Earth's Motions and Seasons

Earth has two motions, and keeping them straight unlocks the whole unit. Rotation is Earth spinning on its axis, once every 24 hours, at 15 degrees per hour, west to east. It gives us day and night. Revolution is Earth orbiting the sun, once every 365.25 days, along a nearly circular, slightly elliptical path. It gives us the year, and, together with the tilt, the seasons.

Because the orbit is slightly elliptical, Earth is actually closest to the sun in early January, in the middle of the northern winter. That one fact kills the most common misconception in astronomy. Distance from the sun does not cause the seasons. There is also real physical evidence that Earth rotates. A Foucault pendulum appears to change its swing direction through the day, and moving air and water curve, which is the Coriolis effect. Both are exactly what a spinning Earth predicts.

Stand outside for a night and the whole sky seems to turn around you. The sun, moon, and stars rise in the east, cross the sky, and set in the west, moving at 15 degrees per hour. None of that is their motion. It is yours. Earth rotating eastward makes everything in the sky appear to move westward. In the Northern Hemisphere the stars trace circles around Polaris, the one point that barely moves, because it sits almost directly above Earth's axis.

The sun's daily path also changes through the year. In New York the noon sun stands highest in late June, lowest in late December, and it rises due east and sets due west only on the equinoxes. Tracking that changing path is the key to the next section.

Earth's axis is tilted 23.5 degrees from vertical, and it points the same direction in space, toward Polaris, all year. As Earth revolves, each hemisphere spends half the year leaning toward the sun and half leaning away. Leaning toward the sun means higher sun angles, more intense light, and longer days, which gives summer. Leaning away gives the opposite, which is winter. Tilt plus revolution makes the seasons, not distance.

Four dates anchor the year. Around June 21, the summer solstice, the northern hemisphere leans toward the sun as far as it can. The noon sun is overhead at 23.5 degrees north, and New York gets its longest day at about 15 hours. Around December 21, the winter solstice, it all reverses. On the equinoxes, about March 21 and September 23, the sun is overhead at the equator and every place on Earth gets 12 hours of daylight. In New York the noon sun is never directly overhead. It peaks at about 72 degrees in June and about 25 degrees in December.

Step through the year and watch what the tilt does to New York. For each month the lab computes the noon sun's altitude at our latitude of about 41 degrees north and the approximate hours of daylight. Find the two months where day and night come out even.

Rotation spins Earth west to east once every 24 hours, at 15 degrees per hour. That gives us day, night, and the sky's apparent westward motion around Polaris. The Foucault pendulum and the Coriolis effect are the physical proof. Revolution carries Earth around the sun once a year on a slightly elliptical orbit. That orbit actually brings us closest to the sun in January, which is exactly why distance cannot explain the seasons. The constant 23.5-degree tilt is what does explain them. Each hemisphere takes its turn leaning toward the sun, which raises the noon sun, lengthens the days, and concentrates the light. Two solstices and two equinoxes anchor the cycle. In New York the noon sun swings from about 72 degrees in June to about 25 degrees in December, without ever standing directly overhead. The moon, the planets, and the rest of the universe are next.

Ten Regents-style questions, one at a time in a focused view, each with an instant explanation. The set reshuffles when you reach the end, so you can keep practicing as long as you like.