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.