Equinoxes are opposite on either side of the equator, so the autumnal (fall) equinox in the Northern Hemisphere is the spring (vernal) equinox in the Southern Hemisphere and vice versa.
In the northern hemisphere, the fall equinox marks the first day of fall (autumn) in what is called astronomical seasons. There’s also another, more common definition of when the seasons start, namely meteorological definitions, which are based on average temperatures rather that astronomical events.
An equinox is an astronomical event in which the plane of Earth’s equator passes the center of the Sun. Equinoxes occur twice a year, around March 21st and September 23rd. The equinoxes are the only times when the subsolar point (the place on Earth’s surface where the center of the Sun is exactly overhead) is on the Equator, and, consequently, the only times when the Sun is at a zenith over the Equator. The subsolar point crosses the equator, moving northward at the March equinox and southward at the September equinox. The equinoxes are the only times when the solar terminator is perpendicular to the Equator. As a result, the northern and southern Hemispheres are equally illuminated.
At an equinox, the Sun is at one of the two opposite points on the celestial sphere where the celestial equator (i.e. declination) and ecliptic intersect. These points of intersection are called equinoctial points: classically, the vernal point and the autumnal point. However, the axes of an equatorial or ecliptic coordinate system may be defined so as to be aligned with the ecliptic and vernal equinox at a fixed point in time (or aligned with an average); therefore due to the Earth’s axial and changes in orbital parameters, the Sun position during equinoxes in an equatorial or ecliptic coordinate system may slightly differ from the aforementioned idealized values.
The oldest meaning of the word “equinox” is the day when daytime and night are of approximately equal duration. The word equinox comes from this definition, derived from the Latin aequus (equal) and nox (night). The equinox is not exactly the same as the day when period of daytime and night are of equal length for two reasons. Firstly, sunrise, which begins daytime, occurs when the top of the Sun’s disk rises above the eastern horizon. At that instant, the disk’s center is still below the horizon. Secondly, Earth’s atmosphere refracts sunlight. As a result, an observer sees daylight before the first glimpse of the Sun’s disk above the horizon. To avoid this ambiguity, the word equilux is sometimes used to mean a day on which the periods of daylight and night are equal. Times of sunset and sunrise vary with an observer’s location (longitude and latitude), so the dates when day and night are closest together in length depend on location.
March Equinox and September Equinox: names referring to the times of the year when such equinoxes occur. These usages are gaining popularity since they are without the ambiguity as to which hemisphere is the context, but are still only appropriate to cultures using the twelve months of the Gregorian calendar year or their linguistic counterparts.
Spring equinox and fall equinox or autumn equinox: these are more colloquial names based on the seasons, and are also therefore ambiguous across hemispheres. Northward Equinox and Southward equinox: names referring to the apparent motion of the Sun at the times of the equinox. The least culturally biased terms. Vernal point and autumnal point are the points on the celestial sphere where the Sun is located on the vernal equinox and autumnal equinox respectively. Usually this terminology is fixed for the Northern hemisphere.
First point of Aires and first point of Libra are names formerly used by astronomers and now used by navigators and astrologers. Navigational ephemeris tables record the geographic position of the First Point of Aries as the reference for position of navigational stars. Due to the precession of the equinoxes, , the astrological signs of the tropical zodiac where these equinoxes are located no longer correspond with the actual constellations once ascribed to them. The equinoxes are currently in the constellations of Pisces and Virgo
On the 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, so night and day are about 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. Day is usually 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 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 0.4 degrees below the horizon, its rays curve over the horizon to the ground. In sunrise/sunset tables, the assumed semi-diameter (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 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.
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 (or vice versa). Far north of the Arctic Circle, at Longyearbyen, Svalbard, Norway, there is an additional 15 minutes more daylight every day about the time of the Spring equinox, whereas in Singapore (which is just one degree of latitude north of the Equator), the amount of daylight in each daytime varies by just a few seconds.
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 time). These times are not exact for several reasons: (1) 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 unparalleled rays creating tangent points beyond an equal-day-night line); (2) Most places on Earth use a time zone which differs from the local solar time by minutes or even hours. For example, if the Sun rises at 07:00 on the equinox, it will set 12 hours later at 19:00; (3) Even people whose time zone is equal to local solar time will not see sunrise and sunset at 06:00 and 18:00. This is due to the variable speed and the inclination of the Earth’s orbit, and is described as the equation of time. It has different values for the March and September equinoxes (+8 and −8 minutes respectively); (4) 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 h 7 min long and the night only 11 h 53 min. Note, however, that these numbers are only true for the tropics. For 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; (5) Night includes twilight. If dawn and dusk are instead considered daytime, the day would be almost 13 hours near the equator, and longer at higher latitudes; and (6) 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 vernal equinox occurs in March, about when the Sun crosses the celestial equator 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 the origin of some celestial coordinate system: (a) in the ecliptic coordinate system, the vernal point is the origin of the ecliptic longitude; and (b) in the equatorial coordinate system, the vernal point is the origin of the right ascension..
Because of the precession of the Earth’s axis, the position of the vernal point on the celestial sphere changes over time, and the equatorial and the ecliptic coordinate systems change accordingly. Thus 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.
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.
