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Catch the Geminids, Then Count Some Stars

by Geoff Chester, USNO Public Affairs | 13 December 2022

by Geoff Chester, USNO Public Affairs | 13 December 2022

Mars, 2022 December 10, 02:52 UT
Imaged from Alexandria, Virginia

The Moon wanes in the morning sky this week, moving southward along the ecliptic as she tours the rising constellations of the spring sky.  Last Quarter occurs on the 16th at 3:56 am Eastern Standard Time.  Early risers can spot Luna between the bright stars Regulus and Algieba in the constellation of Leo, the Lion, on the morning of the 14th.  On the morning of the 18th she may be found to the northwest of the bright star Spica.

The annual Geminid meteor shower reaches its peak activity on the night of the 13th/14th.  This display is often the best of the year, with one or two “shooting stars” per minute as seen from a dark location from late night through the morning hours.  However, this year the waning gibbous Moon rises at around 10:00 pm local time, so many of the shower’s fainter members will be washed out.  That said, the shower’s radiant near the bright star Castor in Gemini should be rising in the northeast during the evening hours and will be fairly high when the Moon comes up.  If you can escape the city lights for a few hours, you might catch a good view of the brighter shower members.  Bring a lawn chair, lots of blankets, and a good hot beverage to fend off the chill.  The Geminids are the spawn of a very unusual object.  Most meteor showers are linked to periodic comets, but the Geminids trace their origin to an asteroid, (3200) Phaethon.  This object was the first asteroid to be discovered by a satellite, and its perihelion is a mere 20.9 million kilometers (13 million miles) from the Sun.  The intense heat from Old Sol cracks the rocky surface of the asteroid, sputtering particles along its orbit.  When Earth crosses the orbital plane of Phaethon every year, we see these particles slam into our planet’s atmosphere where they burn up, producing the bright streak if a Geminid meteor.

You probably haven’t noticed, but the time of sunset is now gradually moving later in the evening.  At the latitude of Washington, DC sunset will occur five minutes later by Christmas Eve.  That said, the latest sunrise is still a few weeks away.  If you’re not a morning person, sunrise will occur ten minutes later than it does now by the end of the year.

This week marks the final monthly campaign for the Globe at Night citizen-science program.  So far this year over 19,000 observations have been contributed by volunteer observers, so the goal of 20,000 reports by the year’s end looks quite attainable.  This month’s target constellation is Pegasus, which is very prominent in the early evening sky.  Finding Pegasus is quite easy this year.  Simply look above the bright glow of Jupiter for a large square asterism made up of four second-magnitude stars.  Most people should be able to see the square from urban and suburban locations, but as you venture to darker locales more stars begin to fill the square’s center.  From very dark sites in West Virginia I have counted nearly a dozen stars in the square, but your counts will be dictated by the brightness of your sky.  Use the Globe at Night web app to compare your view with sample magnitude charts and report your results.  You will be helping scientists determine the amount of light we’re putting into the night sky.

This week affords a chance to see all of the planets known to the ancients.  In the early evening, as twilight fades, look low in the southwest for the bright glow of Venus hugging the horizon in the fading twilight.  Early in the week she sets at around 5:30 pm, but she gains a couple of minutes on the Sun with each passing evening.  If you have binoculars, look about five degrees east of Venus for the glimmer of Mercury, who will cap off the year with his best evening apparition.

Saturn lies about halfway between Mercury and Jupiter in the evening twilight sky.  The ringed planet quickly settles toward the western horizon and sets by 9:00 pm.  If you want to get a good glimpse of him through the telescope, do it as twilight fades before he sinks into more turbulent air currents.

Jupiter still dominates the early evening sky.  The giant planet is now well past opposition, but his apparent disc still shows more detail than any other planet.  Watch the shadow of Old Jove’s largest moon, Ganymede, move across the disc between 7:41 and 10:18 pm EST on the evening of the 15th.  This event should be easy to observe in small telescopes.  For more of a challenge, the shadow of Io will cross Jupiter’s cloud tops between 8:03 and 10:15 pm EST on the 17th.  Between these events Io itself will exit its transit of Jupiter at 8:56 pm.

Ruddy Mars is still very prominent in the later evening sky, drifting westward among the stars of Tauris, the Bull.  We’re now about a week past opposition, and as Earth pulls ahead of Mars in their chase around the Sun, the planet’s ruddy disc is beginning to shrink.  Mars is always hard to observe, but modest telescopes under steady skies should provide good views for a few more weeks..  


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

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