Have you ever seen a big figure 8 on an Earth globe with months and dates on it? That is an analemma. An analemma marks the overhead position of the sun for every day of the year. Earth globes commonly have an analemma straddling the equator.

Because the Earth orbits the sun on a 23½-degree angle, the sun appears to change its latitude in the sky. On March 21, the first day of spring in the northern hemisphere, the sun is overhead at the equator. It’s called the vernal equinox, vernal meaning spring and equinox meaning equal. There are 12 hours of sunlight and 12 hours of darkness, or equal time of day and night.

As the Earth continues its orbit, the sun appears to move farther north of the equator. It reaches its northernmost position on the summer solstice around June 21. It marks the first day of summer in the northern hemisphere. It reaches 23½ degrees north latitude. The Tropic of Cancer is an imaginary line circling the globe parallel to the equator that marks the northernmost overhead path of the sun.

The Earth reaches the opposite point from the vernal equinox in its orbit around the sun. The sun returns to a position overhead on the equator. It is called the autumnal equinox, when we have equal day and night again. It occurs about September 21, and is the first day of the fall season in the northern hemisphere.

As Earth continues its orbit, the sun appears to move farther south of the equator. On the winter solstice, the sun reaches its southernmost point at 23½ degrees south latitude. The Tropic of Capricorn marks the southernmost latitude the sun passes overhead. It occurs about December 21, and marks the first day of winter in the northern hemisphere.

An analemma isn’t a perfect figure 8 shape. An analemma is bigger below the equator. That is because the Earth doesn’t orbit the sun in a perfect circle. The Earth has a slightly elliptical orbit around the sun. In January, the Earth is at perihelion, its closest point to the sun. The sun appears to move faster than average. As a result, the analemma is broader below the equator. In July, the Earth reaches aphelion, its farthest point from the sun. The sun appears to move slower than average. As a result, the analemma appears to be compressed above the equator.

The speed of the sun through the sky is affected by Earth’s elliptical orbit. On the average, at noon the sun is on the meridian, a north-south line dividing the sky in half. Sometimes the sun is a little to the east or west of the meridian at noon. Compare a sundial’s shadow to a very accurate watch at noon. The shadow may be ahead or behind the time on the watch. The shadow caused by the sun will be a few minutes faster or slower at different times of the year.