The Celestial Sea

As you look up into a November sky right at nightfall, you may notice fewer bright stars than at other times of year. No, it’s not just the glare from Houston hiding most of the stars from view–there really are fewer bright stars in the November evening sky than in, say, February or August. To understand why, you need to understand the shape of our galaxy itself.

death cab for cutie:I'll follow you into the dark
Creative Commons License photo credit: visualpanic

Our galaxy, the Milky Way, is a barred spiral galaxy.

Evidence indicates that the Milky Way, like many large galaxies, has a massive black hole at its center. A radio source designated Sagittarius A* could be the black hole itself. (The asterisk is part of the name, which is “Sagittarius-A-star”). Surrounding this black hole is a central bulge where older (and thus redder) stars predominate.  The Bulge of our galaxy is not fully spherical but instead forms a bar a few thousand light years long.  Branching out from this bulge are spiral arms which contain younger (bluer) stars and dust clouds out of which brand new stars form.  Our solar system is about 26,000 light-years from the center to the edge, on the inside edge of the Orion Arm.  The Orion Arm, in turn, is but a spur of the much longer Perseus Arm.  The Milky Way is quite flat–over 100,000 light years wide but only 1,000 light years thick.

The flatness of the galaxy means that most of its stars are near a certain plane in space.  Of course, the galaxy is much thicker than our solar system, so we see our stellar neighbors suurounding us on all sides.  The rest of the galaxy, extending off into the distance, appears to us as a hazy blur in the background, with individual stars (those fairly close to us) in the foreground.  That hazy blur looked like spilled milk to the ancient Greeks, thus the name ‘Milky Way.’  We see more stars near that plane than far from it.

What does this have to do with the dimness of a late November sky at dusk?

Imagine observing our flat galaxy from our vantage point on Earth. When we face into the galactic plane, we see more bright stars, because there are more stars in that direction.  When we face above or below that plane, we see fewer bright stars.

Face west at dusk in late November and early December, and you’ll notice an enormous triangle of three bright stars, all bright enough to appear even in skies lit by Houston.  These stars from the Summer Triangle, so called because it is up all night long from June through mid-August.  This Triangle is also directly in the plane of the Milky Way.  The constellation Sagittarius, which marks the center of the Galaxy, sets just after the Sun this time of year.  Therefore, if you trace a path approximately from the  point of sunset through the Summer Triangle, over to five stars in an ‘m’ shape in the North (that would be Cassiopeia, the Queen), and then over to the northeastern horizon.  This is the plane of the Milky Way across late autumn skies at dusk .

Turn to the south, and you face below the galactic plane (as we’ve arbitrarily defined ‘above’ and ‘below’).  Here is a vast region of sky almost void of bright stars.  One exception is Fomalhaut, low in the southeast at dusk tonight.  Also, Houstonians with a very clear southern horizon can see Achernar very, very low in the south on December evenings.  But that’s about it.  There are many fewer bright stars in this direction than towards the Summer Triangle.  By the way, the brilliant object in the east at dusk tonight, and high in the southeast as dusk in December, is Jupiter. It doesn’t count as a bright star for this sector of the sky.

The Celestial Sea

When ancient Mesopotamians looked up into the dim skies you see at dusk tonight, they imagined the Persian Gulf south of them extended up into the sky, forming a ‘Celestial Sea’.  They therefore placed many water-themed constellations in this part of the sky.  Zodiacal constellations here include Pisces, the Fish, and Aquarius, the Water-Carrier.  Even Capricornus, the Goat, has the tail of a fish because he originally represented Ea, the ancient Babylonian god of the waters.  Under Pisces is the sea monster Cetus, while Piscis Austrinus, the Southern Fish, drinks the water that Aquarius pours.  Eridanus, the River, rises in the southeast, flowing from Orion’s foot into this vast ‘sea.’

predawn
Creative Commons License photo credit: paul (dex)

Contrast this vast, dim region with the much brighter swath of stars that rises in the east later this evening (9-10 pm in late November, earlier in December).  This region of sky includes the brilliant pattern Orion, the Hunter, as well as Sirius, the brightest star we ever see at night.    When these stars rise, we are beginning to face back into the plane of our galaxy, this time looking into our own arm of galaxy at the stars right ‘behind’ the Sun.  (This is why our arm of the Milky Way is called the Orion Arm.)  Winter evening skies are much brighter than those of late autumn.

Go Stargazing! September Edition

Venus and Mars have left Saturn behind in the night sky (check out my earlier blog on the position of the planets). You can spot the star Spica in between Mars and Venus during this time of year. (Spica is similar to Mars in brightness and closer to Venus than to Mars). 

