The seasons on Earth change because the planet is slightly tilted on its axis as it travels around the Sun. This means different points on Earth receive more or less sunlight at different times of year. If Earth were not tilted, the Sun would always appear to be directly above the Equator, the amount of light a given location receives would be fixed, and there would be no seasons. There also would be no need to mark equinoxes or solstices.
The two solstices happen in June (20 or 21) and December (21 or 22). These are the days when the Sun’s path in the sky is the farthest north or south from the Equator. A hemisphere’s winter solstice is the shortest day of the year and its summer solstice the year’s longest. In the Northern Hemisphere the June solstice marks the start of summer: this is when the North Pole is tilted closest to the Sun, and the Sun’s rays are directly overhead at the Tropic of Cancer. The December solstice marks the start of winter: at this point the South Pole is tilted closest to the Sun, and the Sun’s rays are directly overhead at the Tropic of Capricorn. (In the Southern Hemisphere the seasons are reversed.)
The equinoxes happen in March (about March 21) and September (about September 23). These are the days when the Sun is exactly above the Equator, which makes day and night of equal length.
So, in the Northern Hemisphere you have:
- Vernal equinox(about March 21): day and night of equal length, marking the start of spring
- Summer solstice (June 20 or 21): longest day of the year, marking the start of summer
- Autumnal equinox(about September 23): day and night of equal length, marking the start of autumn
- Winter solstice (December 21 or 22): shortest day of the year, marking the start of winter
Read the original article on britannica.com.