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.

How Far are the Stars? (part 1)

Under the Milky Way
Creative Commons License photo credit: jurvetson

During a recent planetarium show, I discussed the stars of the Summer Triangle (up all night long in summer, they are still high in the west at dusk in autumn). I mentioned that Deneb, apparently dimmer than the Triangle’s other two stars Vega and Altair, is actually much larger and gives off much more light. It just seems dimmer because it’s about 100 times farther away. This prompted the question, “How can we tell how far away the stars are?”

This is a very perceptive question.  We obviously cannot directly measure the distance to a star like you might measure the height of a wall.  Neither can we use an odometer like you have in your car, since no one has been to the stars.  By doing a little trigonometry, however, we can get reliable distances to the stars nearest to us.


This animation is an example of
parallax – as the viewpoint moves
side to side, the objects closer to
the camera appear to move faster,
while the objects in the distance
appear to move slower.
Image by natejunk2004
Can’t see the Image? Click here.

This geometric way to measure distance is called parallax and you can illustrate it for yourself quite simply.  Hold your finger in front of your face.  Now close your left eye, leaving the right eye open.  Then close the right eye and open the left.  Repeat this sort of blinking several times and watch how you finger moves back and forth compared to things in the background.  Bring your finger close to your face, and repeat the experiment.  Now hold your finger at arm’s length, and repeat.  Notice how your finger seems to move farther when it is close to your face.  In fact, if you could measure how far your finger moves against the background objects, you could calculate how far it is from your face.

We can do the same thing with nearby stars. If we observe a star at a particular time of year (for example, in January) and then again six months later (in this case, in July), we can define an isosceles triangle where the base is the diameter of Earth’s orbit and the sides are the distance to the star.  The vertex angle of this triangle equals the apparent change in the star’s position due to the Earth’s yearly motion.  One half of this isosceles triangle is a right triangle where one leg is the known Earth-Sun distance (one AU, or astronomical unit), and the hypotenuse is the distance to the star. The angle opposite the one AU leg, which is one half the star’s apparent motion, is the parallax angle p.  Basic trigonometry then yields

sin p = 1 AU/ d,

where d is the distance to the star in question.  Since p is tiny for all stars, the small angle approximation sin p =p is valid.  We can define a standard distance by asking how far away a star would be if it had a parallax of one arcsecond (1/3600 degree).  Plugging d= 1 arcsecond into the equation gives us

d= 206265 AU,

where 206265 represents the conversion factor between radians and arcseconds, given that the approximation sin p =p holds only if the angle is in radians.  We have now defined the parsec, the distance at which a star has a parallax angle of one arcsecond.  It now becomes easy to determine stellar distances compared to this standard distance.  First, measure the parallax of a star in arcseconds.  Then take one over that value, and you have the distance to that star in parsecs.  By the way, although the general public prefers to think of distances to stars in light years, modern astronomers never quote them that way.  The parsec, directly related to a measurable quantity, is a much more preferable unit.  (One parsec is about 3.26 light years.)

This way of measuring distance has a limitation: most stars are too far away to have measurable parallaxes.  An imaginary sphere with a radius of one parsec centered on our Sun would contain precisely one star–the Sun.  The nearest star system to ours, that of Alpha Centauri, is 1.34 parsecs away, and therefore has a parallax of only about 0.75 arcseconds.  More distant stars have much smaller parallaxes, too small for most Earth based equipment to detect.

This began to change in 1989, however, when the European Space Agency (ESA) launched the High Precision Parallax Collecting Satellite, or Hipparcos.  The name was chosen in honor of the ancient Greek astronomer Hipparchus, who put together the first star catalog of the western world.  The first space experiment devoted to astrometry, Hipparcos catalogued 118,218 stars between 1989 and 1993.  The Hipparcos Catalogue was published in 1997.  Among its many scientific results, Hipparcos helped astronomers determine accurate proper motions (a star’s true motion through the galaxy) and was able to measure good parallaxes for stars up to about 1,000 parsecs away.

But, you may wonder, “What about stars more than a few thousand parsecs away from us? “  Keep in mind that our Galaxy is about 100,000 light years, or just over 30,000 parcsecs across.  Most stars, to say nothing of distant galaxies, are so distant that not even Hipparcos can measure their infinitesimal parallaxes.  Fortunately, there are objects known as “standard candles”–celestial objects with a known intrinsic brightness.  Comparing their intrinsic brightness with their apparent brightness in our skies lets us figure out the distances to them.  In a later post, I’ll discuss how we identify and use “standard candles” to determine distances to much more distant stars and even to other galaxies.

Go Stargazing! November Edition

The king of planets, Jupiter, which dominted the evening skies of September and October, is still well placed for obeserving in the evening during this month. It outshines all stars in the sky, so it’s easy to find. Face southeast at dusk.

