It is time for the seasons to change…but why do we have four seasons? Seasons occur because Earth is slightly tilted as it sweeps out its yearly trip around the sun. For part of the year the Earth's North Pole points away from the sun. Part of the time it points toward it. This is what causes our seasons.
When the North Pole points toward the sun, the sun's rays hit the northern half of the world more directly. That means it is warmer and we have summer, even though the earth is farthest away from the sun at that time. Summer is even warmer because of the length of our days and nights. In the summer, daylight lasts longer and nighttime is shorter. The longest day of the year is in the middle of summer. North of the equator, it happens on June 21st or 22nd. It is called the summer solstice.
When the North Pole is pointed away from the sun, the Earth receives less warmth from the sun and we have winter, even though the Earth itself is closest to the sun at that time. Also, the days are shorter during winter and the nights longer. That means there is less time for the sun to warm us during winter days. Short winter days have longer cold nights. The shortest day of the year is in the middle of winter. This happens around December 21st or 22nd north of the equator. It is called the winter solstice.
Twice a year, around March 20 or 21 and September 22 or 23, the sun shines directly on the equator and the length of day and night are nearly equal across the globe. The equinox occurs the moment the sun crosses the equator. On any other day of the year, the Earth's axis tilts a little away from or towards the Sun. The word “equinox” is derived from the Latin words meaning “equal night”. Although day and night are nearly 12 hours long, both are not totally equal because daytime is slightly longer in places that are further away from the equator.
The day between summer and winter when the length of day and night is almost equal is called the autumnal equinox. It is the first day of fall north of the equator and the first day of spring in the southern half of the world.
In between winter and summer there is another equinox. It is called the vernal equinox. Once again, day and night are both about 12 hours long. This day heralds the first day of spring north of the equator and the start of fall to the south.