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The Sky This Week, 2012 February 28 - March 6

Leaping years, leapfrogging planets.

Moon, Jupiter, & Venus
Imaged 2012 FEB 27 from Building 25,
U.S. Naval Observatory 

The Moon waxes in the evening sky this week, passing through the stars of the Great Winter Circle. First Quarter occurs on the 29th at 8:21 pm Eastern Standard Time. On the evening of the 28th look for Luna just southwest of the Pleiades star cluster. On March 1st she is just two degrees from Zeta Tauri, the star that marks the tip of the southern horn of Taurus, the Bull. On the night of the 6th look for her nearly full disk some five degrees below Regulus, brightest star in Leo, the Lion.

Last week’s glorious display of the Moon, Venus, and Jupiter certainly received lots of attention. Thanks to a spate of good weather those of us in the DC area enjoyed the sight for several nights in a row. We’ll get a similar show in another month, but the players will have traded places, with Jupiter hanging below Venus as the Moon glides by between the 25th and 27th. If you missed this month’s show, you’ll have one more chance to catch the night’s three brightest objects in the same patch of sky.

As you undoubtedly know, February 29th makes its (almost) once-every-four-year appearance this week, prompting many people to ponder "Why?" The answer lies in our attempt to carve up the year into an integral number of days. The system of leap years as we know it was first instituted by Julius Caesar c. 46BCE. In the Julian Calendar a leap year occurred every four years, giving a mean duration of a calendar year of 365.25 days over a four-year cycle. The Earth's so-called "Tropical Year" (the time it takes between two successive occurrences of the Vernal Equinox) is 365.2422 years, thus the mean Julian Calendar year exceeded the Tropical Year in length by some 11 minutes. This had the effect of displacing the calendar date of the Equinox by one day every 128 years. By 1582 the cumulative difference between the Julian Calendar and the Equinox was some 10 days (which bollixed up the computation of the date of Easter), prompting Pope Gregory XIII to promote a reform to the calendar which still bears his name. In the Gregorian Calendar leap years occur every four years except in years ending in "00" (e.g. 1700, 1800, 1900, 2100, etc.) However, years ending in "00" that are evenly divisible by 400 (e.g. 1600, 2000, 2400) *are* leap years, which yields 97 leap years in a 400-year cycle. Dividing 97/400 = 0.2425, thus the mean duration of a Gregorian year is 365.2425 days, which is only 26 seconds longer than the Tropical Year. This is the system which has been more or less "universally" adopted. Other calendars have been proposed and rejected in the ensuing 430 years, but so far nobody has come up with a viable system. As with many things in life, change can be a hard thing to accept. It took the British about 180 years to adopt the Gregorian system, and Eastern Orthodox Christians have yet to switch to it. Another major reform seems highly unlikely!

Even though the Moon has left Venus and Jupiter in the western early evening sky, these two bright objects will continue to put on a spectacular show as March opens. Dazzling Venus halves the gap with Jupiter this week, and she will pass the giant planet during the following week. Adding to the mix is the best evening apparition of the elusive planet Mercury, which reaches his greatest elongation east of the Sun on March 5th. Mercury can be seen about 45 minutes after sunset some five degrees above the western horizon as the week opens, and by the week’s end he’s up about ten degrees. Binoculars will help you pick him out; fortunately there are no other bright objects in the area to give him competition. If you have never seen this enigmatic world this will be one of the best times to catch him this year!

Ruddy Mars reaches opposition this week, standing directly opposite the Sun in the sky at 3:10 pm on the 3rd. At this time the red planet rises at sunset and sets at sunrise, and for the next few days presents his largest apparent disc toward Earthbound telescopes. Earth and Mars are closest on the 5th at around noon EST. This year’s opposition occurs close to Mars’ most distant point from the Sun, so his disc is just under 14 arcseconds in diameter. Contrast this with the very close approach of August 27, 2003, when Mars was about as close to Earth as he ever gets. During that opposition Mars’ apparent diameter swelled to nearly 26 arcseconds!

As Mars nears the meridian you can look for Saturn in the southeastern sky. The ringed planet rises at around 9:30 by mid-week, sharing the sky with the bright blue star Spica. We’ll have Saturn to look forward to when Mars shrinks once again to a tiny pink dot in the telescope eyepiece.

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