An equinox is an event in which a planet’s subsolar point passes through its Equator. The equinoxes are the only time when both the Northern and Southern Hemispheres experience roughly equal amounts of daytime and nighttime.
 
On Earth, there are two equinoxes every year: one around March 21 and another around September 22. 
 
Sometimes, the equinoxes are nicknamed the “vernal equinox” (spring equinox) and the “autumnal equinox” (fall equinox), although these have different dates in the Northern and Southern Hemispheres.  The March equinox is the vernal equinox in the Northern Hemisphere, and the autumnal equinox in the Southern. The September equinox is the autumnal equinox in the Northern Hemisphere and the vernal in the Southern.
 
The Science of the Equinoxes
 
During the equinoxes, solar declination is 0°. Solar declination describes the latitude of the Earth where the sun is directly overhead at noon. (The Equator, of course, is 0° latitude.)
 
So, equinoxes are the only times of the year when the subsolar point is directly on the Equator. The subsolar point is an area where the sun's rays shine perpendicular to the Earth's surface—a right angle. Only during an equinox is the Earth's 23.5° axis not tilting toward or away from the sun: the perceived center of the Sun’s disk is in the same plane as the Equator. 
 
Before and after the equinox, the subsolar point migrates north or south. After the March equinox, the subsolar point migrates north as the Northern Hemisphere tilts toward the sun. Around June 21, the subsolar point hits the Tropic of Cancer, (23.5°N). This is the June solstice, after which the subsolar point begins to migrate south. After the September equinox, the subsolar point continues to move south as the Southern Hemisphere tilts toward the sun. Around December 21, the subsolar point hits the Tropic of Capricorn (23.5°S). This is the December solstice.
 
As its name suggests, an equinox indicates equally illuminated hemispheres, with the solar terminator equally dividing the Earth from north to south. (The solar terminator is the shadowed line indicating daylight and sunlight on a globe.)
 
A true equinox would indicate 12 hours of both day and night. Although the equinoxes are as close to this phenomenon as happens on Earth, even during the equinoxes day and night aren’t exactly equal. This is largely due to atmospheric refraction.
 
Atmospheric refraction describes the way light seems to bend or deviate from a straight line as it passes through Earth’s atmosphere. Atmospheric refraction is a result of increasing air density, which decreases the velocity of light through the air. Due to atmospheric refraction, we are able to see the sun minutes before it actually rises and sets. 
 
Equatorial Regions
The Equator, at 0° latitude, receives a maximum intensity of the sun’s rays all year. 
 
As a result, areas near Earth’s Equator experience relatively constant sunlight and little equinoctial variation. Equinoxes and celestial seasons generally have less impact than climate-driven patterns such as precipitation (rainy seasons and dry seasons).
 
Mid-latitudes
Seasonal variation increases with latitude. Atmospheric refraction also increases the disparity in the “equinox” length of day and night. At about 30° latitude, day is about eight minutes longer than night.
 
Polar Regions
Atmospheric refraction is most dramatic in the Arctic and Antarctic, with daylight extending about 12 hours and 16 minutes. In this way, the equinoxes in polar regions signal the slow change from “midnight sun” to “polar night.” 
 
“Midnight sun” describes the phenomenon in which the sun never dips below the horizon, keeping the region bathed in sunlight 24 hours a day. “Polar night” describes the opposite phenomenon, a time in which the sun never rises, keeping the region dark for 24-hour periods. 
 
Equinoctial Disruptions
Satellites are vulnerable to disruptions in the days before and after an equinox. “Sun outages” describe this disruption.
 
In the Northern Hemisphere, sun outages occur in the days before the March equinox and after the September equinox. In the Southern Hemisphere, sun outages occur after the March equinox and before the September equinox.
 
During an equinox, the sun is aligned directly behind satellites in geostationary orbit at the Equator. Situated directly above the subsolar point, the satellites are flooded with direct solar radiation. This solar radiation can interfere with and even stop satellites from transmitting signals. 
 
Many communications satellites orbit around the Equator, and consumers may experience slow Internet connections, radio static, or frozen television screens during equinoctial sun outages.
 
Extraterrestrial Equinoxes
Most planets experience equinoxes. The timing of equinoxes is determined by the planet’s axial tilt and orbital characteristics.
 
On the gas giant Saturn, for example, equinoxes are particularly dramatic. About 15 years separate the equinoxes on Saturn, and the equinoxes last about four days. 
 
Saturn's spectacular ring system orbits in the same plane as the planet's equator. Although the rings extend thousands of kilometers into space, they are actually very thin, only about a kilometer wide. During Saturn's equinoxes, the rings (and Saturn's equator) line up perfectly with the sun. Photos taken from the solar perspective reveal the rings as a razor-thin line.
 
The Culture of the Equinoxes 
 
Like the solstices, equinoxes are historic markers of seasonal change.
 
March Equinox
The March equinox, unofficially marking the spring season, is traditionally observed as a time of rebirth and renewal. For this reason, many cultures have celebrated the March equinox as the first day of the new year. The ancient Babylonian calendar began on the first full moon after the March equinox, and today, many cultural and religious calendars continue to celebrate new year in the spring. 
 
Perhaps the most widespread and well-known equinoctial new year is Nowruz, the first day of the year in the Persian calendar. For more than 3,000 years, Nowruz has been a religious holiday in Zoroastrianism, but today it is marked by secular celebrations throughout Eastern Europe and Central Asia. The Nowruz holiday includes mythical figures (such as Amu Nowruz, sometimes nicknamed the “Iranian Santa Claus”), traditional family gatherings, and the items of “haft-sin.” 
 
