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The Sky This Week, 2012 March 20 - 27

Photogenic evening skies, and equinoxes and calendars...


 Mars, 2012 March 18
Imaged from Alexandria, VA
Bright "patches" near left limb are orographic clouds
forming over the large volcanoes of the "Tharsis" region

The Moon returns to the evening sky this week, giving budding astrophotographers several nights to capture spectacular images with the bright planets Jupiter and Venus during the twilight hours. New Moon occurs on the 22nd at 10:37 am Eastern Daylight Time. You might be able to catch a hair-thin lunar crescent half an hour after sunset about five degrees above the western horizon on the 23rd. By the following night you should have no trouble spotting Luna some 10 degrees below Jupiter. On the 25th the Moon is just over two degrees from Old Jove, and on the 26th she is less than three degrees from dazzling Venus. If you enjoyed the groupings of these evening lights last month, you’ll probably like the upcoming conjunctions even better!

As most of you probably know the Vernal Equinox occurred early in the morning of the 20th. If that seems a little "early" to you, rest assured that it is! This was pointed out to me by a journalist who wanted confirmation that this was indeed the earliest occurrence of the equinox since 1896. Pulling out my trusty copy of Jean Meeus’ Astronomical Tables of the Sun, Moon and Planets I was able to not only confirm this little astronomical tidbit but to also note that the equinox will gradually inch earlier every four years until the year 2096, when it will occur on the 19th at 14:06 Universal Time. It will then creep later for the next eight years, falling on the 21st in 2102 and 2103 before once again resuming its four year creep to earlier times. This will go on until 2492, when the equinox will occur on March 19th at 13:25 UT. Four years later it will fall at 12:50 UT on the 19th, then creep later once again for another eight years. So what is going on here? This is the result of the Gregorian Calendar reform of 1582! Here’s the reason. Since it takes the Earth a bit longer to orbit the Sun than exactly 365 days the equinox occurs just under six hours later on the calendar in each succeeding year. The addition of a leap day every four years almost makes up for this accumulated difference, but over many such four-year cycles the time of the equinox gradually creeps earlier. Under the rules of the Gregorian system, though, most years that end in "00" are "ordinary" years in which February has only 28 days. Under these circumstances the equinox doesn’t "reset" to the calendar for eight years, pushing the equinox date back toward the 21st. Once every 400 years, when a year ending in "00" is evenly divisible by 400 (such as 1600, 2000, and 2400), the eight-year cycle is broken, thus allowing the equinox to creep earlier in its four-year cycle. Sound complicated? It is, but you can rest assured that the average time of the equinox won’t become a full day out of synch with the calendar until sometime around the year 4500!

Returning from the distant future to the here and now, we’ve already noted the upcoming highlight of the early evening sky as the Moon passes the night’s two brightest planets, Jupiter and Venus. Last week the dazzling Venus sped by Old Jove, passing just over three degrees from the giant planet. This week Venus continues to put distance between herself and her paler rival as Jupiter drops ever closer to the encroaching twilight. It is now becoming difficult to observe Jupiter in steady air as the heat of the springtime Sun is liberated by the ground after sunset. My last few views of Jupiter reminded me of looking through ripples in a swimming pool, making it hard to glean any detail on his roiling disc. While he’s still a pretty sight for the unaided eye, his time in the eyepiece is just about over. Venus, although dazzling in brilliance, is almost devoid of detail in the telescope. Her cloud-shrouded disc shows little more than a bright white gibbous phase, rippling in the turbulence of the late twilight sky.

Mars is thus left as the only planet easily observable in the later evening hours. He is riding high in the constellation of Leo, the Lion, and his rusty tint easily distinguishes him from all the other bright objects in this area of the sky. Now that Mars has passed opposition he is beginning to recede from the Earth, and his already small disc is beginning to shrink. His position on the meridian in the late evening hours means that he is still worth a look through the telescope. You may not see more than smudges of detail at first, but steady air and a practiced eye will begin to show some of the red planet’s more interesting details.

As Mars wheels high in the south at around midnight Saturn is rising in the southeast. The ringed planet is accompanied by the first-magnitude star Spica, whose bluish tint contrasts with Saturn’s golden hue. Saturn offers a respite from trying to glean shards of detail on Mars. His rings are visible with just slight optical aid, and in a modest telescope they become a mesmerizing sight. Saturn will reach opposition in another month, so we’ll have all spring to really enjoy him.

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