Cosmic Event Marks Beginning of Autumn Equinox
The cosmic event signals the start of the autumn astronomical season... In the United Arab Emirates, the autumn (or September) equinox is expected on Monday, September 23.
What is the autumn equinox? There are two equinoxes each year, one in March—this is the vernal equinox—and one in the fall. The word 'equinox' means 'equal night'—because it is the point in Earth's orbit around the Sun when the day and night are approximately equal in length (about 12 hours each).
These equinoxes occur on the same days in both the northern and southern hemispheres, but when the northern hemisphere celebrates the vernal equinox, the southern hemisphere observes the autumn equinox.
Why is this important to me? The autumn equinox marks the beginning of astronomical autumn, which is a kind of celestial milestone signaling that cooler weather is approaching. Of course, regional weather patterns, such as the Asian monsoon season, mean that we can probably still expect some high humidity.
Why do equinoxes happen? Here's where it gets a bit more complicated...
It's all connected to the tilt of Earth's axis, which currently (though it varies within a 40,000-year spectrum) sits at a 23.5º angle relative to its orbit around the Sun.
Over the years, the focal point of the Sun, known as the subsolar point, moves along Earth's north-south axis, peaking at the June solstice, while reaching its lowest point at the December solstice.
Equinoxes occur when the Sun's declination is 0º. This means the Sun is directly over Earth's equator at noon, and Earth's axis is neither tilted towards nor away from the Sun. At these points, much of the planet experiences approximately equal daylight hours.
Does it happen on the same day every year? Interesting fact: The exact timing of the equinoxes varies slightly, although they almost always occur between September 22 and 24. This variation is due to our calendars being unable to precisely capture how long it takes Earth to orbit the Sun (365.24 days). That's why we have leap years.
