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The Sky This Week
U.S. Naval Observatory's Weekly Blog
Thankful for the Cornucopia
by Geoff Chester, USNO Public Affairs
24 November 2020
The Moon, Saturn, & Jupiter, 2020 November 19
The Moon brightens the evening hours this week, waxing to her full phase that will occur on the 30th at 4:30 am Eastern Standard Time. November’s Full Moon is known in folklore as the Frosty Moon or the Beaver Moon. The latter name comes from Native American traditions and describes the activity of these North American creatures prepare for their winter hibernation. Beavers were an important source of food and warmth for indigenous people, and they were some of the most important goods that were traded with early European settlers.
This year’s Beaver Moon features a penumbral lunar eclipse. These eclipses occur when the Moon enters the outer or penumbral shadow of the Earth, and most casual observers probably won’t notice much of a change in the Moon’s appearance. However, careful scrutiny will show a slight grey shading of Luna’s northern limb at the time of greatest eclipse, which will be at 4:44 am EST. If you’re out watching the eclipse, look for the bright star Aldebaran about five degrees south of the Moon. On the evening of the 25th Luna glides a few degrees south of ruddy Mars.
The fourth Thursday in November marks the American holiday of Thanksgiving, which starts the season of mid-winter observances in many traditions. Unlike many of the annual holidays which are geared to the stars in the form of seasonal observances, Thanksgiving has no direct ties to skylore, but it does have an interesting indirect connection to a particular star that’s now becoming prominent in the evenings. Capella, the brightest star in the constellation of Auriga, the Charioteer, can be spotted rising in the northeast during the early evening hours and passes almost directly overhead as midnight welcomes a new day. Capella is the sixth-brightest star in the northern sky has a pleasant golden tint that contrasts nicely with the hues of the other bright stars of the Great Winter Circle. In ancient Rome it was known as the “Goat Star”, a name adopted from earlier Greek mythology. To the Greeks the star represented Amalthea, the she-goat that suckled the infant Zeus. The rambunctious toddler accidentally broke off one of Amalthea’s horns and the broken horn, in turn, was transformed into the Cornucopia, or the “Horn of Plenty”. As an icon the Cornucopia, along with the turkey, has come to symbolize the bounty of the Thanksgiving feast. Perhaps this year, instead of watching football, you should take an evening stroll and find Capella shining brightly through the night.
In addition to its mythology, Capella is a fascinating star to study. It’s actually a stellar system consisting of four stars. The brighter component consists of two evolved giant stars that orbit each other once every 104 days. At a distance of just over 43 light-years they are too close to resolve in optical telescopes, but their duplicity is detectable by other means. The other stars in Capella’s system are a pair of faint red dwarf stars that are moving through space with the same motion as the brighter pair. These two stars orbit each other with a period of around 300 years, and they in turn orbit the brighter stars with a period of a few millennia.
Just to the southwest of Capella is a small triangle of stars known as “The Kids”. Two of these stars are eclipsing binary variable stars whose brightness changes predictably. The northernmost of The Kids is Epsilon Aurigae, whose eclipses last for two years and repeat every 27 years. The other eclipsing star is Zeta Aurigae, which dims for about a month every 2.8 years.
You will find Jupiter in pursuit of Saturn in the southwestern sky shortly after sunset. The two planets are slowly converging on a so-called “great conjunction” which occurs at intervals of about 20 years. This year’s event will be very special, though, as Old Jove will pass just six arcminutes south of Saturn on December 21st. That’s close enough for them to appear in the same low-power field of view in a small telescope. Mark your calendars, since the last time they were this close in the sky was the year 1623!
Ruddy Mars gets a visit from the Moon on the evening of the 25th. The red planet still dominates the later hours of the evening, but he has lost some of his luster. He’s now over a full magnitude fainter than he was at opposition in early October.
Venus continues to greet early risers from a perch in the pre-dawn sky. She is gradually creeping closer to the Sun, though, and as the year winds down she will be cloaked in increasing twilight.
sky this week
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