 Cloud structure in The Venusian atmosphere,
revealed by ultraviolet observations

September is the last full month to observe Venus at dusk. That’s because Venus has by now come around to Earth’s side of the sun on its faster, inner orbit.  Thus, Venus now begins to overtake the Earth, passing between the Earth and sun on October 29.  We’ll therefore see Venus shift farther to the left of Mars and then drop down below it.  In October, Venus exits the evening sky quite quickly as it shifts back towards the sun.  September and October 2010 is an excellent period for observing Venus’ crescent phase in telescopes.  Anytime Venus is on our side of the sun, more of its night side faces us, resulting in a crescent like appearance when magnified.

Saturn is far to the lower right of Venus and Mars as you face west at dusk.  You’ll need a horizon clear of tall buildings and trees to see it before it sets.  You’ll also need to look early in the month, as Saturn is practically behind the sun by month’s end.  

Jupiter dominates this month’s skies.  On Tuesday morning, September 21, Earth aligns with the sun and Jupiter, bringing Jupiter to opposition (because the sun and Jupiter are then on opposite sides of the Earth).  On the night of September 20-21 we see Jupiter rise at sundown and set at sunup—Jupiter is up literally all night long.  During the whole month, though, Jupiter is visible virtually the whole night.  It outshines all stars in the sky, so it’s easy to find.  Face east in late evening or south southwest at dawn to see it.  The planet Uranus is less than one degree above Jupiter this month; the two planets are closest on September 18.

The Big Dipper is setting in the northwest at dusk; you now need a horizon clear of trees and tall buildings to get a good look at it. You can extend the curve of its handle to ‘arc to Arcturus’, which is in the west at dusk tonight.  Arcturus, by the way, is the fourth brightest star we ever see at night, but the brightest one Americans ever see on a September evening.

As the Dipper gets lower, look for five stars in the shape of an ‘M’ directly across the North Star from the Big Dipper’s handle.  This is Cassiopeia, the Queen—the ‘M’ is the outline of her throne.  Her stars are about as bright as the North Star and the stars of the Big Dipper, so she’s not too hard to find. 

星空下的汗腾格里峰 / Mt. Khan Tengri under Galaxy
Creative Commons License photo credit: livepine

High overhead, look for the enormous Summer Triangle, consisting of the stars Deneb, Vega, and Altair.   This triangle was up all night long from June to early August, hence its name.  Scorpius, the Scorpion, is in the southwest at dusk.  Sagittarius, the Archer, known for its ‘teapot’ asterism, is to its left.  Between these two star patterns is the center of our Milky Way—the brightest part of that band as wee see it.  On a cloudless night far from the big city, see if you notice the Milky Way glow near the ‘teapot’ of Sagittarius. 

Look for the Great Square of Pegasus rising in the east.  The vast stretch of sky under Pegasus is largely devoid of bright stars—ancients called this the ‘Celestial Sea”. 

Moon Phases in September 2010:

Last Quarter                  September 1, 12:22 am, September 30, 10:52 pm

New Moon                       September 8, 5:29 am

1st Quarter                     September 15, 12:49 am 

Full Moon                        September 23, 4:18 am

At 10:13 pm on Wednesday, September 22, the sun is directly overhead at the equator.  As a result, everyone on earth has the same amount of daylight and the same amount of night.  That’s why it is called the equinox (‘equal night’ in Latin).  In the Northern Hemisphere, we’ve seen the days get a little shorter and the midday Sun a little lower each day since June 21.  For us, the season changes from summer to fall at the equinox.  In the Southern Hemisphere, people have seen the days lengthen and the midday Sun get a little higher each day since June.  For them, the season changes from winter to spring.

Go Stargazing! August Edition

This month the great planet race continues, as Venus, Mars and Saturn form a triangle in the west.  Watch the triangle change shape each night as Venus overtakes Saturn and then Mars!

Venus is by far the brightest of the three planets.  Face west at dusk and look for a point of light that outshines everything in the night sky.

Saturn and Mars are to the upper left of Venus as August opens.  Mars is below Saturn and a bit to its left.  Although these two planets of similar brightness are much dimmer than Venus, they outshine all the other stars near them.

Observe all three carefully throughout August and watch as their configuration changes.  Mars aligned with Saturn last Saturday (July 31) and now begins to move farther to Saturn’s left.  Venus, moving faster than the other two, continues to approach from the right; it will pass Saturn on August 8.  Venus then continues to gain on Mars as they both move away from Saturn.  Venus finally overtakes Mars on August 19-20.  On the night of August 31, Venus and Mars are to either side of the star Spica in Virgo.

Jupiter is now a late evening object, rising by 11 p.m now and by 8:45 p.m. at month’s end.  It outshines all stars in the sky, so it’s easy to find.  Face east in late evening or south southwest at dawn to see it.

The Big Dipper is in the northwest at dusk. You can extend the curve of its handle to ‘arc to Arcturus’ and then ‘speed on to Spica’.  These stars are in the west at dusk tonight.  Arcturus, by the way, is the fourth brightest star we ever see at night, but the brightest one Americans ever see on an August evening.  Spica is in Virgo, the constellation where this month’s ‘planet race’ occurs.

