Go Stargazing! May Edition

May 4, 2009

 The Pleiades
Creative Commons License photo credit: Evilnick

Have you ever wondered when the ‘Dog Days’ begin?  The term is based on Sirius, the Dog Star.  In the time of the Ancient Egyptians, Sirius rose right before sunrise at the summer solstice, after having been invisible for about 2.5 months.  Egyptians began their calendar years with this event, and also considered Sirius a herald of summer.  Ancient Greeks and Romans believed that Sirius, as the brightest star in the night sky, helped cause the hot and oppressive weather of summer when it rose with the Sun.

Because Earth wobbles as it spins, we no longer see the constellations in quite the same places the ancients did (this is called precession of the equinoxes.)  As a result, Sirius now rises with the Sun in mid-August.  So that the term refers to roughly the same time of year, modern folklorists (such as the Farmer’s Almanac) take the Dog Days to end with the reappearance of Sirius and to begin when Sirius leaves the evening sky.  May is the month to watch Sirius slowly go away.  It’s easily visible in the southwest tonight and next week.  As May continues, notice Sirius gets lower and lower each night.  By Memorial Day, you’ll need perfect viewing and no trees or buildings to the southwest in order to see it.  How long can you follow it?  All the way to May 31? Once it’s gone, the Dog Days are upon us.

Saturn remains well placed in the evening sky this month.  Look for it in the south at dusk.  Mercury is briefly visible at dusk for the first week in May.  Mercury is bright enough to appear in twilight while most stars aren’t.  Look low in the west northwest at dusk, right over the point of sunset. A compact cluster of stars called the Pleiades is nearby. 


 The Constellation Orion
Creative Commons License photo credit: Wisze

Jupiter, in the south-southeast at dawn, is the brightest thing in that part of the sky unless the Moon is nearby (as it is on May 17).  Venus is a dazzling morning star this month.  Look east right at day break for the brightest thing there except for the Moon.  Venus remains the ‘morning star’ for the rest of 2009.  Mars remains close to the horizon at dawn much of the spring, and takes longer to fully emerge into the morning sky.   

Look in the south at dusk for stars in the shape of a backwards question mark, with a right triangle to the left of that.  These are the stars in Leo the Lion.  Saturn is under the ‘right angle’ in that right triangle.  The Big Dipper is highest on spring evenings.  From the Big Dipper’s handle, you can arc to Arcturus.  Arcturus, in the east at dusk, is the fourth brightest star we ever see at night and will be the brightest one left once Sirius sets. Continuing the curve of the Big Dipper’s handle past Arcturus, you can “speed on to Spica“, a star low in the southeast at dusk.  Spica represents a stalk of wheat held by Virgo the Virgin, who is the harvest goddess.

Dazzling Orion leaves the evening sky this month; you can see him only right at dusk in early May.  His belt now points right to Aldebaran, the eye of Taurus the Bull, which sets with Orion in the west.  The Dog Stars Sirius and Procyon also set in the west, to Orion’s left.  Sirius is the brightest star we ever see at night.  Now above Orion are two stars of similar brightness less than five degrees apart.  These are Castor and Pollux, marking the heads of Gemini, the Twins.

Moon Phases in May 2009:

1st Quarter                         May 1, 3:44 pm
                                         May 30, 10:22 pm

Full                                    May 8, 11:01 pm

Last Quarter                       May 17, 2:27 am

New                                   May 24, 7:11 pm

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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