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Just why, for instance, can evening skywatchers in the Northern Hemisphere enjoy Orion the Hunter only during the coldwintry months? During balmy summer evenings it is not Orion, but the stars ofScorpius, the Scorpion, that dominate the southern sky. Spring evenings provideus with a view of the sickle of Leo, the Lion.
Yet on fall evenings, it's theGreat Square of Pegasus that vies for the stargazer's attention. The change is subtle. Were we to watch the night skyon any one night from dusk to dawn we would notice certain stars rising from above the easternhorizon in the evening hours. They would sweep across the sky during the night,finally setting beneath the western horizon by dawn.
No big deal here, since,after all, the sun does the same thing during the daylight hours.
It's causedby Earth's rotation. Those stars that were low over the western horizon duringthe early evening hours would, within a matter of a few weeks, disappearentirely from our view, their places being taken up by groups which a few weeksearlier were previously higher up in the sky at sundown.
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In fact, it would seemthat with the passage of time, all the stars gradually shift westward while newstars move up from the eastern horizon to take their places. If we were to synchronize our clocks using the motions ofthe stars as a reference we would discover that the Earth would complete asingle turn on its axis not in 24 hours, but actually 23 hours and 56 minutes,or four minutes shy of 24 hours. This would be a day based upon the apparentmovement of the stars in our sky, which astronomers call a "sidereal"day from the Latin word for star.
While this is happening, all of us are being carried aroundthe sun on an annual journey almost million miles long.
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Our orbit is almosta circle and as seen from the sun the Earth would move about one degree eachday, since we take about days to go around a circle of degrees. As seenfrom Earth — from our vantage point — the sun seems to move and it changes itsplace in the sky by that one-degree per day, as measured against the backgroundof stars.
Of course, we can't see the stars in the daytime butastronomers can measure the position of the sun. The direction of the sun'sapparent motion is eastward among the stars.
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Since the daily turning of the sky caused by the Earth's rotation appears to move westward, this slight motionof the sun is what makes a day as measured by the sun called a solar day longer: the Earth must turn about one degree or about 4 minutes more than afull circle to complete a hour day as measured by the sun. That slight shift each day is what makes the different starsand constellations appear at different times of theyear.
The sun slowly changes its position, but so slowly that the starswhich are up when the sun is down also change. If you want to try an experiment, look outside some clearevening from a location you can find again. Notice the exact time that aparticular star is directly aligned with some object, like a telephone pole ora roof.
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At least seven explanations of what the wise men saw have been offered as listed in the Christmas section of my Astronomical Companion. Fred Schaaf is working on a book that will reveal the result of his study.
Fred is the author of more than a dozen books on astronomy — the one I refer to most often is his biography as I want to call it of The Brightest Stars. He is a notably sensitive and appreciative soul, and I know his Christmas star book will radiate the wonder with which he is filled by what he sees in the skies directly or through his telescope. I hope it appears in good time for next Christmas, and I will reserve my thought on the topic till after he has had his say.
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Hammock in the snow, and bird-feeder. Thank you, Guy, Anthony and Jack for your much appreciated comments. Schaaf is able to convey the big picture — how the constellations and the planets move through the sky season by season, our experience of the seasons here on Earth, and how our current scientific understanding of the cosmos relates to the sky lore of our ancestors — with admirable simplicity and wonder.
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