Skywatching December 2012


The moon appears near Jupiter on December 25, 2012 as it did in
November as seen in this image.

Moon Phases
Last Quarter — December 6
New Moon — December 13
First Quarter — December 19
Full Moon — December 28

The Winter Solstice occurs on December 21, 2012 at 5:12 a.m. CST. Mid-latitudes experience 9 hours of daylight and 15 hours of night on this date.

Evening Sky

Bright Jupiter dominates the evening sky during December.  On December 3, Earth passes between the sun and this giant planet, known as opposition.  Jupiter rises at sunset in the east, appears higher in the eastern sky each hour, reaching the southern sky at midnight, and getting lower toward the western horizon as the early morning hours progress.  Jupiter appears in front of the stars of Taurus, with the bright star Aldebaran nearby.  On December 7, Jupiter appears about 5 degrees from Taurus’ brightest star.

Throughout the month Jupiter appears to be moving eastward (retrograde) as compared to the stars.  The chart above shows Jupiter on December 1, 10, 20, and 30, 2012 as it appears to move backwards.  This retrograde motion is due to the faster moving Earth catching up to and passing Jupiter.  This occurs with all the planets that lie outside Earth’s orbit.  This illusion is most pronounced in the planets closest to Earth (Mars and Jupiter).  When Earth moves between a planet and the sun, we call this opposition, as the sun and planet are opposite in the sky.

At the same time, Asteroid Vesta retrogrades, passing opposition on December 9.  Vesta retrogrades farther during the month, because it is closer to us.  Vesta is just beyond the limit of eyesight and its motion is best observed with binoculars.

Binoculars will permit the best viewing of the Hyades and the Pleiades, two widely spaced star clusters in Taurus.  The Pleiades has composed of bluish white stars while the Hyades’ stars are more yellow and orange.

On December 25, the moon and Jupiter make a bright pair as they did in November as shown in the image at the top of this review.

Mars is the other planet in the evening sky.  It sets shortly after sunset.  On December 15, the moon appears nearby.

Morning Sky

Three planets appear in the eastern morning sky with Jupiter appearing low in the west before sunrise.  Brilliant Venus is in the southeastern sky throughout the month, although it rises earlier each day.  On December 1, it rises 2 hours, 20 minutes before sunrise and diminishes to 1 hour, 30 minutes by month’s end.  Saturn appeared in the eastern just last month after passing behind the sun.  It appears above Venus throughout the month.  Elusive, Mercury makes a brief appearance after it passed between Earth and the sun on November 15.

The chart above shows the three planets and the background stars on December 1 at 6 a.m. CST.

On December 4, Mercury reaches its greatest angular separation from the sun.  The chart shows its position on this date at 6 a.m. CST.

The waning crescent moon moves through the the eastern sky early in the month:

  • December 9, the moon appears near the star Spica.
  • December 10, the moon appears below Saturn.
  • December 11, the moon appears near Venus.

Mercury then moves rapidly back into bright twilight and disappears into the sun’s glare to reappear in the evening sky in 2013.  Venus continues its rapid eastward movement during the month.

On December 23, it passes near Antares, the brightest star in Scorpius.

By month’s end Venus appears lower in the southeastern sky as the sky brightens.

This December morning celestial display is worth watching.

As seen from above, the chart above shows the relative positions of the visible planets on December 15, 2012.  On this diagram, the planets move counter-clockwise.  Notice how Earth is nearly between Jupiter and the sun.  Saturn, Venus, and Mercury are on the same side of the solar system, the earth’s morning sky.  Mars appears in its orbit with no other planets is in its vicinity. (Click this image and all images to see them larger.)

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