The Summer Solstice
- 2007 20 Jun
Back in my public school days, I was taught that the Earth's axis is inclined "23 1/2 degrees" to the plane of its orbit around the Sun. Like many of the things I learned in public school, this was an interesting factoid, but didn't help me very much in understanding what was going on in the sky. But this number "23 1/2 degrees" turns up in many interesting ways, especially around the solstices.
On June 21, the Earth reaches a point in its orbit where the North Pole is leaning in the general direction of the Sun. As seen from the Earth's surface, the Sun appears to reach the northern extreme of its annual circle through the sky. So on this day, the Sun appears to "stand still" and not move further North. For this reason, the Romans called it the "solstice," which means in Latin, "The Sun stands."
On the June solstice, the Sun reaches its furthest extent to the North in the sky. For folks in the northern hemisphere, the Sun is highest overhead at Noon, and the Noon shadows are the shortest of the year. Also, the morning Sun is seen to rise furthest to North than any day of the year, and also sets furthest to the North. For this reason, the June solstice is "The Longest Day of the Year," and "The First Day of Summer" for everyone in the Northern Hemisphere.
Mirror of the World
Just as the Earth has North and South Poles and an Equator, the sky is also regarded as having Celestial North and South Poles, and a Celestial Equator. And as the Earth has "latitude" to measure positions between the Equator and the Poles, the sky has "declination" to measure the positions in the sky between the Celestial Equator and the Celestial Poles.
The ancients realized that the sky is like a mirror of the world, since the declinations of the sky are straight overhead at the same number latitudes of the Earth. For example, if one could be at the North Pole, a latitude of 90 degrees North, one would see straight overhead the North Celestial Pole, at a declination of 90 degrees North. Likewise for the South Pole and its celestial counterpart. Similarly, when at the Equator (Zero latitude), one would see straight overhead the Celestial Equator (Zero declination).
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% 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 it's 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, nightime 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."
You might want to use a globe to help visualize these variations in latitude.
This article is from the Classical Astronomy Update, a free email newsletter for Christian homeschoolers. Jay Ryan is also the author of "Signs & Seasons," an astronomy homeschool curriculum. For more information, visit his web site www.ClassicalAstronomy.com.