The traditional East Asian calendars divide a year into 24 solar terms (節氣). Solar terms in East Asia Main article: Chunfen Names referring to the apparent motion of the Sun at the times of the equinoxes. Northward equinox and southward equinox.Names to indicate in which constellations the two equinoxes are currently located. One disadvantage is that due to the precession of the equinoxes the astrological signs where these equinoxes are located, do not correspond any longer with the actual constellations. Alternative names for the previous set, but removing the problem that the vernal equinox may be dependent on a specific hemisphere. First point of Aries and first point of Libra.The name of the other equinox is used less often. As such the vernal equinox is the equinox where the Sun passes from south to north, and is a zeropoint in some celestial coordinate systems. Although in principle they are subject to the same problem as the spring/autumn names, their use over the centuries has fixed them to the viewpoint of the northern hemisphere. These names are direct derivatives of Latin ( ver = spring, autumnus = autumn), and as such more apt to be found in writings. The names are also not useful for other planets ( Mars, for example), even though they have seasons. These names are still not universal, however, as not all people on Earth use a solar-based calendar where the equinoxes occur every year in the same month (they differ in the Jewish calendar, for example). An alternative to the previous set, but without the ambiguity for which hemisphere they are intended. The seasons of the northern hemisphere and southern hemisphere are opposites (the spring equinox of one hemisphere is the autumn equinox of the other) so these names can be ambiguous. These names can be used when one wants to relate the equinox to a season. Spring equinox and autumn or fall equinox.The real equality of day and night only happens at places far enough from the equator to have at least a seasonal difference in daylength of 7 minutes and occurs a few days towards the winter side of each equinox. These effects together make the day about 14 minutes longer than the night at the equator, and longer still at sites toward the poles. 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 As with the Calendar Wikia, the text of Wikipedia is available under Creative Commons License. The list of authors can be seen in the page history. This page uses content from the English Wikipedia.
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