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The Strange, Shifting Days

When there aren’t 24 hours in a day

The full moon rises at sunset Friday and sets at daybreak Saturday morning. Look for it less than about two degrees from Aldebaran, the heart of Taurus the bull. December’s full moon is known as the Cold Moon, the Long Night Moon and the Moon Before Yule. And as we approach winter solstice, these are the longest nights of the year.
    In fact, Sunday marks a turning point in the tug between light and dark, with the earliest sunset of the year at 4:45. Why, you may ask, is the earliest sunset separated from the shortest day of the year by two weeks? Two factors come into play, one celestial the other of our own contrivance.
    Earth’s elliptical orbit around the sun creates a skew in the exact point of solstice — the overall shortest day — and the earliest sunset and the latest sunrise, which won’t occur until January 4 for us along Chesapeake Bay.
    Additionally, modern timekeeping bases each day on a 24-hour cycle. However, the time between one sunrise and the next — or one sunset and the next or high noon from day to day — seldom adds up to an even 24 hours. The cycle of a solar day is dictated by the time it takes for the earth to make one full rotation. And the speed of earth’s rotation changes, spinning faster when closer to the sun, as it is this time of year, and spinning slightly slower when it is farther away in June, July and August.
    So while the time of sunset will hold for the next week or so, we will continue to lose another six minutes of sunlight in the early mornings before reaching solstice December 21 and more still until January 4.
    Venus is slowly pulling away from the sun, appearing for a few minutes in our early evening sky before disappearing below the southwest horizon. At week’s end this evening star sets roughly 40 minutes after the sun, but each night she appears a little higher and remains visible more than a minute longer than the night before. Even so, you may need to scour the horizon with binoculars to see this planet so early in its evening apparition.
    Mars, too, comes into view as the sun sets, quite a bit higher than Venus but also dimmer. Shining at first-magnitude, the red planet is as bright as the average star — enough to be seen during full moon. While pulling away from earth, it nonetheless maintains its brightness through December and remains in view until setting around 8pm.
    Around 10pm, Jupiter rises in the east-northeast, the brightest object at that time other than the moon. By dawn it is high in the south, with the much fainter blue-white star Regulus 10 degrees to its lower left. The night of the 10th, Jupiter trails the waning gibbous moon by about the same distance. The two travel together throughout the dark hours, shining high in the southwest as dawn approaches.
    If you’re up in the hour before dawn, you may be able to spot the return of Saturn. It is quite low in the east-southeast as the sky begins to lighten, another target perhaps requiring binoculars to see amid the growing glare. But each morning it creeps a little higher and by month’s end is visible a full hour before sunrise.