Friday September 23rd is the start of Fall here in the Northern Hemisphere, an event that has a specific moment, in this case 2:05 am on the West Coast. What happens at this moment? Can you 'see' an Equinox?
The two Solstices of the year, in December and June, get much more fanfare because they mark more dramatic transitions, both in the weather and in the slow change of the seasons. The longest day and shortest day of each year are much easier to understand. But the balance points in the equation of Earth's orbit, the Equinoxes, get much less attention because there is much less drama. But for me, there is a lot going on. From an observational point of view, there are three things to look for.
1. Sunrise is precisely due East and Sunset precisely due West, the two days of the year this happens.
2. The Sun is above the horizon 12 hours and below the horizon 12 hours (give or take a few minutes owing to the bending of light around the horizon as experienced at Sunrises and Sunsets).
3. The Sun's height in the sky at local noon, as measured in degrees above the southern horizon, is 90 degrees minus your latitude on Earth. For example, in San Francisco, we are about 38 degrees north of the equator, so the Sun's height at local noon is 90 - 38, or 52 degrees above the horizon.
There is a fourth, more subtle effect that happens around the time of the Equinox. The length of the day is changing most rapidly around this time. From the Summer Solstice to the Winter Solstice, the length of the day is continually reducing. On the Equinox, it is 12 hours, but from one day to the next the length changes about 2 1/2 minutes, around 16 minutes during the full week. That is noticeable, and if you get up at the same time each day, you are certainly aware of the changing light of the morning (or lack of light at this point in time).
The combination of the Earth's annual circuit around the Sun, the tilt of the Earth's axis, and the slightly eccentric orbit of the Earth around the Sun all contribute to very interesting effects that are most pronounced around the interesting transition points of Equinoxes and Solstices. Take a moment this Friday to appreciate what you are observing in the sky around you.
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