Astronomy news for the week starting Friday, February 25, 2000.

The Moon, now gone from the evening sky, passes through third quarter the night of Saturday the 26th just about the time of moonrise in North America, and then wanes through crescent, each day becoming closer to the eastern horizon in morning twilight. Two days after the quarter it passes apogee, when it is farthest from Earth.

Though Mars still hangs with us deep in western twilight amidst the dim stars of Pisces, the planetary night still belongs to bright Jupiter and Saturn, the pair to the south of the classic figure of Aries. Slowly drawing together, Mars and Jupiter will be in conjunction in twilight in early April. Mercury, now gone entirely from the evening sky, passes inferior conjunction with the Sun (when it is between us and the Sun) on Wednesday, March 1. The last time it was in inferior conjunction, last October 15, it transited the solar surface. This time it will pass 4 degrees to the north of the Sun. The morning planetary sky features only Venus, which is getting more difficult to see. Rising now in twilight, appearing rather reddish through the thickness of the horizon atmosphere, it presents a pretty sight low in the southeast as sunrise approaches.

This week harbors a most unusual day, leap day, February 29, 2000. Leap days are required every four years to reconcile the days to the year. There are 365.24219...days per year. The extra calendar day every four years gives a four-year average of 365.25 days, quite close to the true year. The difference, however, accumulates with time. The traditional leap year cycle was introduced in the time of Julius Caesar, when the first day of spring was set at March 25. By 1580, spring was beginning on March 12. To re-align the calendar, and to reconcile it with the seasons, the astronomers of Pope Gregory XIII reset the first day of spring on March 21 and then dropped three leap years every 400 years, in century years not divisible by 400. England and its colonies went over to the Gregorian calendar in the eighteenth century. Over a 400 year period the Gregorian calendar averages 365.2425 days per year, very close to the true value. The year 1900 was not a leap year, and neither will be 2100. But 2000 will be, as it is evenly divisible by 400. This particular leap day, Tuesday February 29, takes place only once in 400 years!

Be sure to admire the winter stars. Orion stands tall to the south at 7 PM, but is slipping west to make way for the spring stars, Leo climbing the eastern sky, the Dipper standing on its handle in the northeast.
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