Equinox Warning: You are not logged in. Your IP address will be publicly visible if you make any edits. If you log in or create an account, your edits will be attributed to your username, along with other benefits.Anti-spam check. Do not fill this in! ===Geocentric view of the astronomical seasons=== {{main|Sun path}} {{Unreferenced section|date=December 2011}} In the half-year centered on the June solstice, the Sun rises north of east and sets north of west, which means longer days with shorter nights for the northern hemisphere and shorter days with longer nights for the southern hemisphere. In the half-year centered on the December solstice, the Sun rises south of east and sets south of west and the durations of day and night are reversed. Also on the day of an equinox, the Sun rises everywhere on Earth (except at the poles) at about 06:00 and sets at about 18:00 (local solar time). These times are not exact for several reasons: * Most places on Earth use a [[time zone]] which differs from the local solar time by minutes or even hours. For example, if a location uses a time zone with reference meridian 15° to the east, the Sun will rise around 07:00 on the equinox and set 12 hours later around 19:00 . * Day length is also affected by the variable orbital speed of the Earth around the Sun. This combined effect is described as the [[equation of time]]. Thus even locations which lie on their time zone's reference meridian will not see sunrise and sunset at 6:00 and 18:00 . At the March equinox they are 7–8 minutes later, and at the September equinox they are about 7–8 minutes earlier. * Sunrise and sunset are commonly defined for the upper limb of the solar disk, rather than its center. The upper limb is already up for at least a minute before the center appears, and the upper limb likewise sets later than the center of the solar disk. Also, when the Sun is near the horizon, atmospheric refraction shifts its apparent position above its true position by a little more than its own diameter. This makes sunrise more than two minutes earlier and sunset an equal amount later. These two effects combine to make the equinox day 12{{sup|h}} 7{{sup|m}} long and the night only 11{{sup|h}} 53{{sup|m}}. Note, however, that these numbers are only true for the tropics. For [[Middle Latitudes|moderate latitudes]], the discrepancy increases (e.g., 12 minutes in London); and closer to the poles it becomes very much larger (in terms of time). Up to about 100 km from either pole, the Sun is up for a full 24 hours on an equinox day. * Height of the horizon changes the day's length. For an observer atop a mountain the day is longer, while standing in a valley will shorten the day. * The Sun is larger in diameter than the Earth, so more than half of the Earth is in sunlight at any one time (because non-parallel rays create tangent points beyond an equal-day-night line). ====Day arcs of the Sun==== {{main|Sun path}} Some of the statements above can be made clearer by picturing the day arc (i.e., [[sun path|the path]] along which the Sun [[diurnal motion|appears to move]] across the sky). The pictures show this for every hour on equinox day. In addition, some 'ghost' suns are also indicated below the horizon, up to 18° below it; the Sun in such areas still causes [[twilight]]. The depictions presented below can be used for both the northern and the southern hemispheres. The observer is understood to be sitting near the tree on the island depicted in the middle of the ocean; the green arrows give cardinal directions. * In the [[northern hemisphere]], north is to the left, the Sun rises in the east (far arrow), [[culmination|culminates]] in the south (right arrow), while moving to the right and setting in the west (near arrow). * In the [[southern hemisphere]], south is to the left, the Sun rises in the east (near arrow), culminates in the north (right arrow), while moving to the left and setting in the west (far arrow). The following special cases are depicted: <gallery widths="240px" heights="180px"> File:equinox-0.jpg |'''Day arc at 0° latitude (equator)'''<br />The arc passes through the [[zenith]], resulting in any purely vertical object (such as an [[obelisk]] or [[pillar]]) having no shadow at high noon. File:equinox-20.jpg|'''Day arc at 20° latitude'''<br />The Sun culminates at 70° altitude and its path at sunrise and sunset occurs at a steep 70° angle to the horizon. Twilight still lasts about one hour. File:equinox-50.jpg|'''Day arc at 50° latitude'''<br />Twilight lasts almost two hours. File:equinox-70.jpg|'''Day arc at 70° latitude'''<br />The Sun culminates at no more than 20° altitude and its daily path at sunrise and sunset is at a shallow 20° angle to the horizon. Twilight lasts for more than four hours. File:equinox-90.jpg|'''Day arc at 90° latitude (pole)'''<br />If it were not for atmospheric refraction, the Sun would be on the horizon all the time. </gallery> Summary: Please note that all contributions to Christianpedia may be edited, altered, or removed by other contributors. If you do not want your writing to be edited mercilessly, then do not submit it here. 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