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First Day of Fall
 

Autumn Begins with the September Equinox

The astronomical start of the fall season is marked by the positions of Earth and the sun.

September brings the last day of summer and the first day of fall. Here is a list of the precise moments and dates when fall arrives for the northern hemisphere:

  • September 23, 2007 at 5:51 am EDT
  • September 22, 2008 at 11:44 am EDT
  • September 22, 2009 at 5:18 pm EDT
  • September 22, 2010 at 11:09 pm EDT

Equinox

On the equinox, the sun rises directly in the east and sets directly in the west. This is true for both the fall and spring equinoxes. So on two days of the year you can find the exact cardinal directions of East and West by using the sun. The ancients did this when they set up their calendars at Stonehenge and the Mayan pyramid, among others.

The autumnal equinox for the northern hemisphere is the vernal equinox for the southern hemisphere. Another way to say this is that when fall begins for the northern hemisphere, spring begins for the southern hemisphere. Keeping this in mind can help you remember why we have seasons. If part of the globe has opposite seasons than the other half, then the reasons for the seasons is NOT because of how close or far away we are from the sun. The reason is related to the tilt of the Earth on its axis.

As the north pole begins to tilt away from the sun, cooler weather comes to the northern hemisphere because the sun is no longer giving its direct rays to this part of Earth. In winter when the north pole is tilted its farthest away from the sun, we have the least amount of daylight hours and the coldest weather. You can even notice the angle of the sun and how it never gets as high above the horizon.

Equal Day and Night?

Although it seems logical that equinox would refer to day and night being equal on these two days of the year, this is not the case. In the Northern Hemisphere on the fall equinox, daylight hours will still be longer than nighttime by about seven to 10 minutes. A couple days after the equinox, day and night are finally equal. You can check for the exact date where you are by looking up sunrise and sunset times. For example, in Washington D.C. the sun rises at 6:59 am and sets at 6:59 pm on September 26. The reason the equality in day and night does not occur on the equinox has to do with the sunrise and sunset being measured by the edges of the sun and not the geometric center and the diffraction of light as it passes through Earth's atmosphere.

Balancing an Egg?

An old wives' tale says that on the equinoxes, when the Earth is "balanced" and all things are equal, it is possible to balance an egg on one end. TA Reminder

Don't forget to put fresh batteries in your smoke detectors on the first day of fall!


 


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