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Equinoxes and Solstices
Informally, the Equinoxes in March and September are the times of the year when the length of day and night are the same and when solar declination is 0.More rigorously, the (Vernal) Equinox in March is defined as the 0 point in reckoning the progress of the earth in orbiting the sun. When the earth orbits 180 degrees away from this point the September Equinox occurs.
The June and December Solstices occur when the earth has reached 90 degrees and 270 degrees respectively in it's orbit around the sun.
Informally the Solstices the times of year when the day is of minimum and maximum length and and when solar declination is at it's minimum and maximum.
Calculate it!
The Sun API provides the sdxFindAnnFeatx function which returns the times of Equinoxes and Solstices and many other annually occurring solar events.