Sky watch: Sunlight as measuring stick

Though much of our nation remains covered by snow, spring is definitely on its way. In fact, you've probably noticed that our daytime hours are becoming longer. But have you also noticed that the sun is climbing higher in our midday sky?

You'll find that the sun will continue its daily climb until the first day of summer, when days will be longest, and the sun will stand quite high above the southern horizon at noon. From that day on, the sun will begin its six-month descent, reaching its lowest yearly elevation on the first day of winter.

Astronomers know each of these dates as a "solstice," a word that comes from Latin for "sun stands still." And it's true that, on those days, the sun does stand still - at least in its annual oscillation in height above our southern horizon.

On the summer solstice, the sun reaches its highest point in the midday sky, turns around, and begins its long descent toward winter. On the winter solstice, it seems to stop its long descent and begin its ascent once again.

The shadows cast by the sun also appear to change during the year. Around the summer solstice, midday shadows appear quite short, while around the winter solstice, they appear very long.

Anyone familiar with basic trigonometry knows to calculate the height of any object simply by knowing the angle at which sunlight falls on it and the length of the shadow it casts. But what if the very thought of trigonometry makes your eyes roll back into your head? This time of year, you can do the same thing without math . . . at least for a few days.

Twice during the sun's annual oscillation in the midday sky, the sun reaches a point 45 degrees above the southern horizon. At those times - and for a few days before and after - the length of all midday shadows equal the height of the objects that cast them.

The dates that this happens depend on location. In the New York area, it will happen at the end of the first week of March, and in Chicago and the Midwest around mid-March.

For example, suppose you'd like to monitor the growth of a tree in your backyard. Simply measure the length of its noontime shadow along the flat ground on these days each year, and you'll be able to keep track of its progress.

You can look up the maximum midday height the sun reaches at your location at aa.usno.navy.mil/data/docs/mrst.php. Simply enter your town and state, along with a range of dates, and you will get a table that shows the time that the sun "transits" the midday sky and its height at that moment.