LEAD: This year's fall equinox arrives WEDNESDAY, Sept. 23, at 4:21 a.m. EDT. The name equinox comes from the Latin words for "equal" (aequus) and "night" (nox). The length of day and night is the same on this date: 12 hours each.
1. Looking at the Northern Hemisphere, night is on the left and day is on the right.
2. Advancing towards December, the Northern Hemisphere night becomes longer and days become shorter. Shorter days mean less solar energy and consequently colder days.
3. It is the relative tilt of Earth as it goes around the sun that causes the seasons.
TAG: By Dec. 21, 2015, Earth’s North Pole will be tilted 23.5 degrees away from the sun.
NOTE: Time-lapse video assembled from images taken by EUMESAT's Meteosat-9 satellite in 2010 and 2011. For more information about the images, see links below.