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The Hunter and His Hound

by Geoff Chester, USNO Public Affairs | 12 December 2023

by Geoff Chester, USNO Public Affairs | 12 December 2023

Orion and Canis Major, imaged 2016 January 3 from Mollusk, Virginia
with a Canon EOS Rebel T2i DSLR

The Moon returns to the evening sky this week, waxing to her First Quarter phase, which will occur on the 19th at 1:39 pm Eastern Standard Time.  As Luna climbs through the dim autumnal constellations, she passes just over two degrees south of Saturn on the evening of the 17th.

For those who are keeping track of the time of sunset here in the Washington area, Old Sol gradually begins to set later beginning on the evening of the 13th, when he drops below the horizon at 4:47 pm EST.  By Christmas Eve sunset will be five minutes later than its earliest occurrence, but the time of latest sunrise is still a few weeks from now.  The shortest day of the year will still occur on the winter solstice, which will occur on the 21st.

The annual Geminids meteor shower peaks on the night of the 13th-14th.  These meteors are associated with an asteroid, (3200) Phaethon, which is likely the dusty remnant of an extinct comet nucleus.  It is the year’s most consistently prolific shower, and a single observer at a dark site may see upwards of 60 “shooting stars” per hour before midnight, with up to 150 per hour between 2:00 and 3:00 am local time.  This year conditions are close to ideal, with no Moon to brighten the sky.  The radiant point lies near the stars Castor and Pollux in Gemini, which are high in the eastern sky by 10:00 pm.  Geminids are usually quite bright with a yellowish tint, and their relatively slow speeds allow them to persist for a few seconds.

Observing the Geminids is quite simple.  Find a dark location, lie back in a lawn chair, and look up.  Dress for temperatures some 20 20 30 degrees colder than forecast, and bring a hot beverage to enjoy.  Unlike August’s Perseid meteor shower, you shouldn’t need mosquito repellant for the Geminids!

While you’re enjoying the Geminids, take the time to appreciate the colorful stars that accompany Gemini as the Twins march across the sky.  Castor and Pollux form a part of a large circle of bright stars that surround the constellation of Orion, the Hunter.  Popularly known as the Great Winter Circle or the Winter Hexagon, the 25 brightest stars in the sky fall on or within the figure’s boundary.  Orion is the central figure here, sporting the bright stars Betelgeuse and Rigel.  Note the strong color contrast between these luminaries.  Betelgeuse has a distinct reddish tint, while Rigel blazes with an icy blue glow.  Between these two stars is Orion’s distinctive belt, whose three members are some of the most luminous stars in the galaxy.  The “Belt Stars” serve as a convenient signpost to other bright stars in the circle.  A southeast-pointing line through the Belt points to the brightest star in the sky, Sirius, in the constellation of Canis Major, the Greater Dog.

I’ve always liked to compare Rigel and Sirius on a cold, dark night.  Both stars gleam like luminous sapphires against the velvet blackness of space.  At first glance they appear similar, although Sirius is noticeably brighter.  Apart from their colors, though, they are as different as they could be.  Sirius is similar to our Sun but is twice as massive.  It fuses its hydrogen fuel at a hotter temperature, and so emits about 25 times the light of the Sun.  It is also one of the closest stars to the Sun, just 8.5 light years away.  Rigel is a massive giant star, some 20 times the mass of the Sun, and its fusion core is running at a furious pace.  It pumps out the luminosity of well over 120,000 Suns from a distance that is some 100 times more remote than Sirius.  If we could have Rigel and Sirius exchange places, from our vantage point Sirius would a star of 9th magnitude and require a telescope to be seen at all; Rigel would be nearly as bright as the full Moon! 

Saturn continues to recede in the southwestern sky.  The ringed planet is now well past the meridian as evening twilight fades, but there’s still time to enjoy a telescopic view of him in a dark sky.  Don’t wait too long, though, since he sets at around 10:00 pm by the week’s end. 

Jupiter appears high in the east soon after sunset, and dominates the evening sky thereafter.  The giant planet outshines everything else in the night sky except the Moon and Venus, so he’s very hard to miss.  He is a treat for almost any kind of optical aid.  A steadily-held pair of binoculars will show his four bright Galilean moons, and small telescopes will reveal their rigid obedience to their massive master.  The motions of the moons are so regular that they can be used as a time reference, and it was diligent observing of the moons that led the 17th Century astronomer Ole Rømer to deduce that light moved at a finite speed.

Venus continues her southward journey along the ecliptic, passing from the constellation of Virgo into Libra.  The latter is the only inanimate constellation of the Zodiac, whose stars once “belonged” to Scorpius.  On the morning of the 18th Venus passes between Libra’s two brightest stars, Zubenelgenubi and Zubeneschamali.


Commander, Naval Meteorology and Oceanography Command | 1100 Balch Blvd. | Stennis Space Center, Mississippi 39529

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