Hubble Images Suggest Rogue Asteroid Smacked Jupiter
Creative Commons License photo credit:
NASA Goddard Photo and Video

Mars remains very low in the southwest at dusk; it is of only average brightness and hard to recognize.  It will be even harder to see in the months to come, as Earth passes around the far side of the sun from it.

Venus passed between the Earth and the sun on Oct. 29, an alignment known as inferior conjunction.  In other words, Venus has just ‘lapped’ us on its faster, inner orbit.  As a result, in November 2010 we see Venus emerge quickly into the morning sky.  Face southeast at dawn, especially beginning around mid-month, and you can’t miss it.  If you are a consistent early riser, you can do an experiment.  Observe the southeast horizon before dawn (5:30-5:40 a.m. once we fall back) and see for yourself when you can first see Venus.

Saturn is in the southeast at dawn, above the much brighter Venus.  Look for the ringed planet low in the east at dawn.

Look for the enormous Summer Triangle, 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 east 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.  In fall, while the Dipper is low, Cassiopeia rides high. The vast stretch of sky under Pegasus is largely devoid of bright stars—ancients called this the ‘Celestial Sea.”  The only first magnitude star in the entire region is Fomalhaut, in the Southern Fish.  Jupiter’s stark brilliance is even more remarkable against this dim backdrop.  Taurus, the Bull rises in the northeast.  Look for the Pleiades star cluster at the feet of Perseus, low in the northeast just after dusk.

Moon Phases in November 2010:

New Moon                      November 5, 11:51 p.m.

1st Quarter                     November 13, 10:39 p.m.

Full Moon                       November 21, 11:27 a.m.

Last Quarter                  November 28, 2:36 a.m.

Sunday, Nov. 7 is the first Sunday of the month.  Accordingly, we fall back from Daylight Saving Time to Standard Time on that date.  Don’t forget to set all your clocks back on Saturday, Nov. 6 before going to bed.  Enjoy your extra hour of sleep!


*Time* Ticking away...
Creative Commons License photo credit: Michel Filion

Go Stargazing! October Edition

Venus passes between the Earth and sun on October 29, an alignment known as inferior conjunction. ‘Superior’ conjunction occurs when Venus passes around the far side of the sun. As a result, in October 2010 we see Venus stop its apparent forward motion and shift back towards the sun—it will soon leave our evening skies.  For now, you can look for Venus low in the southwest at dusk. After next week, however, Venus sets during twilight.

Mars is above Venus (and much, much dimmer) as October opens; it remains low in the southwest at dusk after Venus is gone.

Saturn aligned with the sun on September 30 (i.e., it was at conjunction), so we haven’t gotten a good look at it in a while.  Near the end of this month, though, you can begin to look for the ringed planet low in the east at dawn.

Jupiter dominates this month’s evening skies.  Up literally all night long late last month, the king of planets is now well placed for observing in convenient evening hours.  It outshines all stars in the sky, so it’s easy to find.  Face southeast at dusk, and you can’t miss it.

The Big Dipper happens to be to the lower left of the North Star at dusk this month; you’ll need a clear northern horizon to get a good look at it.  Sagittarius, the Archer, known for its ‘teapot’ asterism, is in the southwest.  Look for the enormous Summer Triangle, consisting of the stars Deneb, Vega, and Altair, high in the west.   As familiar summer patterns shift to the west, the constellations of autumn take center stage.  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, Cassiopeia rides high. The vast stretch of sky under Pegasus is largely devoid of bright stars—ancients called this the ‘Celestial Sea.”  The only first magnitude star in the entire region is Fomalhaut, in the Southern Fish.  Jupiter’s stark brilliance is even more remarkable against this dim backdrop.

Comet  Hyakutake

A comet may become visible to the naked eye later this month.  If you recall comets Hyakutake and Hale-Bopp from the ‘90s, this one won’t be quite that bright, but it should be visible from dark sites when no moon is out, and definitely visible in binoculars.  It is Comet Hartley 2, and it makes its closet approach to Earth, at just 0.12 AU, on October 20.  On that date it will appear near the star Capella in Auriga. Therefore, it will rise in the northeast at dusk on that evening and be visible all night long for us.

Moon Phases in October 2010:

New Moon                       October 7, 1:44 p.m.

1st Quarter                     October 14, 4:25 p.m.

Full Moon                        October 22, 8:37 p.m.

Last Quarter                  October 30, 7:46 a.m.

Saturday, October 16, is our annual Astronomy Day at the George Observatory. Come join us anytime from 3 to 10 p.m.  On Astronomy Day, it is free to look through even the main domes at George.  Before dusk, we will have solar observing, Challenger Center simulations, outdoor and indoor presentations (beginning at 4) and many other activities!  Surf to www.astronomyday.info for more information.