- Wednesday, June 11, 2003
23 1/2 Degrees
On the June solstice, the Earth's axis, which is inclined 23 1/2 degrees, is leaning toward the sun. As seen from the Earth, the sun reaches a declination of 23 1/2 degrees to the north of the Celestial Equator. On this day, the sun is seen directly overhead at noon at a latitude of 23 1/2 degrees north. This latitude is called the Tropic of Cancer. On the tropic, the shadows at noon disappear when the sun is directly overhead.
The word "tropic" comes from the Greek word "trope," which means "turning." This word turns up in one verse of the New Testament:
"Every good gift and every perfect gift is from above, and cometh down from the Father of lights, with whom is no variableness, neither shadow of turning." - James 1:17
After the solstice, the sun "turns" once again toward southern declinations. So the Tropic of Cancer is the "turning point" of the sun's northerly extent over the Northern Hemisphere.
As one moves north of the Tropic of Cancer, the sun does not reach as high at noon on the solstice. In South Florida for example, the noon sun is very high in the sky, but not precisely overhead, since there are very short shadows at noon.
Over the mid-temperate latitudes around 40 degrees latitude north, such as New York, Washington, Chicago and Cleveland, the noon sun is about 17 percent toward the south. But as one moves further north, the days are longer on the June solstice, and the sun rises and sets further to the north.
The Arctic Circle is at latitude 66 1/2 degrees north, which is 23 1/2 degrees from the North Pole. At the Arctic Circle, the daylight is so long on the June solstice that the sun does not set! The solstice day is 24 hours long. The sun rises due north at midnight, circles the sky all day, and sets again due north at the next "midnight." And since the days change slowly, there is constant daylight in the Arctic for much of the late spring and early summer.
At the North Pole, the "days" are six months long. The sun rises on the Equinox in March, and sets on the Equinox in September. The June solstice is the "noon" of this six-month polar day. On this day, the sun reaches its highest point in the polar sky, 23 1/2 degrees above the horizon.
Meanwhile, at the South Pole, the June solstice is the "midnight" of the six-month polar night, and the sun cannot be seen at all from here during June. At the Antarctic Circle, nighttime is 24 hours in length. The Antarctic sun can be seen briefly rising due north, and promptly setting from the same place. The Antarctic Circle is at latitude 66 1/2 degrees south, which is 23 1/2 degrees from the South Pole.
For folks in the Southern Hemisphere, the June solstice is the shortest day of the year. As one moves north from the Antarctic Circle, the period of daylight is longer, but it will still be shorter than 12 hours on this day.
After the June solstice, the sun will turn south again through the sky. A half-year from now, on the December solstice, the sun will reach a declination of 23 1/2 degrees south. On this day, the sun will pass directly overhead on the Tropic of Capricorn, at latitude 23 1/2 degrees south. And on this day, the Southern Hemisphere will have its longest day, while the Northern Hemisphere will have its first day of winter.
Jay Ryan is the creator of "The Classical Astronomy Update," a free e-mail newsletter for helping Christian homeschool families learn more about what's up in the starry sky. If you would like to receive the Update, please drop Jay an e-mail firstname.lastname@example.org.
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