SEPTEMBER 22 – First Day of Autumn (Equinox)

SEPTEMBER 22, 2012* - First Day of  Autumn


Equinox occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the Sun being vertically above a point on the 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 are approximately equally long. It may be better understood to mean that latitudes +L and -L north and south of the equator experience nights of equal length.

The word is also used for the same event happening on other planets and in setting up a celestial coordinate system; see equinox (celestial coordinates).

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: the vernal point and the autumnal point. By extension, the term equinox may denote an equinoctial point.

An equinox happens each year at two specific moments in time (rather than two whole days), when there is a location on the Earth's Equator where the centre of the Sun can be observed to be vertically overhead, occurring around March 20/21 and September 22/23 each year.

 occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the Sun being vertically above a point on the 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 are approximately equally long. It may be better understood to mean that latitudes +L and -L north and south of the equator experience nights of equal length.

The word is also used for the same event happening on other planets and in setting up a celestial coordinate system; see equinox (celestial coordinates).

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: the vernal point and the autumnal point. By extension, the term equinox may denote an equinoctial point.

An equinox happens each year at two specific moments in time (rather than two whole days), when there is a location on the Earth's Equator where the centre of the Sun can be observed to be vertically overhead, occurring around March 20/21 and September 22/23 each year.

Length of equinoctial day and night

On a day of the equinox, the centre 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 centre 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 centre 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.

The date at which the time between sunset and sunrise crosses 12 hours , is known as the equilux. Because sunset and sunrise times vary with an observer's geographic location (longitude and latitude), the equilux likewise depends on location and does not exist for locations sufficiently close to the equator. The equinox, however, is a precise moment in time which is common to all observers on Earth.


Heliocentric view of the seasons

The Earth's seasons are caused by the rotation axis of the Earth not being perpendicular to its orbital plane. The Earth's axis is tilted at an angle of approximately 23.44° from the orbital plane; this tilt is called the axial tilt. As a consequence, for half of the year (i.e. from around March 20 to around September 22), the northern hemisphere tips toward the Sun, with the maximum around June 21, while for the other half of the year, the southern hemisphere has this honour, with the maximum around December 21. The two instants when the Sun is directly overhead at the Equator are the equinoxes. Also at that moment, both the North and South Poles of the Earth are just on the terminator and day and night are divided equally between the hemispheres.

The table above gives the dates and times of equinoxes and solstices over several years. A few remarks can be made about the equinoxes:

    * Because the Sun is a spherical (rather than a single-point) source of light, the actual crossing of the Sun over the Equator takes approximately 33 hours.
    * At the equinoxes, the rate of change for the length of daylight and night-time is the greatest. At the Poles, the equinox marks the start of the transition from 24 hours of nighttime to 24 hours of daylight. High in the Arctic Circle, Longyearbyen, Svalbard, Norway has an additional 15 minutes more daylight every day around the time of the Spring equinox, whereas in Singapore (which is virtually on the Equator), the amount of daylight each day varies by just seconds.
    * It is 94 days from the June solstice to the September equinox, but only 89 days from the December Solstice to the March equinox. The seasons are not of equal length, because of the variable speed of the Earth in its orbit around the Sun.
    * The instances of the equinoxes are not fixed, but fall about six hours later every year, amounting to one full day in four years. They are reset by the occurrence of a leap year. The Gregorian calendar is designed to follow the seasons as accurately as is practical, which is good, but not absolutely perfect. Also see: Gregorian calendar#Calendar seasonal error.
    * Smaller irregularities in the times are caused by perturbations of the Moon and the other planets.
    * Currently, the most common equinox and solstice dates are March 20, June 21, September 22 and December 21; the four-year average will slowly shift to earlier times in coming years. This shift is a full day in about 70 years (compensated mainly by the century "leap year" rules of the Gregorian calendar). This also means that in many years of the twentieth century, the dates of March 21, June 22, September 23 and December 22 were much more common, so older books teach (and older people may still remember) these dates.
    * Note that the times are given in UTC (roughly speaking, the time at Greenwich, ignoring British Summer Time). People living farther to the east (Asia and Australia), whose local times are in advance, will see the seasons apparently start later; for example, in Tonga (UTC+13), an equinox occurred on September 24, 1999, a date which will not crop up again until 2103. On the other hand, people living far to the west (America) whose clocks run behind UTC may experience an equinox as early as March 19.

Geocentric view of the seasons

In the half year centred 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 centred 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 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 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:

    * Most places on Earth use a time zone which is unequal to the local time, differing by up to an hour or even two hours, if daylight saving time (summer time) is included. In that case, the Sun could rise at 08:00 and set at 20:00, but there would still be 12 hours of daylight.
    * Even those people fortunate enough to have their time zone equal to the local time will not see sunrise and sunset at 06:00 and 18:00 respectively. This is due to the variable speed of the Earth in its orbit, and is described as the equation of time. It has different values for the March and September equinoxes (+8 and -8 minutes respectively).
    * Sunrise and sunset are commonly defined for the upper limb of the solar disk, rather than its centre. The upper limb is already up for at least one minute before the centre appears, and likewise, the upper limb sets one minute later than the centre of the solar disk. Due to atmospheric refraction, the Sun, when near the horizon, appears a little more than its own diameter above the position than where it is in reality. This makes sunrise more than another two minutes earlier and sunset the equal amount later. These two effects add up to almost seven minutes, making the equinox day 12 h 7 min long and the night only 11 h 53 min. In addition to that, the night includes twilight. When dawn and dusk are added to the daytime instead, the day would be almost 13 hours.
    * The above numbers are only true for the tropics. For moderate latitudes, this discrepancy increases (for example, 12 minutes in London) and closer to the Poles it gets very large. 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 on both the sunrise and sunset sides changes the day's length. Going up into the mountains will lengthen the day, while standing in a valley with hilltops on the east and the west can shorten the day significantly. This is why settlements in east-west running valleys are more favourable (daylight-wise) than north-south running valleys.

From Wikipedia, the free encyclopedia


*The First Day of Autumn will be as follows:
                      2011 – September 23
                      2012 – September 22
                      2013 – September 22
                      2014 – September 23