Summer Solstice

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StarDate: June 21, 2009

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The Sun is at a standstill right now. Not its motion across the sky, mind you, but the points at which it rises and sets on the horizon. This "standstill" is known as the solstice, and it marks the beginning of summer here in the northern hemisphere and winter in the southern hemisphere.

The Sun moves across the sky because Earth's axis is tilted a little. At the summer solstice, the north pole is tilted toward the Sun, so the Sun stands farthest north in the sky, and sets farthest north along the horizon. At the winter solstice in December, when Earth is on the opposite side of its orbit, the south pole tilts toward the Sun, so the Sun appears farthest south in the sky.

Because of all that nodding up and down, the sunrise and sunset points move north and south along the horizon. Around the equinoxes, in March and September, the Sun scoots along in a hurry. But at the solstices, it slows down -- like a basketball that's reached the top of its arc. For several days, its rising and setting points hardly move at all -- in other words, the Sun appears to stand still.

Within a few days, though, it'll start to slip southward again, heading toward its next "standstill" in December.

The exact time of the solstice, by the way, was 12:46 Central Daylight Time this morning. The summer solstice is the longest day of the year, so most of the United States will see a good 14 hours of sunlight or more.

Tomorrow: the tiniest black holes.

Script by Damond Benningfield, Copyright 2009

For more skywatching tips, astronomy news, and much more, read StarDate magazine.

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