Seeing Stars with James Wooten: Comet ISON, Winter Solstice & the Geminid meteor shower

December 4, 2013

ev.owaThis is it. The final stargazing report of 2013. So let’s get to it, shall we?

Venus remains in the west at dusk for one more month. It outshines everything but the Sun and Moon, so you can begin observing it during deep twilight. Shortly after the new year begins, Venus shifts from the evening to the morning sky.

Jupiter will be up literally all night long early next month. In December 2013, then, it is not up at dusk but rises during the evening. Now you can see it rise in the northeast at about 8 p.m., just as Venus sets. By New Year’s Eve, Jupiter rises by 5:50 p.m., during twilight.

Mars remains in the morning sky. It continues to brighten a bit in the south at dawn.

Saturn has reappeared in the pre-dawn sky. Face southeast right before sunup to see it.

In December, the Big Dipper is below the horizon at dusk. As the Big Dipper sets, though, Cassiopeia rises. This is a pattern of five stars in a distinct W shape which lies directly across the North Star from the Big Dipper. Look for Cassiopeia high in the north on fall and winter evenings.

The Summer Triangle sets in the west. Watch for the Great Square of Pegasus almost overhead at dusk now and in the west by Christmas. Taurus, the bull, rises in the east. Look for the Pleiades star cluster above reddish Aldebaran. Dazzling Orion, the hunter, rises shortly after dusk (by month’s end, it is already up at dusk). As Orion enters the evening sky, we transition from the relatively dim evening skies of autumn to the brilliant stars of winter.

Unfortunately, it appears that Comet ISON did not survive its close passage to the Sun this past Thanksgiving. At 12:48 p.m. CST on Thurs., Nov. 28, ISON passed just 1.7 solar radii above the Sun’s surface. This proved to be too close, as the Sun’s gravity tore ISON apart, causing it to shed much of its gas and dust. This left only a small remaining fragment to continue on ISON’s orbital path, a fragment too small to put on a naked-eye show on December mornings. Binocular observers, though, can still give it a try.

You can still find information showing ISON’s position, or you can view the full path.

If ISON survives perihelion this Thanksgiving (it has about a 50/50 chance), we could see it quite well between Thanksgiving and Christmas. More on this in the December update.

Moon Phases in December 2013:

New: December 2, 6:21 pm
1st Quarter: December 9, 9:12 am
Full: December 17, 3:28 am
Last Quarter: December 25, 7:49 am

At 11:11 a.m. on Sat., Dec. 21, the Sun is directly overhead at the Tropic of Capricorn, the farthest point south where this is possible. This puts the Sun as low as possible in our sky, and marks the winter solstice. In the Southern Hemisphere, the Sun is as high as possible in the sky — this is the summer solstice for them.

Although the winter solstice is the shortest day, the earliest sunset occurred on about Dec. 2, and the latest sunrise will occur Jan. 10. That’s because the Earth speeds up on its orbit as it approaches perihelion (closest approach to the Sun) next month. This acceleration shifts sunrise, local noon, and sunset slightly later each day this month and next. The effect is smaller than that of the Sun taking a lower path across the sky, which normally dominates in causing earlier sunsets and later sunrises. But the Sun’s apparent path varies very little near the solstice itself, allowing the secondary effect of the Earth approaching the Sun to predominate. For most people, then, (those who witness sunset but sleep through sunrise), days will seem to lengthen throughout December, although they don’t really begin lengthening until Dec. 21.

The Geminid meteor shower peaks this month, as it does every December. Along with the Perseids in August, the Geminids are one of the two most reliable meteor showers, producing on average about 100 meteors per hour. The Geminids are unique among meteor showers because they are associated not with a comet but with an asteroid, 3200 Phaethon. This means that with Geminids, we see significant activity much earlier in the night than with other showers.

Most meteor showers peak in the hours immediately before dawn. This is because what plows through the debris field is the leading edge of the Earth, and that’s the side going from night into day. Since Phaethon is an asteroid, however, debris along its orbital path forms a shallower angle to Earth’s orbital path, meaning that we begin to face into the debris field as early as 9 or 10 p.m. Meteors will seem to “radiate” from the constellation Gemini, hence the name of the shower. However, they may appear anywhere in the sky.

As always, you see more meteors the farther you are from big city lights, which hide dimmer ones. Our George Observatory will be open at 5 p.m. on Friday night, Dec. 13, to 1 a.m. on Saturday morning, Dec. 14, for observing this meteor shower.

On most clear Saturday nights at the George Observatory, you can hear me do live star tours on the observation deck with a green laser pointer. If you’re there, listen for my announcement.

Authored By James Wooten

James is the Planetarium Astronomer at the Houston Museum of Natural Science. He teaches students every school morning in the planetarium, and also answers astronomy questions from the public.

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