Go Stargazing! September Edition

September 3, 2008

Jupiter remains well placed for observing this month.  It outshines everything else in the night sky, unless the Moon or Venus (which is only visible in the early evening) is present.  It is therefore easy to find in the south at dusk. 

Saturn is out of sight right now, as it is directly behind the Sun from our perspective on September 3 (This alignment is called conjunction). However, Saturn emerges into the morning sky by the end of the month. Venus is beginning to re-emerge into the evening sky. Although it is low in the west southwest in the evening twilight, Venus outshines everything in the sky except the Sun and the Moon, so viewers with clear views to the west southwest should be able to find it.  Mars disappears into the Sun’s glare this month, and will remain out of sight into 2009. 

Inside The Forums at Caesar's Palace
Creative Commons License photo credit: SykoSam

The brightest star in the sky this evening is Arcturus, which you can find low in the west by extending the arc of the Big Dipper’s handle (‘arc to Arcturus’).  Arcturus, the fourth brightest star we see at night, is the brightest star left right now, since the top three are not visible in Houston during September. 

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, especially later in the month.  Scorpius, the Scorpion, is in the southwest at dusk.  Sagittarius, the Archer, known for its ‘teapot’ asterism, is to Scorpius’ left (Jupiter is in Sagittarius).  Look for the enormous Summer Triangle, consisting of the stars Deneb, Vega, and Altair, overhead at dusk.  This triangle was up all night long in June and July, hence its name.  In the east, the Great Square of Pegasus has entered the sky, heralding the approaching autumn. 

Moon Phases in September 2008:

1st Quarter            September 7, 9:04 pm
Full                        September 15, 4:14 am
Last Quarter          September 22, 12:05 am
New                       September 29, 3:12 am

At 10:44 am on Monday, September 22, the Sun is directly overhead at the equator.  This marks the autumnal equinox, the official start of autumn.  On this day, everyone on the planet has the same amount of daylight.

Ever since June 21, the Sun has appeared slightly lower in the sky each day.  Also, we’ve been having a little less daylight each day since June 21.  From September 22 forward, nighttime is longer than daytime in the Northern Hemisphere.  The reverse is true in the Southern Hemisphere.  There the days have been getting longer and the Sun slightly higher in the sky since June 21.  After September 22, daytime is longer than nighttime in the Southern Hemisphere; that day is the vernal (spring) equinox for them.

Harvest Moon
Creative Commons License photo credit: maxedaperture

The Full Moon of September 15 is the closest Full Moon to the equinox.  Accordingly, it is the Harvest Moon.  The plane of our solar system in the sky, called the ecliptic, makes a very shallow angle to the horizon on September evenings.  As a result, the Moon, which we see roughly along that path, rises at almost the same time several nights in a row near the Full Moon.  Harvesters could work late into the night by moonlight with little darkness between sunset and moonrise for several days. 

Want to learn more about Astronomy?
How many planets do we really need anyway?
Why is July called July?
Want to see the longest solar eclipse of the century? China, 2009.

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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