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How Many Days in A Year?

When I was in fifth grade, my teacher happened to cover both multiplication of fractions and Astronomy at about the same time. When she came to the point of assigning class projects, I was given the task of computing how many days each planet had in its year. I dutifully computed incorrect numbers based on the false assumption that a day on Jupiter or Mercury was as long as a day on Earth. It took many years before I began to understand how completely naive my calculations had been.

For example, Jupiter has a period of 117/8 Earth years. Multiplying this by 365¼ yields about 43371/3 days in a Jovian year. Jupiter's rotation is listed at just under 10 hours. So an Earth day is about 22/5 days on Jupiter. If you make this adjustment you'll get something like 10497 Jovian days in a Jovian year. However, even this isn't a very good calculation, because Jupiter, being almost totally gaseous rotates at varying speeds at varying latitudes. On Jupiter, the number of days in the year depends on where you live.

On some planets, the day and the year are intertwined in marvelous ways. Mercury goes around the Sun in just under 88 days. It rotates about its axis (its day) in about 582/3 days. If you divide its day by its year you realize that two Mercurian years is three Mercurian days long. Venus is even stranger. Venus spins backwards on its axis and its day (243 Earth days) is longer than its year (2242/3 Earth days). What is really odd is that three days on Venus come very close to being two years on Earth. Is this just a coincidence or some cosmic synchrony? I don't know, but I find it fascinating. I doubt that my fifth grade teacher had any idea where my class project would lead but I do hope that today's fifth graders are as lucky finding such a teacher.

For several months, there have been few or no planets in the evening sky. This drought is slowly coming to an end as Spring moves in Summer. By the end of May, Neptune and Uranus rise shortly after 10:30 and 11:30 PM. A month later they will be rising almost two hours earlier, just about the time the sky gets dark. Jupiter rises near midnight near the middle of June with Saturn rising an hour and a half later. Mercury is making a brief appearance in the Western sky shortly after sunset. For the remainder of the year, we will have several planets in the evening sky. Mars will remain a morning planet from late May throughout the remainder of the year. Venus will be a morning planet from now until late October when it overtakes the Sun and becomes an evening planet.

In the eastern sky, the so called Summer Triangle makes its first appearance of the early evening sky. By midnight it will be riding high. This Triangle is an "asterism" which means a grouping of stars not recognized as one of the 87 official constellations. The Triangle is made up of three very bright stars Vega, Deneb and Altair in almost a perfect right Triangle. Vega is at the top of Lyra (the Lyre). Deneb and Altair are respectively the tail stars in Cygnus (the Swan) and Aquila (the Eagle).

In the North, Ursa Major (the Big Bear) which we all know as the Big Dipper is in a position to hold water. The two stars in the bowl of the Big Dipper opposite the handle, point to Polaris (the pole star which is almost exactly due North). Simple find these two stars and move up five time as far as they are separated from each other. If you follow the arc of the Big Dipper's handle, you will come to an orange yellow star called Arcturus - the main star of Boötes (the Shepherd). If you continue along this same sweep you will come to a bluish star called Spica in Virgo (the Maiden). Just a bit north of Virgo and next to Boötes is Coma Bernices (Bernice's Hair).

Virgo appears as a tall woman in classical representations, but to me she appears to look a lot like Alice in Wonderland in her pinafore dress and apron. Somehow this Alice is turned on her side. Boötes may be a shepherd, but he looks exactly like a diamond shaped kite to me, complete with kite tail and line. Coma Bernices is full of quite faint stars. You don't have to have a super imagination to see these stars as twinkling glints shining from Bernice's windblown hair.

Leslie Coleman
Leslie Coleman
Entry Date:
May 1, 1998
Published Under:
Leslie Coleman's Columns
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