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The Sky This Week, 2009 July 24 - 31

Don't overlook the Moon!

The Moon waxes in the evening sky this week, passing from crescent to gibbous as she wends her way through the southern reaches of the sky.  First Quarter occurs on the 28th at 6:00 pm Eastern Daylight Time.  Look for Luna several degrees southwest of fading Saturn in evening twilight on the 24th.  On the 27th she lies just four degrees south of the bright blue-tinted star Spica.  As the week draws to its close look for the Moon near the ruddy star Antares.  Luna is west of the star on the 30th, just east of it on the 31st.  


The return of the Moon to the evening sky fittingly marks the 40th anniversary of the safe return of the Apollo 11 crew.  Ten more men walked on the desolate face of our only natural satellite over the course of the next three years, and for many of us the legacy of Apollo is a continued interest in the exploration of other worlds.  We amateur astronomers often say that the Moon is “looked over, then overlooked” by casual observers.  It’s no wonder, since the Moon pretty much defines sterility and lack of long-term change.  The face we look at now is virtually identical to the one Galileo pondered 400 years ago and will probably look the same 400 years from now, so why bother with it?  The main attraction that I find in the Moon is the abundance of surface features that are visible with all of my telescopes, from my humble 80mm aperture “grab-and-go” refractor to my 14.5-inch Dobsonian.  Observing the slow advance of the terminator over the lunar surface shows an ever-changing palette of shadows, sun-drenched crater rims, and peaks.  With a little practice it’s possible to become very familiar with this alien landscape, and I probably know the names of more features on the Moon than I do of places in my neighborhood.  If you keep in mind that the apparent disc of Jupiter at its best is about the same apparent size of a medium-diameter lunar crater, you’ll understand just how much fun a peek at the ol’ Moon can be.


It’s finally time to bid Saturn a fond farewell.  By the end of the week he sets just as evening twilight ends, winding up an interesting season for Earth-based observers.  His rings are now just about edge-on to both Earth and Sun, but unfortunately for us the actual ring-plane crossing won’t be visible due to the planet’s proximity to Old Sol in the sky.  We’ll have to wait until 2025 for the next ring-plane crossing, but that one will also be almost impossible to see!


Fortunately, Jupiter is eagerly waiting in the wings for Saturn’s exit.  The giant planet rises at 9:00 pm by the end of July, and then it will only be two weeks to his opposition.  Old Jove is rebounding from his southerly declinations of the past few years and is now headed northward as he slowly meanders through the stars of Capricornus.  During the past week an Australian amateur astronomer discovered a prominent dark spot not far from Jupiter’s south pole.  He suspected from the start that he may have witnessed an impact similar to one of the fragments of Comet Shoemaker-Levy 9 in 1994, and sure enough observations by professional telescopes have confirmed his suspicions.  Jupiter was evidently whacked by a comet or small asteroid sometime between the 19th and 20th.  If the S-L9 impacts are any reliable guide, the dark cloud caused by this one should persist for a week or two before dissipating in Jupiter’s active jet streams.


The pre-dawn sky finds dazzling Venus putting more distance between herself and ruddy Mars.  This week the red planet passes 5ive degrees north of the star Aldebaran, which nearly matches the planet in hue and brightness.  Venus hurtles into the constellation Gemini, and early risers with clear eastern horizons may notice the bright “shoulder” stars of Orion peeking over the horizon below Venus as twilight gathers.