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The Sky This Week
U.S. Naval Observatory's Weekly Blog
Catch a Falling Star, and Look For One That Winks At You
by Geoff Chester, USNO Public Affairs
08 December 2020
The Milky Way in Perseus and Cassiopeia, imaged 2019 September 28
from Blackwater Falls State Park, Davis, West Virginia
captured with a Canon EOS Rebel SL2 DSLR
The Moon wanes in the morning skies this week, moving southward along the ecliptic as she passes through spring’s rising constellations. New Moon occurs on the 14th at 11:17 am Eastern Standard Time. Those who happen to be located in southern Chile and Argentina will be treated to a total solar eclipse at that time. For 2 minutes and 10 seconds the Sun’s face will be completely obscured by the Moon in one of Nature’s most spectacular sights. Can’t get to Patagonia by next week? Not a major problem. You’ll only need to wait until April 8th, 2024 for a 4.5-minute eclipse that crosses the U.S. from Texas to the Great Lakes and northern New England.
Another of Nature’s spectacles is gearing up for a splendid viewing opportunity over the weekend. The annual Geminid meteor shower will peak on the night of December 13-14, and there will be no moonlight to interfere with the view. This is one of the most consistent showers from year to year, with up to 100 meteors visible per hour from dark locations. The shower members appear to radiate from a point in the sky near the bright star Castor in the constellation of Gemini, and unlike many meteor showers the radiant is well above the horizon in the evening hours. By 10:00 pm local time the radiant is about 40 degrees above the horizon, and it is almost directly overhead at 2:00 am. Geminids are characterized by relatively slow, bright meteors that can be vividly colored. Unlike summer’s Perseids, these slower meteors don’t tend to leave persistent smoke trains behind them. While almost all of the year’s periodic showers are produced by the passage of comets through the inner solar system, the Geminids are the spawn of an asteroid, (3200) Phaethon. Discovered in 1983, this was the first asteroid to be found by a space-based instrument, and it has the distinction of having the closest perihelion distance of any named asteroid. It is now thought to be a defunct comet nucleus, sputtering off material as it orbits the Sun every 1.43 years.
This is the final week in 2020 to participate in the
Globe at Night
citizen-science sky awareness program. From now until December 15th you are encouraged to go outside, look up, and count stars. This month’s featured constellation is Perseus, the Hero, which may be found high in the northeastern sky in the mid-evening hours. Perseus lies just west of a line that connects the bright yellow-hued star Capella with the W-shaped asterism formed by Cassiopeia. Look for a grouping of stars that resemble a wish-bone with the bright star Mirfak at the intersection of the wish-bone’s tines. To make an observation, simply visit the
web app on the Globe at Night website
and compare your view of Perseus with the charts on the site. The site’s sponsors are hoping to collect a total of 30,000 observer reports for the year.
The second-brightest star in Perseus is usually Algol, which is normally a second-magnitude object. However, every 2.8 days the star fades to magnitude 3.4. It is the prototype of a class of variable stars known as eclipsing binaries in which a brighter star is periodically eclipsed by a darker companion. Its variability was first documented by Medieval Persian astronomers who named it “Al Ra’s al Ghul”, the Demon’s Head. The demon it represented was the Gorgon Medusa in Greek mythology, a creature who had snakes for hair and the ability to turn people to stone if they looked into her eyes. The clever Perseus was able to kill Medusa by looking at her reflection in his polished bronze shield before lopping off her head, which the Hero subsequently carried around with him as a grisly trophy. This week Algol reaches one of its minima on the evening of the 8th at 9:38 pm EST. Five hours later it will have regained its regular brightness.
Bright Jupiter continues to inch up on Saturn during evening twilight into the first hour of darkness. You can easily spot Jupiter in the southwest shortly after sunset, and Saturn should appear shortly thereafter. Jupiter will pass Saturn in two weeks in what is often called a “Great Conjunction” since it only occurs every 19.76 years. This particular one will be truly memorable as the two objects will appear extremely close together on the evening of the 21st. The last time they appeared this close together and were easily visible was the year 1226 CE!
Mars continues to shine through the night, but his brightness is gradually fading. Fortunately he is located in a part of the sky that is bereft of bright stars, so he stands out despite his waning glory. You can also identify him by his strong ruddy tint, unmatched by any other object in the sky. If you have been following him through the telescope since his close opposition two months ago you will see that his disc is now almost half as big as it appeared at that time.
Bright Venus can be spotted in the gathering morning twilight as she moves through the stars of the constellation of Libra. She gets a visit from the waning crescent Moon on the mornings of the 12th and 13th.
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