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The Sky This Week, 2010 March 16 - 23

The equinox (at last!) and Saturn's opposition
Mars, 2010 March 8, 01:19 UT
Imaged with the U.S. Naval Observatory's 1895-vintage
12-inch Clark/Saegmüller refractor telescope

The Moon waxes in the evening sky this week, wending her way up through the departing constellations of the Great Winter Circle.  First Quarter occurs on the 23rd at 7:00 am Eastern Daylight Time.  Look for Luna's slender crescent above Venus at dusk on the 17th.  By the week's end she is closing in on ruddy Mars high among the stars of Gemini, the Twins.

The beginning of astronomical spring, marked by the occurrence of the Vernal Equinox, occurs at 1:32 pm EDT on the 20th.  After the snows of the past month this is an extra-welcome season here in the Washington area this year.  From an astronomical point of view, the equinox is the moment in time when the ecliptic longitude of the center of the Sun's disc reaches zero degrees on the celestial sphere.  Despite the implication of the word “equinox” (“equal night” in Latin) the length of day on the 20th actually exceeds that of night by some eight minutes.  The actual date when there are exactly 12 hours of day and night is the 17th.  This seeming discrepancy is caused by the way we measure sunrise and sunset, which are both determined by the geometric appearance and disappearance of the Sun's upper limb.  Since Old Sol subtends a disc about half a degree across, the center of the disc lags behind the limbs.  Were the Sun a point source of light its rise and set times would be 12 hours apart on the actual date of the equinox.

Each successive evening this week brings the bright planet Venus into more prominence.  After a visit by the slender Moon on the 17th, the dazzling planet slowly inches higher in the west each night.  As the week begins she sets about 15 minutes before the end of evening twilight, but by week's end that gap is less than five minutes.  Through the telescope Venus presents a small featureless disc that's about the same size as that of her ruddy rival Mars.

Last weekend's switch to Daylight Time has given Mars something of a reprieve in the evening sky.  The red planet transits the meridian between 9:30 and 9:45 pm, standing almost directly overhead as he does so.  Even though his disc is now just 10 arcseconds across, his placement in the sky and the tranquil clear air of the spring sky has given me some of the best views of his dusty surface that I've had this year.

Daylight Time postpones Saturn's entry into the sky by an hour, but the ringed planet becomes a prominent object in the southeastern sky by late evening.  He reaches opposition on the evening of the 21st, when he rises at sunset and sets at sunrise.  His rings are tilted at a very shallow angle to our line of sight, so they now appear as little more than a pair of spikes protruding from the sides of his disc.  Look for an interesting phenomenon during this time.  Through the telescope, the ball of the planet shows its extremely oblate figure.  The planet's poles are comparatively dark, while his equatorial regions are now quite bright.  Under these conditions small telescopes often give the appearance that the planet is almost square!