this into consideration, it pumps out some 3,000 to 5,000 times our sun’s energy level.
Achernar’s mass is six to eight times that of our sun, and its average diameter is nearly eight to 10 times that of the sun. But, while our sun spins on its axis once about every 25 days, Achernar completes one rotation in slightly more than two of our days, which means it basically rotates 15 times faster than our sun. This fast rotation produces an odd, flattened shape, first discovered by the European Southern Observatory’s Very Large Telescope (VLT) in 2003.
If you could look directly at Achernar, it would look a little like a blue M&M, while our sun is more yellow and sphere-shaped. This flattening of Achernar makes an exact surface temperature for this star hard to determine, because the flattening actually causes the star’s poles to be hotter than the equator.
Following behind Achernar is another stunningly bright star, Canopus. These two stars, along with the earth’s brightest star, Sirius, are known as the beacons of the south and have historically been well known navigational stars for those traveling in the Southern Hemisphere. Sirius, also called the dog star, is above Canopus and to the left.
Canopus follows Achernar’s path and sets about 3:10am – in almost the same spot that Achernar touched the horizon. Sirius sets west-southwest at 4:45am.
Thank you for keeping up with the Night Sky articles. If you are out later on in the week, each star rises about four minutes earlier each day than written here, and the moon rises 50 minutes later. Night Sky is researched and compiled by Lisa Davis-Burnett. Earthsky.org is a key resource for information and images. Questions or comments? Email This email address is being protected from spambots. You need JavaScript enabled to view it.
