First Day Of Fall
Today is the autumnal equinox, the first day of fall for Earth’s northern hemisphere. At this point in the planet’s annual orbit, the sun appears directly over the equator, producing essentially the same amount of time in daylight and darkness over today’s 24 hours. Then, for the northern half of the planet, daylight hours steadily decrease until the shortest day of the year, the winter solstice, in December. Time in daylight will then start to increase again, eventually reaching the vernal equinox next spring.
According to the National Oceanic and Atmospheric Association, our planet will appear to have no tilt with respect to the sun. Earth’s tilt aligns with its cycle around the sun twice a year, in March and September.
Those in the Southern Hemisphere, or south of the equator, will be experiencing the vernal or spring equinox, which marks the beginning of spring on Sept. 22.
The word “equinox” comes from the Latin words “aequalis” and “nox,” meaning “equal night.” On the autumnal (and the spring) equinox, day and night are roughly 12 hours long in most of the world.
Day and night aren’t exactly 12 hours long on the day of the equinox because the Earth’s atmosphere refracts, or bends, light in an optical illusion that results in more daylight. Because of this, the date when day and night are of exactly equal length is usually a few days after the autumnal equinox.
The tilt of Earth’s axis, which is 23.5 degrees parallel to its orbit, causes seasonal changes, according to the NASA Earth Observatory. The axis of the planet points in the same direction as it spins around the sun.
