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Keeping Our Calendar Straight

11 minutes a year can really add up

The moon wanes through late-night and early-morning skies this week, reaching last quarter Tuesday. The moon rises Thursday around 9pm, with the bright star Spica trailing about 10 degrees behind. Far to the west of the moon is Jupiter, the next-brightest object. Friday night Spica rises ahead of the moon, but now the two are less than five degrees apart.
    The moon rises just before midnight Sunday followed only minutes later by the red planet Mars, roughly five degrees to the southeast. As sunrise approaches Monday you’ll find them high in the south.
    The moon rises around 1am Tuesday, and now it’s six degrees to the left of Mars. Another red light, Antares, the heart of Scorpius, shines to the moon’s lower left, and it, the moon and Mars form a tight triangle. Ten degrees east of the moon is golden Saturn. The moon and Saturn are spectacular Wednesday before dawn, with the moon just two degrees above ­Saturn.
    Venus still glimmers low above the southeast horizon in the half-hour before daybreak. You may even spot Mercury lower still, though you may need binoculars.
    Monday marks Leap Day, that time every four years when we recalibrate our calendars to celestial time. You see, it takes the earth a little more than 365 days to orbit the sun, so we add a 366th day on each Leap Year to keep things in synch. You might think that Leap Year is a modern development. In fact, it was first enacted by Julius Caesar more than 2,000 years ago.
    Those early astronomers were able to track the earth’s annual passage around the sun to within 11 minutes — pretty good considering the telescope would not come along for another 1,400 years. While 11 minutes may seem insignificant over a typical year’s 525,600 minutes, it adds up to a full day every 130 years. By the 1500s, the vernal equinox fell on March 11 rather than the 21st.
    Enter Pope Gregory XIII, who in 1582 wiped from that year the days of October 4th through 15th. Further, he ordained that Leap Years would continue in years divisible by four except those ending in 00 — unless those 00 years were themselves divisible by 400. So back in 2000 we observed Leap Year, but in the year 2100 we will not. This reduces the difference between a solar year and our calendar year to 26 seconds, one day every 3,000 years!
    Again, pretty accurate computations at a time when the abacus was the most advanced mathematical instrument.