In the east, look for the enormous Summer Triangle, consisting of the stars Deneb, Vega, and Altair.   This triangle is up all night long from June to early August, hence its name.  Scorpius, the Scorpion, is in the south at dusk.  Sagittarius, the Archer, known for its ‘teapot’ asterism, is to its left.  Between these two star patterns is the center of our Milky Way—the brightest part of that band as wee see it.  On a cloudless night far from the big city, see if you notice the Milky Way glow near the ‘teapot’ of Sagittarius.  In late evening, look for the Great Square of Pegasus rising in the east.

Moon Phases in August 2010:

Last Quarter                        August 3, 12:00 a.m.

New Moon                              August 9, 10:08 p.m.

1st Quarter                           August 16, 1:14 p.m.

Full Moon                              August 24, 12:05 p.m.

Perseid Meteor 8/12/08
Creative Commons License photo credit: aresauburn™

On Friday morning, August 13, the Earth passes through a stream of debris left long ago by Comet Swift-Tuttle.  This produces the Perseid Meteor Shower, one of the best meteor showers each year.   The Perseids occur every year at about this time, producing on average about one meteor per minute.  Keep in mind that even a short period such as a minute can seem longer if you are waiting for something to happen.  Since Earth is running into the meteors, not the other way around, the leading edge of the Earth encounters the shower.  This is the side going from night into day.  Accordingly, we see more meteors as dawn approaches.  Big city lights or the Moon can limit the meteors you see by dimming out fainter ones.  This August, however, the New Moon is on the 10th, giving us a skinny crescent on the 12th which sets long before the shower really gets going.  The main challenge, then, is to avoid city lights.

If skies that night are clear, our George Observatory will open Thursday night, August 12 at 9pm and remain open until dawn for observing the shower.  If you come out to George or go elsewhere, you’ll want to lie on your back (to see as much of the sky as at once as possible) and orient yourself towards the constellation Perseus.  (The shower is called the ‘Perseids’ because they seem to radiate from that constellation.)  Perseus rises in the northeast at dusk and is high in the north at dawn.

Go Stargazing! July Edition

During July, you can watch a great planet race, as Venus closes in on Mars while they both close in on Saturn!

Saturn is now in the south southwest at dusk.  Look just to the west of due south, about 2/3 of the way up from the horizon to the zenith.

Venus remains high in the evening sky during July.  Face west at dusk and look for a point of light that outshines everything in the sky but the Sun and the Moon.

Mars is also in the western sky.  Look in the west at dusk for a reddish point of light between Venus and Saturn.

Observe all three carefully throughout July and watch as they get closer together.  By July 31, Mars will have caught up to Saturn, with Venus only about 7.5 degrees away.  Keep watching next month as Mars moves ahead of Saturn and Venus passes them both.

Jupiter is in the south at dawn this month.  It outshines all stars in the sky, so it’s easy to find.  By July 31, Jupiter rises at about 11 p.m.; it will be a late evening object next month.

In the west, a distinct backwards question mark shape outlines the mane and forepaws of Leo, the Lion.  Three stars forming a right triangle are to its upper left; they mark Leo’s hindquarters.  This month, the Lion serves as the backdrop for the great planet race described above.  The Big Dipper is high in the northwest at dusk. You can extend the curve of its handle to ‘arc to Arcturus’ and then ‘speed on to Spica.’  These stars high in the west and southwest, respectively, by dusk tonight.  Arcturus, by the way, is the fourth brightest star we ever see at night, but the brightest one Americans ever see in all of July.

In the east, look for the enormous Summer Triangle, consisting of the stars Deneb, Vega, and Altair.   This triangle is up all night long in July, hence its name.  Scorpius, the Scorpion, is in the southeast at dusk.  Sagittarius, the Archer, known for its ‘teapot’ asterism, is to its left.  Between these two star patterns is the center of our Milky Way—the brightest part of that band as wee see it.  On a cloudless night far from the big city, see if you notice the Milky Way glow near the ‘teapot’ of Sagittarius.

Summer Triangle

Moon Phases in July 2010:

Last Quarter                       July 4, 9:36 am

New Moon                            July 11, 2:40 pm

1st Quarter                         July 18, 5:11 am

Full Moon                            July 25, 8:36 pm

Flag of Turkey
Creative Commons License photo credit: steelight

The new moon of Sunday, July 11, will align precisely with the sun and Earth, casting its shadow on the Earth.  This will cause a total solar eclipse.

Unfortunately, the shadow’s path is entirely over the South Pacific Ocean.  Easter Island and certain islands of French Polynesia are the only land where totality can be seen.  Even partial phases are visible only from South America.

On Tuesday, July 6, Earth is as far from the sun as it will get this year, a position called aphelion.  Remember, the Earth’s orbit is not quite a circle but an ellipse.  We are therefore slightly closer to the Sun in January than in July.  Also, remember that the difference between our January and July distances from the Sun is small.  When it comes to making us hotter or colder, the effect of our axial tilt dominates.