AUTUMN (A/K/A FALL)
Autumn or fall is one of the four temperate seasons. Autumn marks the transition from summer into winter, in September (Northern Hemisphere) or March (Southern Hemisphere) when the arrival of night becomes noticeably earlier.
The equinoxes might be expected to be in the middle of their respective seasons, but temperature lag (caused by the thermal latency of the ground and sea) means that seasons appear later than dates calculated from a purely astronomical perspective. The actual lag varies with region. Some cultures regard the autumnal equinox as “mid-autumn”; others with a longer lag treat it as the start of autumn. Meteorologists (most of the temperate countries in the southern hemisphere) use a definition based on months, with autumn being September, October and November in the northern hemisphere, and March, April and May in the southern hemisphere.
In North America, autumn is usually considered to start with the September equinox. In traditional East Asian solar term, autumn starts on or around August 8th and ends on about November 7th. In Ireland, the autumn months according to the national meteorological service are September, October and November. However, according to the Irish Calendar which is based on ancient Gaelic traditions, autumn lasts throughout the months of August, September, and October, or possibly a few days later, depending on tradition. In Australia, autumn officially begins on March 1st and ends May 31st. According to United States tradition, autumn runs from the day after Labor Day (the Tuesday following the first Monday of September) through Thanksgiving (the fourth Thursday in November), after which the holiday season that demarcates the unofficial beginning of winter begins.
The alternative word fall for the season traces its origins to old Germanic languages. The exact derivation is unclear, with the Old English fiæll or feallan as well as and the Old Norse fall all being possible candidates. However, these words all have the meaning “to fall from a height” and are clearly derived either from a common root or from each other. The term came to denote the season in 16th century England, a contraction of Middle English expressions like “fall of the leaf” and “fall of the year”.
Association with the transition from warm to cold weather, and its related status as the season of the primary harvest, has dominated its themes and popular images. In Western cultures, personifications of autumn are usually pretty, well-fed females adorned with fruits, vegetables and grains that ripen at this time. Many cultures feature autumnal harvest festivals, often the most important on their calendars. Still extant echoes of these celebrations are found in the mid-autumn Thanksgiving holiday of the United States and Canada, and the Jewish Sukkot holiday with its roots as a full-moon harvest festival of “tabernacles” (living in outdoor huts around the time of harvest). There are also the many North American Indian festivals tied to harvest of autumnally ripe foods gathered in the wild, the Chinese Mid-Autumn or Moon festival, and many others. The predominant mood of these autumnal celebrations is gladness for the fruits of the earth mixed with a certain melancholy linked to the imminent arrival of harsh weather. While most foods are harvested during the autumn, foods particularly associated with the season include pumpkins (integral parts and apples, which are used to make the seasonal beverage apple cider. I also go apple picking in the fall at several local farms and get apples to make homemade apple sauce and pies.
Autumn in poetry has often been associated with melancholy. The possibilities of summer are gone, and the chill of winter is on the horizon. Skies turn grey, and many people turn inward, both physically and mentally.
In the Anglo sphere, most notably in Anglo-America, autumn is also associated with the Halloween season (which in turn was influenced by Samhain, a Celtic autumn festival), and with it a widespread marketing campaign that promotes it, in the U.S.A. The television, film, book, costume, home decoration, and confectionery industries use this time of year to promote products closely associated with such a holiday, with promotions going from early September to October 31st, since their themes rapidly lose strength once the holiday ends, and advertising starts concentrating on the Christmas season.
Autumn has a strong association with American football, as the regular season begins during September and ends with playoff competition in December or January, in the winter season. Canadian football, on the other hand, begins in the summer, but extends its season through the autumn season and into November. A normal activity for high schools in the US is attending Friday night football games in Autumn, while Sunday afternoons are reserved for the professional game, particularly the National Football League, and Saturdays are traditionally used for college football. The sport is generally geared around fall weather and playing in cold elements. Autumn also has strong ties to post-season baseball, with the autumnal equinox occurring with about a week left in the regular season, depending on scheduling. Autumn baseball oftentimes signifies excitement in the air for fans who root for teams on the cusp of making the post-season, as well as those that made it. The World Series, baseball’s championship series which determines the champion of Major League Baseball for that season is held in mid-to-late October (sometimes spilling over into November to accommodate longer series) and is nicknamed the “Fall Classic”.
Autumn, particularly in most parts of the United States, also has a strong association with the start of a new school year, particularly for children in primary and secondary education. “Back to school” advertising and preparations usually occurs in the weeks leading to the start of the fall season. Since 1997, Autumn has been one of the top 100 names for girls in the United States. In Indian mythology, autumn is considered to be the preferred season for the goddess of learning Saraswati, who is also known by the name of “goddess of autumn”.
Although colour changes in leaves occur wherever deciduous trees are found, coloured autumn foliage is noted in various regions of the world: most of Anglo-America, Eastern Asia, Europe, parts of Australia and New Zealand’s South Island. Eastern Canada and New England are famous for their autumnal foliage, and this attracts major tourism (worth billions of U.S. dollars) for the regions. I remember when we (my brother and I) were growing up always being fascinated by the changing colours of the leaves. We always raked them up and put them in piles for the town to collect. We also used old clothing and other things we found around to stuff with the leaves we had raked up. Our family also took weekend outings to different areas to see the leaves in all their glorious colors. In school we sometimes brought the best colored leaves in to use in class projects or in art class.