In the northern hemisphere the evenings become crisp and cool. The leaves are turning colors and giving us that beautiful reminder that the summer has ended. The autumnal equinox is upon us. There are some very clear features of the commencement of the autumnal equinox. The science behind the autumnal equinox can be easy to understand and fascinating.
– Tilt of the earth
The tilt of the earth on its axis provides us seasons. When tilt of the northern hemisphere is inclined away from the sun, we experience winter. When the tilt is inclined toward the sun we enjoy summer. During the autumnal equinox and vernal equinox the earth is neither inclined toward or away from the sun. For a brief time both hemispheres receive the rays from the sun equally.
– Date of the Autumnal Equinox
The exact date and time of the autumnal equinox varies due to the fact that our Gregorian calendar is not exactly 365 days. The time at which the sun passes directly over the equator is the date of the equinox. Usually it falls on September 22nd or 23rd. Occasionally it will also fall on the 24th of September.
– The position of the sun
Regardless of where you are on the globe, the position of the sun indicates the beginning of a new season. During the autumnal equinox the sun will rise due east and set due west. This rare occurrence only happens during the fall (autumnal) and spring (vernal) equinox. If you happened to be on the equator the sun would pass directly overhead. Whereas if you were on the north pole the sun would barely peek out from the horizon, signaling the long six months of darkness to begin.
– Equal night and day
Because the sun is rising due east and setting due west, the amount of darkness during the nighttime and light during the daytime should be as close to equal as possible. Those on the equator should experience a very close to equal amount of sunlight and darkness. Thus we call the word equinox meaning equal night. Of course, geologic features such as mountains, and atmospheric diversions do change the amount of sunlight somewhat.
The cooler days of fall along with the falling leaves and bountiful harvest are marked by the autumnal equinox. What a great time of year to watch the signs that accompany the equinox and know more of the science that is behind this annual change.
For more information go to http://earthsky.org/astronomy-essentials/everything-you-need-to-know-about-the-autumnal-equinox-of-2012