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.

Does autumn equinox occur on Sept. 22 or 23?

That depends on what time zone you’re in.

• The autumn equinox occurs at 12:55 a.m. Eastern Daylight Time on the morning of Sept. 23, but that’s 11:55 p.m.
• on the evening of Sept. 22 in Central Daylight Time.
• In the Mountain and Pacific time zones, fall also starts on the 22nd (at 10:55 and 9:55 p.m. daylight saving time, respectively).

Why does fall start at such an odd time? And what is the equinox?

The seasons’ starting times are governed by the Earth’s motion around the sun — or equivalently, from our point of view, the sun’s annual motion in Earth’s sky. These cycles go about their business with no regard for our timekeeping convenience. The start of fall (for the Northern Hemisphere) is defined as the moment when the sun passes over Earth’s equator heading south — a moment called the autumnal equinox. This moment can come at any time of day or night. The sun appears to move north and south in our sky during the year because of what some might consider an awkward misalignment of our planet.

Earth’s axis is tilted with respect to our orbit around the sun. So when we’re on one side of our orbit, the Northern Hemisphere is tipped sunward and gets heated by more direct solar rays, making summer. When we’re on the opposite side of our orbit, the Northern Hemisphere is tipped away from the sun. The solar rays come in at a lower slant to our part of the world and heat the ground less, making winter.

For a sky watcher on Earth (at north temperate latitudes), the effect is to make the sun appear to move higher in the sky each day from December to June, and back down again from June to December. An equinox comes when the sun is halfway through each journey. This celestial arrangement makes several other noteworthy things happen on the equinox date:

• Day and night are almost exactly the same length; the word “equinox” comes from the Latin for “equal night.” (There are two reasons why a look in your almanac will reveal that day and night are not exactly 12 hours long at the equinox. First, sunrise and sunset are defined as when the sun’s top edge crosses the horizon, not when the sun’s center crosses the horizon. Second, the Earth’s atmosphere distorts the sun’s apparent position slightly when the sun is very low. Have these facts on hand when you get the inevitable calls at the equinox from people saying your sunrise and sunset times are not 12 hours apart.)
• The sun rises due east and sets due west. The fall and spring equinoxes are the only times of the year when this happens.
• If you were standing on the equator, the sun would pass exactly overhead in the middle of the day. If you were at the North Pole, the sun would be skimming the horizon about to begin the six-month polar night.
• In the southern hemisphere, the September equinox marks the start of spring, and the March equinox marks the start of fall. Summer for kangaroos begins in December, winter in June.
• Eggs do not balance on end more easily at the equinox than at other times! Actual tests have demolished this bit of New Age goofiness; the ability of eggs to balance depends on tiny irregularities on their shells (and how persistent the would-be balancer is), not on what day it is. “This perennial silly-season story has nothing to do with how eggs balance,” said Sky & Telescope senior editor Alan MacRobert, “and everything to do with how some media can’t say no to a wacky story even if it’s wrong.”