Dennis’ Local Almanac

By DENNIS McTIGHE

Equinox marks the start of autumn

Last Friday at 11:30 p.m., we moved into the fall or autumnal equinox. The fall equinox is the halfway point between the summer and winter solstices and marks the precise moment when the sun appears directly over the Earth’s equator.

The exact time of the equinox varies each year. While the autumnal equinox usually appears on September 22 or 23, it occasionally falls outside these dates. An early September 21 equinox last happened more than 1,000 years ago, but it will happen again later this century in 2092 and 2096, according to www.timeanddate.com.

On the other hand, a late September 24 equinox occurred as recently as 1931, but it won’t happen again until 2303.

During the September equinox, the sun’s direct rays cross the Earth’s equator into the Southern Hemisphere where spring is beginning. In the Northern Hemisphere, we’ll continue to lose daylight until the winter solstice, as the sun traces a shorter and lower path across the sky. The reduced sunlight is the main reason trees change color before dropping their leaves for the winter.

The location of sunrise and sunset will also edge closer to the southern horizon until December. During the equinox, the sun rises due east and sets due west everywhere on Earth except near the north and south poles.

Equinoxes and seasons happen because the Earth doesn’t orbit the sun completely upright. Because it is tilted on its axis by about 23.5 degrees, the Northern and Southern Hemispheres receive different amounts of sunlight throughout the year. On the spring and autumnal equinoxes, however, both hemispheres receive equal amounts of the sun’s energy, causing day and night to be nearly equal everywhere on Earth.

Though “equinox” comes from the Latin words “acquus” (equal) and “nox” (night”), the Earth has more than 12 hours of daylight during the equinox. Laguna has about 12 hours and 7 minutes with a sunrise at 6:48 a.m. and a sunset at 6:55 p.m. However, the “equinox” – the day when sunrise and sunset are exactly 12 hours apart actually happens three days later. It will be on Monday, Sept. 25 here in Laguna, so not until next March 16 will the sun grace our skies for at least 12 hours. There are two reasons we have more than 12 hours of sunlight on the equinoxes.

One is how we define the term “sunrise” and “sunset” to measure the length of a day. The sun appears as a disk, not a discrete point in the sky like a nighttime star. Sunrise occurs the moment the sun’s upper edge appears on the horizon, while sunset doesn’t happen until the sun’s upper edge completely dips below it. If you’ve ever watched a sunset, you’ve likely noticed that it takes a few minutes for the sun to fully disappear from the sky. Moreover, the Earth’s atmosphere can refract or bend the sun’s light. This optical illusion allows us to see the sun at sunrise and sunset when it’s technically below the horizon. Together, these two factors add several minutes of daylight to the equinox. Take notes, there’ll be a quiz in the morning.

See ya in the fall, y’all! ALOHA!