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The Sky This Week, 2016 February 23 - March 1

Why we "leap".
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Jupiter & satellites, 2016 FEB 21, 04:16 - 04:48 UT

The Moon moves into the morning sky this week, waning as she moves through the constellations of the late spring and early summer sky. Last Quarter occurs on March 1st at 7:11 pm Eastern Standard Time. Luna rises with Jupiter on the evening of the 23rd, then passes the bright star Spica in the wee hours of the 26th and 27th. She closes out the week near ruddy Mars before dawn on the 29th and March 1st.

This week is a special one that only comes along once every four years, almost. It is a leap year, when February has one extra day tacked on to give us a year of 366 days’ duration. The reason for this is that the Earth takes a bit more than 365 days to complete one orbit around the Sun, so to keep our civil and astronomical calendars in synch we have to periodically add a day. The concept of leap days goes back to the time of Ptolemy, but it was the Roman emperor Julius Caesar who first gave us our modern calendar with a leap year occurring every four years. This four-year cycle approximated four trips of the Earth around the Sun with an excess error of about 11 minutes per year, which was a good approximation for the time. The problem with this approximation was that the calendar "slipped" a day with respect to the seasons after 130 years. Nonetheless, the Julian Calendar remained in widespread use for the next 1600 years. By the mid-16th Century, however, the vernal equinox was falling on March 11, 10 days before its "traditional" date of March 21st. Since the date of Easter and all of the "moveable feasts" observed by Christians were tied to the equinox, this caused considerable confusion among clergy and lay people. In 1582 Pope Gregory XIII reformed the calendar after years of study by deleting 10 days between October 4th and 15th and changing the "rules" for leap years. Under the Gregorian reform leap years occurred in years evenly divisible by 4 except for years that ended in "00". These years were ordinary years unless they were evenly divisible by 400. Under this system the year 2000 is a leap year, but 1900 and 2100 are not. This system results in a 400-year cycle of 97 leap years, reducing the error of the calendar year to an excess of around 26 seconds compared to the astronomical year. Under this system the calendar will shift one day every 3000 years, which should give future calendar reformers plenty of time to come up with a new and even better system. Despite its obvious advantages, the Gregorian Calendar was not widely accepted outside of Catholic countries. England and its colonies grudgingly accepted it in 1752, and countries that are dominated by the Orthodox Christian church still use the Julian Calendar to fix their important dates.

Our early evening sky is still dominated by the bright constellations of winter, but shortly after sunset you’ll see a very bright object ascending in the eastern sky. Jupiter is racing into the evening sky, rising about four minutes earlier on successive nights. The giant planet will reach opposition on March 7, so he is in the prime-time of his 2016 apparition. Old Jove presents the largest apparent disc size of all of the planets, which makes him the best target for the small telescope after the Moon. Despite his vast distance of over 650 million kilometers (400 million miles) the planet’s enormous girth allows us to see a stunning variety of features in his roiling atmosphere. A six-inch telescope will reveal many of these features, while smaller instruments should reveal his dark equatorial cloud bands and dancing Galilean moons.

Mars receives a visit from the waning Moon as the week closes. The red planet is moving eastward toward the stars in the "head of the constellation of Scorpius, which is led by the rose-tinted star Antares. Mars and Antares provide a fine color complement in the pre-dawn sky, and you’ll have a good chance for the next several weeks to compare the planet with the "rival of Mars".

Saturn is also visible in the hours before sunrise, and after Jupiter is the next best planetary sight in the small telescope. The ringed planet is over twice as far from us as Jupiter, but even a small telescope will easily show his amazing rings. Compare all three planets on the next clear morning, and watch their movements over the next few months.

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