Alphabetic Index : A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
An equinox occurs twice a year (around 20 March and 22 September), when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the center of the Sun being in the same plane as the Earth's equator. The term equinox can also be used in a broader sense, meaning the date when such a passage happens. The name "equinox" is derived from the Latin aequus (equal) and nox (night), because around the equinox, the night and day have approximately equal length.
At an equinox, the Sun is at one of two opposite points on the celestial sphere where the celestial equator (i.e. declination 0) and ecliptic intersect. These points of intersection are called equinoctial points: classically, the vernal point (RA = 00h 00m 00s and longitude = 0º) and the autumnal point (RA = 12h 00m 00s and longitude = 180º). By extension, the term equinox may denote an equinoctial point.
The equinoxes are the only times when the subsolar point is on the Equator. This point (the place on the Earth's surface where the center of the Sun can be observed exactly overhead) crosses the Equator moving northward at the March equinox and crosses the Equator moving southward at the September equinox.
The equinoxes are also the only times when the terminator is inclined 90° to the Earth's Equator (while at solstices, that inclination reaches its minimum of 66.5°, corresponding to 90° minus Earth's axial tilt).
Another meaning of equinox is the date at which day and night are of equal length. Because times of sunset and sunrise, unlike the phenomenon described in preceding paragraphs, vary with an observer's geographic location (longitude and latitude), these dates likewise depend on location and do not exist for locations sufficiently close to the Equator. To avoid this ambiguity, the term equilux is sometimes used in this sense.
Illumination of Earth by the Sun at the March equinox.
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 (white).
Diagram of the Earth's seasons as seen from the north. Far right: December solstice.
Diagram of the Earth's seasons as seen from the south. Far left: June solstice.
When Julius Caesar established his calendar in 45 BC, he set March 25 as the spring equinox. Between the 4th and 16th centuries, the calendar drifted with respect to the equinox, such that the equinox was occurring on about 21 March in AD 300 and by AD 1500 it had reached 11 March.
In more detail, the gradual shift to March 11 induced Pope Gregory XIII to create a modern Gregorian calendar. The Pope was moved by the desire to restore the edicts concerning the date of Easter of the Council of Nicaea of AD 325. (Incidentally, the date of Easter itself is fixed by an approximation of lunar cycles used in the Hebraic calendar, but according to the historian Bede the English name "Easter" comes from a pagan celebration by the Germanic tribes of the vernal (spring) equinox.) So, the shift in the date of the equinox that occurred between the 4th and the 16th centuries was annulled with the Gregorian calendar, but nothing was done for the first four centuries of the Julian calendar. The days of 29 February of the years AD 100, AD 200, AD 300, and the day created by the irregular application of leap years between the assassination of Caesar and the decree of Augustus re-arranging the calendar in AD 8, remained in effect. This moved the equinox four days earlier than in Caesar's time.Names
On a day of the equinox, the center of the Sun spends a roughly equal amount of time above and below the horizon at every location on the Earth, night and day being of roughly the same length. The word equinox derives from the Latin words aequus (equal) and nox (night); in reality, the day is longer than the night at an equinox. Commonly, the day is defined as the period when sunlight reaches the ground in the absence of local obstacles. From the Earth, the Sun appears as a disc rather than a single point of light, so when the center of the Sun is below the horizon, its upper edge is visible. 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. In sunrise/sunset tables, the assumed semidiameter (apparent radius) of the Sun is 16 minutes of arc and the atmospheric refraction is assumed to be 34 minutes of arc. 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. These cumulative effects make the day about 14 minutes longer than the night at the Equator and longer still towards the Poles. The real equality of day and night only happens in places far enough from the Equator to have a seasonal difference in day length of at least 7 minutes, actually occurring a few days towards the winter side of each equinox.Geocentric view of the astronomical seasons Main article: Geocentric view of the seasons
|This section does not cite any references or sources. (December 2011)|
In the half year centered on the June solstice, the Sun rises and sets towards the north, 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 and sets towards the south 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 06:00 in the morning and sets at 18:00 in the evening (local time). These times are not exact for several reasons, one being that the Sun is much larger in diameter than the Earth, so that more than half of the Earth could be in sunlight at any one time (due to unparallel rays creating tangent points beyond an equal-day-night line); other reasons are as follows:
Some of the statements above can be made clearer by picturing the day arc (i.e., the path the Sun tracks along the celestial dome in its diurnal movement). 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 Northern and Southern hemispheres. The observer is understood to be sitting near the tree on the island depticted in the middle of the ocean; the green arrows give cardinal directions.
The following special cases are depicted:
Day arc at 0° latitude (Equator) The arc passes through the zenith, resulting in almost no shadows at high noon.
Day arc at 20° latitude 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.
Day arc at 50° latitude Twilight lasts almost two hours.
Day arc at 70° latitude 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; in fact, there is barely any night.
Day arc at 90° latitude (Pole) If it were not for atmospheric refraction, the Sun would be on the horizon all the time.
The vernal equinox occurs when the Sun crosses the celestial equator in March on its way from south to north. The term "vernal point" is used for the time of this occurrence and for the direction in space where the Sun is seen at that time, which is used as the origin of some celestial coordinate systems:
Because of the precession of the Earth's axis, the position of the vernal point changes with respect to the celestial sphere over time and as a consequence, both the equatorial and the ecliptic coordinate systems change over time. Therefore, when specifying celestial coordinates for an object, one has to specify at what time the vernal point and the celestial equator are taken. That reference time is called the equinox of date.
The autumnal equinox is at ecliptic longitude 180° and at right ascension 12h.
The upper culmination of the vernal point is considered the start of the sidereal day for the observer. The hour angle of the vernal point is, by definition, the observer's sidereal time.
The same is true in western tropical astrology: the vernal equinox is the first point (i.e. the start) of the sign of Aries. In this system, it is of no significance that the equinoxes shift over time with respect to the fixed stars.
Using the current official IAU constellation boundaries — and taking into account the variable precession speed and the rotation of the ecliptic — the equinoxes shift through the constellations as follows (expressed in astronomical year numbering in which the year 0 = 1 BC, −1 = 2 BC, etc.):
A number of traditional spring and autumn (harvest) festivals are celebrated on the date of the equinoxes.Neopaganism
|This section does not cite any references or sources. (September 2012)|
Equinox is a phenomenon that can occur on any planet with a significant tilt to its rotational axis. Most dramatic of these is Saturn, where the equinox places its normally majestic ring system edge-on facing the Sun. As a result, they are visible only as a thin line when seen from Earth. When seen from above—a view seen by humans during an equinox for the first time from the Cassini space probe in 2009—they receive very little sunshine, indeed more planetshine than light from the Sun.
This lack of sunshine occurs once every 14 years and 266 days. It can last a few weeks before and after the exact equinox. The most recent exact equinox for Saturn was on August 11, 2009. Its next equinox will take place on April 30, 2024.
One effect of equinoctial periods is the temporary disruption of communications satellites. For all geostationary satellites, there are a few days around the equinox when the sun goes directly behind the satellite relative to Earth (i.e. within the beam-width of the ground-station antenna) for a short period each day. The Sun's immense power and broad radiation spectrum overload the Earth station's reception circuits with noise and, depending on antenna size and other factors, temporarily disrupt or degrade the circuit. The duration of those effects varies but can range from a few minutes to an hour. (For a given frequency band, a larger antenna has a narrower beam-width and hence experiences shorter duration "Sun outage" windows.