Where Time and Space Meet

This Saturday marks the astronomical beginning of a new season: Dec. 22 is the winter solstice.

Although a solstice is not an especially visual event, its semi-annual arrival celebrates one of the relationships at the heart of astronomy’s appeal.

The deep sky is where time and space comingle: the position of the sun is a tick on our clock, the shape of the moon is a page on our calendar. We embrace this delightful confusion at the start of both summer and winter: A solstice is, by definition, both a particular point in time and a particular point in space.

At 1:08 a.m., (Eastern Standard Time) on Saturday, the rays of our sun will slam directly and perpendicularly on the Tropic of Capricorn: a line of latitude essentially 23 ½ degrees south of Earth’s equator. This is as low on the Earth as the sun’s rays ever point.

If you were to visit the tropic and lie face-up on the beach at noon this Saturday, you would find the sun all the way at the tippy-top ("zenith") of the sky. Its beams would shine straight down on your forehead. Winter trips to "the tropics" (that 47 degree band between Cancer and Capricorn) do, after all, make sense. For all other latitudes on Earth, the sun will be lower in the sky and its rays will graze rather than attack Earth’s surface. For us here at approximately 42 degrees above the equator, we ought not look straight up nor lie down in order to find the sun — even if the ground happens to be clear of snow. The sun simply will not climb any higher than 25 degrees above the horizon (that is, 90 - 42 - 23 degrees). And that’s at a mid-day peak. This low altitude and shallow angle becomes a nadir for the entire year.

Since the sun has to reach this (minimal) maximum via a gently curving journey across the southern sky, it cannot embark from anywhere terribly far east nor end up anywhere terribly far west. To stay smooth, in other words, a low and bounded arc must be a short arc. So the day of the winter solstice is one that bring us fewer minutes of daylight than almost all other days of the calendar year. Due to the artificially un-smooth character of time zones, among other details, the winter solstice does not perfectly coincide with latest sunrise nor with earliest sunset.

On Saturday, the first day of astronomical winter, the sun will rise at 7:19 a.m., and set at 4:24 p.m. Sunset time will have already reached an earliness peak back on Dec. 15, when it set at 4:21 p.m. Sunrise will not reach a lateness peak until Jan. 9, when it will rise at 7:22 a.m. Each of these events will occur dramatically south of due east and due west, respectively. Picturing the sun’s position in our sky, rather than counting minutes of daylight, thereby provides a fundamental way to grasp the meaning of a solstice.

For sunlight to hit us in the above fashion, the sun itself must aim from a specific location.

This position, currently found way in the western portion of the constellation Sagittarius, is an established and defined point. That point is literally called "the winter solstice." When the sun is way up there, the trajectory of its rays inspire candles, coats and cozy-nesting way down here. Unless, of course, "way down here" means way "down under."

Daniel Yaverbaum is in charge of The Berkshire School’s observatory. Readers with sky questions may e-mail them to compass@lakevillejournal.com.