Go Stargazing! September Edition

September 2, 2009

Io Close-Up with New L8 Jupiter
Creative Commons License photo credit: FlyingSinger

Jupiter is the brightest thing in the evening sky this month, unless the Moon is out.  Face southeast and look for the brightest point of light there.  Remember, Jupiter outshines everything in the sky except the Sun, the Moon, and Venus, so if you’re looking in the right direction, you can’t miss it.

Right around midnight on the night of September 2-3, there is a rare sight for those with a telescope: all four Galilean moons will vanish!   This won’t happen again until 2019.  Typically, when you observe Jupiter through a telescope, there are four points of light in line with Jupiter and to its left and/or right.  These are the Galilean satellites, which Galileo discovered in 1610.  If you see less than four moons, it’s because one or more of the moons is either behind Jupiter’s disk (occulted), in Jupiter’s shadow (eclipsed), or in front of Jupiter’s disk (in transit) and not noticeable due to Jupiter’s brightness. At dusk on the 2nd, Callisto is already occulted by Jupiter, leaving three moons visible.  At 10:44 pm, Io goes behind Jupiter.  Then the other two begin to transit Jupiter—first Europa at 11:00 and then Ganymede at 11:46.  At that point all four Galilean satellites are hidden either by Jupiter’s disk or by its glare.

Although Europa and Ganymede are too stongly backlit to be seen, you might notice their shadows on Jupiter’s disk.  Because the Earth is no longer aligned with the Sun and Jupiter, the shadows seem to lag a bit behind the moons themselves.  Europa’s shadow appears at 11:56 pm, and Ganymede’s at 1:46 am.

At 1:32 am on Thursday morning, September 3, Io emerges from Jupiter’s shadow and is again visible.  By 1:50, Europa has crossed to the other side of Jupiter’s disk; its transit is over.  Ganymede finishes its transit at 3:24.  Callisto comes out of Jupiter shadow at 3:44.  Observe Jupiter in your telescope at dawn on the 3rd, and you’ll see all four Galilean moons again.

Creative Commons License photo credit: shish0r

Venus is a dazzling morning star this month.  Look east right as day begins to break for the brightest thing unless the Moon is nearby.  Venus remains the ‘morning star’ for the rest of 2009.  Mars is a little higher in the east at dawn than it has been.  Still, it remains fairly dim.  Look for Mars above Venus in the east.  Saturn is now behind the Sun from our perspective, and thus invisible.  On September 4, the Earth is exactly in Saturn’s ring plane, and the rings actually vanish from view!

The Big Dipper is lower in the northwest than earlier in the summer; you may need a clear northwest horizon to see it, especially later in the month.  From the Big Dipper’s handle, you can ‘arc to Arcturus’.  Arcturus, in the west at dusk, is the fourth brightest star we ever see at night and will be the brightest star in our evening skies during all of September.  In the southwest as night falls is Antares in Scorpius, the Scorpion.  This is a red supergiant star about 700 times as wide across as our Sun.  To the Scorpion’s left, look for eight stars in the shape of a teapot.  These stars are the bow and arrow of Sagittarius, the Archer.  High overhead, the Summer Triangle dominates the evening sky.  Vega is the brightest of the triangle’s three stars, followed by Altair in Aquila and Deneb in Cygnus.   Rising in the east on September evenings is the Great Square of Pegasus, heralding the upcoming autumn.

Moon Phases in September 2009:

Full                                    September 4, 11:03 am
Last Quarter                  September 11, 9:16 pm
New                                   September 18, 1:43 pm
1st Quarter                     September 25, 11:48 pm

At 4:22 pm on Tuesday, September 21, the Sun is directly overhead at the equator.  This, then, is the autumnal or fall equinox.  Everyone in the world gets the same amount of sunlight on this day.  Beginning on this date, night is longer than day in the Northern Hemisphere.  You probably already noticed that the midday sun is no longer as high as it was back in June and July.  It will continue to shift further to the south until December.  In the Southern Hemisphere, the situation is reversed.  They’ve been noticing that the midday sun is getting higher in the northern sky since June.  For them, the higher Sun will change winter into spring.

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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