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Old Today, 04:10 PM
Location: Roaring '20s
1,771 posts, read 454,152 times
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Originally Posted by orbiter View Post
Originally Posted by Sun Belt-lover L.A.M. View Post
Those are Roman seasons, not solar ones. In the solar ones (which is what I'm looking for start/ends to), the respective solstice/equinox is the midpoint of that season.
Those are generally called astronomical seasons. The solar seasons were in common use in medieval Europe. Thus, the term Midwinter or Midwinter's Day for the winter solstice.

The midpoint of solar seasons depends on what you want. The numerical midpoints are easy - just split the difference. If you want to get exact about it, add up the length of an astronomical season, divide by two, and then count the result forward from the beginning of that season (or back from the end).

In the graphic above, the duration of each season is laid out to the nearest minute (for Vancouver, anyway). Dividing that in half gives us 44 days, 11 hours, 45 minutes. Adding that to 8:19 pm on December 21 gives is a solar season midpoint of 8:04 am, February 5th (if my mental calculations are correct). Again, this is for Vancouver. You'll have to do this yourself for wherever you happen to be.

However, it's not quite that simple. That numeric halfway distance is misleading. Notice how winter is the shortest astronomical season, while summer is the longest, with spring and autumn in between (but spring still longer than autumn)? That's because Earth, like all bodies in the solar system, orbits the Sun not in a circle but in an ellipse (technically, a circle is just a special kind of ellipse, but never mind that). And orbiting bodies move fastest through space the closer they are to the bodies around which they orbit and slowest the furthest away they are from that body. And the Earth is currently (as in, this millennia) closest to the Sun in January and furthest from the Sun in July - by about three million miles. The exact date wobbles around through the first week of the month due to our irregular calendar as well as variable gravitational interactions with other planets (mostly Venus and Jupiter) and the Moon; this year it was on January 5th. That's called Earth's perihelion; Earth's aphelion (when it is furthest from the Sun) falls on July 4 this year. Earth's orbital velocity is now slowing and will continue to do so until July 4, when it will begin increasing until it reaches perihelion again. The result of this is that the Earth will complete 1/8th of its revolution around the Sun, marked from its perihelion, before 8:04 am, February 5th.

So, how the heck do you calculate that? You don't! However, I did a little digging. The midpoint of solar seasons occur at 45 degrees (spring), 135 degrees (summer), 225 degrees (autumn) and 315 degrees (winter) of solar longitude. And the internet - yay! - hosts a solar longitude calculator. Turns out that 315 degrees of solar longitude occurs as precisely 2020-02-04 15:27:22 UTC. That's Coordinated Universal Time, also known as Greenwich Mean Time. You'll need to know your local UTC offset. For example, I live in North America's Central Time Zone, so my offset is -6 (ie, exactly 315 degrees of solar longitude will occur at precisely 2020-02-04 09:27:22 CST) now, during Standard Time. When Daylight Saving Time kicks in my offset will be -5.

Here's the calculator. Just plug in the relevant solar longitude (again, 45 for spring, 135 for summer, 225 for autumn, 315 for winter) and the year (twice) and you'll get your precise solar season midpoint in UTC. Then adjust per your offset.
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