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.