Go Stargazing! December Edition

Jupiter dominates this month’s evening skies.  It outshines all stars in the sky, so it’s easy to find.  Face south at dusk and look for the brightest thing there.

Mercury has emerged into the evening sky, and is visible at the beginning of this month.  Look low in the southwest at dusk, right over the point of sunset.  By mid-month, Mercury is again lost in the Sun’s glare; it re-aligns with the sun (is at inferior conjunction) on Dec. 19.

Venus remains a dazzling morning star.  Face southeast at dawn, and you can’t miss it.

Saturn is in the southeast at dawn, above the much brighter Venus.

mars-06-crop
Creative Commons License photo credit: chipdatajeffb

Mars is now lost in the sun’s glare; it will remain invisible to us all winter as Earth passes around the far side of the sun from it.

Look for the enormous Summer Triangle in the night sky, consisting of the stars Deneb, Vega, and Altair, in the west.  These stars were up all night long back in June and July, hence the name. The Great Square of Pegasus, not quite as bright as the Summer Triangle, is high in the south at dusk.  The star in its upper left hand corner is also the head of Andromeda.  Rising after Andromeda is Perseus, the hero that saved her life.  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.  Taurus, the Bull rises in the northeast.  Look for the Pleiades star cluster at the feet of Perseus.  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.

Moon Phases in December 2010:

New Moon                             December 5, 11:36 a.m.

1st Quarter                            December 13, 7:58 a.m.

Full Moon                              December 21, 2:14 a.m.

Last Quarter                         December 27, 10:19 p.m.

Eclipse burning bright
Creative Commons License photo credit: ericskiff

The full moon of early Tuesday, December 21, enters the Earth’s shadow, causing a total lunar eclipse.  This eclipse is visible in its entirety from all of North America, including Houston.  The moon first encounters the Earth’s shadow (umbra) at 12:32 a.m.  This marks the beginning of the partial eclipse.  The moon takes just over an hour, until 1:40 a.m., to enter the shadow.  That is when totality begins.  In this eclipse, the Moon does not quite cross the center of Earth’s shadow but instead passes through the northern part of it.  Even so, the moon takes 74 minutes to cross to the other side of the shadow, so totality lasts from 1:40 to 2:54 a.m.  By 4:02 a.m., the moon has re-emerged from the shadow, and the eclipse is over.  Remember, seeing a lunar eclipse requires no special equipment at all; anyone who sees the moon sees the eclipse.  The only thing that could stop us from seeing this would be a cloudy night on December 20-21, 2010.  The next total lunar eclipse we see here in Houston occurs just after midnight on April 15, 2014.

At 5:42 p.m. on Tuesday, December 21, the sun is overhead at the Tropic of Capricorn, the most southerly latitude where the sun can be overhead.  This is therefore the winter solstice for those of us in the Northern Hemisphere, and the summer solstice for people south of the equator.

At Houston’s latitude, the earliest sunset of the year occurs Thursday, December 2.  Of course, days continue to shorten until the solstice, which makes sunset earlier and sunrise later.  However, Earth is also accelerating as it approaches perihelion (closest approach to the sun) in early January.  This causes sunrise, local noon, and sunset to occur slightly later each day.  This close to the solstice, the second effect actually predominates, so sunset gets a little later during December even while the days are getting shorter.  As you head out to ring in the New Year, notice that sunset on New Year’s Eve is about 10 minutes later than it is now.

2009 Leonid Meteor (cropped, afterglow closeup)
Creative Commons License photo credit: Navicore

The Geminid meteor shower peaks every year in mid-December, this year on the 14.  This shower and the Perseids in August are the two most reliable showers of the year, producing about 1 or two meteors per minute.  The Geminids are not as popular, though, because of colder nights (yes, sometimes even in Houston) and a greater chance of cloudy skies.  Still, it’s worth a look if the skies are clear.  Unlike most meteor showers which are comet debris, the Geminids originate from an asteroid (3200 Phaethon.  The shallower angle between this debris path and Earth’s orbit means that Earth rotates us towards the debris field before midnight.  We can thus observe meteors from late evening all the way until dawn.  Meteors will seem to radiate from the constellation Gemini, hence the shower’s name.

Go Stargazing! January Edition

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?
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Go Stargazing! November Edition

mars-06-crop
Creative Commons License photo credit: chipdatajeffb

The “main event” of November evenings occurs in the southwest at dusk, where you can watch Venus close in on Jupiter.  Look towards the southwest right as night falls for the two brightest objects in the sky, other than the Moon.  The brighter one low in the southwest is Venus, which outshines everything in the sky except the Sun and the Moon.  Jupiter is closer to due south at dusk and is the dimmer of the two, although it still outshines all the stars we see at night. 

Venus and Jupiter begin the month just under 30 degrees apart (your fist at arms length blocks about 10 degrees).  However, Venus will close that gap quite noticeably each night, until it appears directly under Jupiter on November 30.  The two planets will then be about 2 degrees apart.  When two or more planets are roughly in the same line of sight, astronomers say they are in conjunction. Saturnis still visible at this time of year, it resides high in the east and can be seen around dawn.  Mars is lost in the Sun’s glare this month, and will remain out of sight into 2009. 

Look for the enormous Summer Triangle, consisting of the stars Deneb, Vega and Altair, high in the west.  The Great Square of Pegasus is high in the east at dusk.  The star in its upper left hand corner is also the head of Andromeda.  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, while the Dipper is low and out of sight, Cassiopeia rides high.

Our Milky Way Galaxy..
Creative Commons License photo credit:
Sir Mervs (byaheng bicol)

You’ll notice that November skies at dusk, especially to the south and east, contain many fewer bright stars than skies of summer or winter.  This is because we are facing out of the galaxy plane when we look in that direction.  The Summer Triangle and Cassiopeia are in the galaxy plane, where most bright stars are.  Looking away from that, we see a large are of dim stars known to the ancients as the ‘Celestial Sea’.  By late evening (10 pm now, 8 pm by the 30th), dazzling Orion rises in the east, reminding us that winter is on the way.


Moon Phases in November 2008:

1st Quarter          November 5, 10:03 am
Full                      November 13, 12:18 pm
Last Quarter         November 19, 3:32 am
New                     November 27, 10:55 pm

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