Spring and Autumn Equinox

This Sunday, we have the first official day of Autumn 2024.  Here are some interesting facts and figures around the autumn equinox:

* Although we designate an entire day for the equinox, the instance will occur at the exact same moment of time for every location on earth.  For an autumn equinox, that time will be the moment the sun crosses the celestial equator – the imaginary line above the equator – from north to south

     * For September 2024, the autumn equinox moment will occur at 7:43am CDT on Sunday the 22nd

     * The exact date and time of the equinox will vary from year to year since the earth’s yearly orbit is not complete in a whole number of days, but rather in a fraction of whole days (365.24).  The fall equinox will occur between September 21 and 24 depending on the various time zones across the planet.

     * A September 21st equinox can be rare for North America.  The last equinox on September 21st (central time zone) occurred over 1000 years ago, but will happen again in the year 2092 and 2096

     * The word “equinox” comes from Latin’s “aequus” and “nox”, meaning equal and night respectively.  However, because of our definition of sunrise/sunset, and the earth’s atmosphere refracting the sun, the day will contain a bit more daylight than darkness

     * Regardless of your location on the earth this day, the sun rises exactly due east and sets exactly due west

     * The Earth does not move at a constant speed in its elliptical orbit, due to its distance away from the sun and resulting gravitational forces.  Currently, in the winter, the earth is closest to the sun and therefore gains gravitational forces and moves faster in its orbit.  

     * Therefore, the seasons are not of equal length.  Roughly speaking for the northern hemisphere, summer is 93.6 days in length, the spring season is 92.8 days, autumn is 89.8 days, and thankfully, the winter season is the shortest at 89.0 days

     * The inconsistent speed of the elliptical orbit also creates variability around the amount of daily daylight loss or gain throughout the seasons

     * The amount of tilt in the earth’s axis obviously does not alter over time.  However, the relation of the earth’s tilt to the sun does alter on a daily basis.  The seasons change because of this changing position in regard to the sun (see illustration).  In September and March during an equinox, the earth’s tilt is directly perpendicular to the sun (not away from the sun nor towards the sun).  A solstice occurs when the tilt is at its maximum distance away from the sun or towards the sun (December and June).

https://www.farmersalmanac.com/spring-equinox-first-day-spring https://www.timeanddate.com/calendar/seasons.html




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