Go Stargazing! January Edition

January 6, 2009

Mercury and Jupiter begin this month together low in the southwest at dusk.  The two were side by side on New Year’s Eve; now Mercury is slightly higher in the sky than Jupiter.  Mercury is at greatest elongation (apparent distance from the Sun in our sky), and therefore highest above the southwest horizon, on January 4.  After that, is seems to double back towards the Sun and starts becoming harder to see.  Meanwhile, Jupiter just gets slightly lower each evening until it also drops into the Sun’s glare.  How deep into January can you follow them?

The departure of Mercury and Jupiter leaves Venus as the planet of January evenings.  Look west-southwest right as night falls for the brightest thing there except for the Moon, which is nearby at the beginning of the month. 

Two factors make Venus much higher in the sky now than in December or November.  First, Venus is at greatest elongation on January 14, just as Mercury is on the 4th.  Secondly, the plane of our solar system in our sky, called the ecliptic, intersects our horizon at a steeper and steeper angle each night as we go from the winter solstice to the spring equinox.  More and more of Venus’ apparent distance from the Sun is also height in the sky.  Also, Venus, on its faster orbit, is coming around to our side of the Sun (and will pass us in March).  Therefore Venus, which outshines everything in the sky except the Sun and the Moon, is getting even brighter this month as it approaches us. 

Saturn is now high in the southwest at dawn.  It will be rising in the east in late evening by month’s end.  Mars remains lost in the Sun’s glare this month.

12 segundos de oscuridad
Creative Commons License photo credit: Libertinus

Facing north, you’ll see five stars in a distinct ‘M’ like shape—this is Cassiopeia, the Queen.  Her stars are about as bright as those in the Big Dipper, and she is directly across the North Star from that Dipper.  In fall and early winter, while the Dipper is low and out of sight, Cassiopeia rides high.

Dazzling Orion rises in the east, reminding us that winter is on the way.  His belt points up to Aldebaran, the eye of Taurus, the Bull.  The Dog Stars Sirius and Procyon are below Orion in the east.  Sirius is the brightest star we ever see at night.  To Orion’s left as he rises are two stars of similar brightness less than five degrees apart.  These are Castor and Pollux, marking the heads of Gemini, the Twins.

Moon Phases in January 2009:

1st Quarter        January 4, 5:55 am
Full Moon          January 10, 9:27 pm
Last Quarter      January 17, 8:46 pm
New Moon         January 26, 1:55 am

Eclipse solaire
Creative Commons License photo credit: luc.viatour

The New Moon of January 26 blocks the Sun and thus causes an eclipse of the Sun.  The eclipse happens when it’s nighttime here, though; only those around the Indian Ocean see a partial eclipse.  What’s more, the Moon is near apogee (farthest distance from Earth) and appears slightly smaller in the sky.  Therefore, it can’t block the Sun completely, and people directly in the eclipse path see a small ring of the Sun around the Moon at maximum eclipse.  This type of partial eclipse is an annular eclipse.  The path of annularity is over the southern Indian Ocean; it does not touch land until it reaches Indonesia.

That same New Moon is also the second New Moon following the winter solstice.  Accordingly, it marks the Chinese New Year.  The Year of the Rat becomes the Year of the Ox on this date. 

Earth makes its closest approach to the Sun, called perihelion, at about 6pm on Saturday, January 3.  The Earth is about 98% of its average distance from the Sun (about 93 million miles).  Aphelion is on July 3, when Earth will be at 101.6% of its average distance from the Sun.  This is not enough of a distance to affect our seasons. 

Rangitoto @ Dawn
Creative Commons License photo credit: Chris Gin

The latest sunrise of the year occurs on the morning of January 10.  We are still close enough to the winter solstice that the Sun’s apparent path across the sky on January 10 is only slightly higher than on December 21.  Meanwhile, Earth has just passed perihelion a week earlier.  As a result, the Earth is moving a little faster than usual. 

The effect isn’t much (Earth’s orbit is nearly circular), but it’s enough to make both sunrise and sunset a little later each day this month and next.  With the Sun’s apparent height in the sky not changing that much until late January, the small effect of Earth’s acceleration near perihelion dominates.  Since most of us sleep through sunrise and witness sunset, the days seem be slightly lengthening much more than the actually are in early January.

Want to Learn More About Astronomy?
Read about the Big Bang and the timeline of the universe.
Learn how the days of the week got their names.
Discover the origin of Halloween.

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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