Their combination means that when the upper limb of Sun is on the visible horizon its center is 50 minutes of arc below the geometric horizon, which is the intersection with the celestial sphere of a horizontal plane through the eye of the observer. In sunrise/ sunset tables, the assumed semi-diameter (apparent radius) of the sun is 16 minutes of arc and the assumed refraction is 34 minutes of arc. Furthermore, the atmosphere refracts light so, even when the upper limb of the Sun is below the horizon, its rays reach over the horizon to the ground. From Earth, the Sun appears as a disc and not a single point of light so, when the center of the Sun is below the horizon, the upper edge is visible. Commonly the day is defined as the period that sunlight reaches the ground in the absence of local obstacles. In theory, the day is longer than the night.

The word equinox derives from the Latin words aequus (equal) and nox (night). In a wider sense, the equinoxes are the two days each year when the center of the Sun spends an equal amount of time above and below the horizon at every location on Earth. More technically, the equinox happens when the Sun is at one of two opposite points on the celestial sphere where the celestial equator and ecliptic intersect. On these dates, night and day are nearly of the same length and the Sun crosses the celestial equator (i.e., declination 0). Far left: June solstice File:Equinox-0.jpgĭay arc at 0° latitude, equator File:Equinox-20.jpgĭay arc at 20° latitude File:Equinox-50.jpgĭay arc at 50° latitude File:Equinox-70.jpgĭay arc at 70° latitude File:Equinox-90.jpgĪn equinox in astronomy is the event when the Sun can be observed to be directly above the Earth's equator, occurring around March 20 and September 23 each year. Far right: December solstice File:South season.jpgĭiagram of the Earth's seasons as seen from the south. File:North season.jpgĭiagram of the Earth's seasons as seen from the north. The Earth in its orbit around the Sun causes the Sun to appear on the celestial sphere moving over the ecliptic (red), which is tilted on the equator (blue). Illumination of the Earth by the Sun on the day of equinox, (ignoring twilight). UT date and time of solstices and equinoxes

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Equinox fall 2017