Weather

Autumnal equinox occurred at 4:20 a.m as fall begins

In the news business there are certain stories done every year. There will always be feel-good stories around the holidays, peppered with images about people’s hardships. Late in the winter you can count on hearing that spring training is underway, in spite of snow likely still on the ground. When summer arrives, there’s the predictive narrative about the first hot day of the year, the obligatory feature about holiday and weekend traffic, marathons, local festivals, and of course the end of summer report complete with images of parents buying back to school supplies.

In the weather business there are also plenty of repeatable topics. There are the firsts: frost, snow, 70, 80, and 90-degree days. Then there are the seasons. There are eight seasons to talk about each year. We have the four meteorological seasons of spring, summer, winter and fall and the same four astronomical seasons.

Turn, Turn, Turn
Meteorological seasons are based on temperature whereas astronomical seasons are calculated using the position of the Earth in its orbit around the sun. Wednesday at 4:21 a.m. eastern daylight time marks the point of the autumnal equinox. This is the time when the earth is aligned with the sun in such a way that the sun is directly overhead at the equator. It also marks the time you can probably safely take out the air conditioners. Even if we get some heat and humidity from this point forward, it only lasts 24 hours or so.

The best way to explain this whole idea of changing seasons is to use some images. The first image shows the orbit of the Earth during the year. As the Earth, which is tilted on its axis, revolves around the sun, different parts of the planet get different amounts of light.

fall image explainer.jpg

The two exceptions to this rule are the equinoxes. During the vernal (spring) and autumnal equinoxes all parts of the planet receive roughly 12 hours of daylight and 12 hours of darkness.

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Look at the sunrises and sunsets for various spots in the world today. Notice how close to a 6 a.m. sunrise and 6 p.m sunset everywhere has. These numbers become slightly closer on Wednesday.

sunrise sunset fall.png

Equal Light
The next picture shows where it will be dark and light tomorrow at the moment of the autumnal equinox. At this time the sun is exactly overhead where it is noon on the equator. I circled the spot in the Indian Ocean off Africa. If you were in a boat in that spot at noon tomorrow (local time) the sun would be exactly overhead. The reality is it would be basically overhead all along the equator as noon occurred across the globe.
The other aspect of this picture to notice is the dark rectangle representing night.

fall time 923.png

Notice how symmetrical this shape is. As it moves across the globe, we all have equal darkness and equal daylight. We’ve been losing daylight for 3 months at an increasingly faster rate to get us to this point. We will continue to lose light at nearly 3 minutes a day, but the pace of the loss is slowing slightly. By the time we reach the first day of winter (see below), the darkness takes on a bell shape with the northern hemisphere experiencing total darkness as the highest latitudes and the southernmost reaches of the planet enjoying 24 hour daylight.

dec solistic 1021.png

Summer Look
In six months, the entire picture inverts itself and the northern hemisphere enjoys the maximum daylight while those areas south of the equator will see more darkness during a 24 hour day. If you don’t like all this shifting of light over the course of the year, move to the middle. Places on the equator like Quito in Ecuador, have nearly 12 hours of daylight and darkness every day of the year.

summer stolfds.jpg

Jewish High Holy Day
As fall arrives on Wednesday, it’s also the high holiday of Yom Kipper for those of us in the Jewish faith which arrives Tuesday at sundown. Many Jews will fast for 24 hours and go to temple. If you are unfamiliar with this holiday, think of it like this: It’s kind of like Catholic confession rolled into one long day, done together in a temple or synagogue. This is a solemn holiday, and if you want to say something to any friends who are Jewish, wish them “an easy fast”.

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