For some people — mainly kids and their parents — summer ends on the first day of school. Other people feel like summer ends on Labor Day weekend. Scientifically, however, summer ends and autumn begins each year on September 22, or sometimes, on September 23. (It’ll occur on September 23 in 2023.) The day on which autumn arrives is called the autumnal equinox. 

This year, autumn in Tucson will begin on Wednesday, September 22, at 12:21 p.m. local time. But what is it about this date that makes it the first day of autumn? 

The autumnal equinox happens when daytime and nighttime are the same length in both hemispheres. The word equinox means “equal night” in Latin, which is exactly what happens on September 22. The 24-hour day will be divided evenly into 12 hours of daylight and 12 hours of nighttime all over the Earth. The day and time of the equinox always varies from year to year because, while we think of a year as being 365 days, the earth takes a little more than that — 365.25 days, to be more exact — to orbit the sun and so time of the equinox is not exactly the same each year. 

Why is there more daylight in summer and more night in winter? The Earth is tilted by 23.5 degrees in relation to the sun. Consequently, either the Southern Hemisphere (the part of the globe south of the equator) or the Northern Hemisphere (the part of the globe north of the equator) usually tilt a little more towards the sun. But twice a year, in the spring and fall, the sun rises and sets directly over Earth’s equator. 

The graphic below shows how, for those who live in the northern hemisphere, the earth is tilted away from the sun in winter, and towards the sun in summer.

The autumnal equinox is a result of the sun crossing the earth’s equator from north to south so that, for six months, the sun will spend less time shining on the northern half of the earth. The sun will again start spending more time shining on the northern hemisphere beginning around March 20, the spring equinox (known as the vernal equinox) when the day and night will again be the same length. 

Just as the autumnal equinox marks the end of summer and the beginning of autumn, the vernal equinox marks the end of winter and the beginning of spring. On the two annual equinox days, the sun will rise exactly due east and set precisely due west, no matter where on earth you live.

The amount of time the sun shines on the northern hemisphere will continue to get shorter each day until the winter solstice, the shortest day and the longest night in the northern hemisphere. After the winter solstice, daylight hours will once again begin to increase.

After a hot Arizona summer, we’re probably all looking forward to cooler weather. Although it won’t get cooler for a while, at least summer is officially over, so bring out your jackets and make some hot chocolate because, this month, fall officially arrives!