Seasons and the Solstice
Why do we have seasons?
Ask this question to a room full of people, and you might get answers like, "Because the Earth rotates on its axis around the sun," or, "Because the Earth orbits around the sun in an ellipse; therefore, the Earth is closer to the sun in the summer and farther away in the winter." Actually, the answer lies more in the fact that the Earth's axis spins on a 23.45-degree tilt. Huh?
The Earth, while traveling in orbit around the sun, also spins on its axis, which you can think of as an invisible line that goes through the Earth's center. The northernmost point of this axis is the North Pole, and the southernmost point is the South Pole. The axis is not perpendicular to the plane of the Earth's orbit around the sun, but rather it is at a 23.4-degree tilt. Because of the tilt of the Earth on its axis, the different hemispheres tilt towards the sun at different angles at different times of the year, and that leads to different day lengths and temperatures. (If the Earth rotated on an axis perpendicular to the plane of the Earth's orbit around the sun, there would be no differences in the lengths of daylight throughout the year, and we would not have seasons.)
When the North Pole is tilted toward the sun, from midspring to midsummer, the Northern Hemisphere gets the most direct light from the sun, and that leads to longer periods of daylight and warmer temperatures. On the other hand, during this time, the Southern Hemisphere experiences shorter periods of daylight and cooler temperatures. When the South Pole is tilted toward the sun, from midautumn to midwinter, the Southern Hemisphere experiences longer periods of daylight and warmer temperatures.
What is the solstice?
The summer solstice in the Northern Hemisphere occurs when the sun reaches its most northerly point. This year, the summer solstice occurs on June 21. It is the longest day of the year for the Northern Hemisphere, and it marks the beginning of summer.
The winter solstice in the Northern Hemisphere takes place when the sun reaches its most southerly position. This year, the winter solstice occurs on December 21. It is the shortest day of the year and marks the beginning of winter.
Loosely translated from Latin, the word "solstice" means "sun stands still." For several days before and after each solstice, the sun appears to stand still in the sky, meaning that its noontime elevation does not seem to change from day to day. In fact, around the time of the summer solstice in the Northern Hemisphere, the North Pole has several days of total sunlight, while the South Pole will have long periods of total darkness. The opposite happens around the time of the winter solstice.
How does the solstice differ from the equinox?
The equator is an invisible line that circles the Earth at its widest point; it is the same distance away from each pole. During the solstice, the sun is at its greatest distance from the equator (either north or south). When the sun is lined up directly above the equator, the length of the day is the same all around the Earth. This happens twice a year, and these days are referred to as the "equinoxes." In March, the day is referred to as the spring or vernal equinox in the Northern Hemisphere, and in September, the day is referred to as the fall or autumnal equinox.