Haft-sin are seven symbolic foods associated with Nowruz, all beginning with the Arabic or Persian letter sin: sabze (sprouts, symbolizing rebirth); samanu (sweet pudding, symbolizing wealth); senjed (dried, date-like fruits, symbolizing love); seer (garlic, symbolizing health); seeb (apples, symbolizing beauty); somac (red sumac fruit, symbolizing the color of sunrise); and serkeh (vinegar, symbolizing age and wisdom). 
 
Outside new year celebrations, the March equinox is celebrated in holidays all over the world. Vernal Equinox Day is a national holiday in Japan. The Jewish festival of Passover begins the night of a full moon after the March equinox. The date of Easter, one of the most important holidays in the Christian calendar, is calculated using the March equinox. 
 
September Equinox
Fewer events mark the September equinox. Perhaps the most familiar of these are Rosh Hashanah and Yom Kippur, Judaism’s “High Holy Days.”
 
Rosh Hashanah is the Jewish new year, marked around the September equinox. Rosh Hashanah is calculated as 163 days after the first day of Passover (which itself is calculated by the March equinox.) The sounding of the shofar, a ram’s horn used as a trumpet for thousands of years of Jewish ritual, welcomes Rosh Hashanah.
 
Yom Kippur, the holiest day in the Jewish calendar, falls about 10 days after Rosh Hashanah. Yom Kippur is a solemn holiday known as the “Day of Atonement.” It is traditionally observed with a daylong fast and prayers for forgiveness.
 
Most cultural events associated with the September equinox are tied to autumnal harvest festivals. Chuseok, celebrated over a three-day period in the Koreas, is one of the most familiar of these folk festivals. Sometimes nicknamed “Korean Thanksgiving,” Chuseok is a celebration of family and Korea’s rich agricultural heritage.
equinox
The solar terminator appears vertical during a March equinox.

Shadowy Snake
Ancient Mayan architects may have honored the equinoxes in the design the enormous stone pyramid nicknamed "El Castillo" at Chichen-Itza, Mexico. Every equinox, the light of the sun casts shadows down the steps of the pyramid, making it look like a slithering serpent. "El Castillo" is a temple honoring the serpent god Kukulcan.

Archaeologists and anthropologists have not determined if this phenomenon was intentional or a happy accident.

Burning of the Socks
Not all festivals surrounding the equinox are ancient. Annapolis, Maryland, celebrates The Burning of the Socks every March equinox. Chesapeake Bay boaters traditionally only wear socks between winter and spring. The Burning of the Socks signifies the warmer, sock-less spring season.

Chuseok Spam
Chuseok, the huge Korean holiday around the September equinox, is a time for family gatherings and gift-giving. In addition to traditional autumn foods such as apples and nuts, one of the most popular Chuseok gifts are boxed sets of Spam.

Songpyeon
Songpyeon are an iconic food associated with Chuseok, the Korean holiday around the September equinox. Songpyeon are rice cakes made with freshly harvested rice and filled with sesame seeds, chestnuts, or red bean paste. The treats are steamed and traditionally served on layers of pine needles. Songpyeon symbolize important elements of Chuseok. The bite-sized food encourages sharing, and the rice and fillings are all harvested in late summer and autumn. Finally, the scent of pine, a sturdy evergreen, symbolizes autumn and the slow onset of the cold season on the Korean peninsula.

align
Verb

to put in a straight line.

Antarctic
Noun

region at Earth's extreme south, encompassed by the Antarctic Circle.

Noun

region at Earth's extreme north, encompassed by the Arctic Circle.

Noun

layers of gases surrounding a planet or other celestial body.

atonement
Noun

Jewish and Christian belief about how God forgives his followers.

autumnal equinox
Noun

autumn day, usually around September 22, when day and night are of generally equal length.

axial tilt
Noun

angle between an object's axis of rotation and its orbital axis, perpendicular to the orbital plane. Earth's axial tilt is about 23.5 degrees.

Noun

an invisible line around which an object spins.

Babylonia
Noun

(~1890 BCE to 539 BCE) political and cultural empire in southern Mesopotamia, what is today Iraq, roughly dating from the end of the Sumerian Empire to conquest by the Persians.

Christian
Noun

people and culture focused on the teachings of Jesus and his followers.

Chuseok
Noun

Korean harvest festival.

Noun

all weather conditions for a given location over a period of time.

consumer
Noun

person who uses a good or service.

culture
Noun

learned behavior of people, including their languages, belief systems, social structures, institutions, and material goods.

Noun

number of things of one kind in a given area.

deviate
Verb

to turn aside or digress.

disparity
Noun

difference or inequality.

disrupt
Verb

to interrupt.

dry season
Noun

time of year with little precipitation.

Easter
Noun

most important celebration in the Christian religion, marking the day when the deity Jesus rose from the dead.

Noun

imaginary line around the Earth, another planet, or star running east-west, 0 degrees latitude.

equinoctial
Adjective

having to do with an equinox or the equinoxes.

Noun

period in which daylight and darkness are nearly equal. There are two equinoxes a year.

evergreen
Noun

tree that does not lose its leaves.

fast
Verb

to not eat.

gas giant
Noun

one of the four enormous outermost planets in the solar system (Jupiter, Saturn, Neptune, Uranus), composed mostly of gases instead of rock. Also called a Jovian planet.

geostationary orbit
Noun

orbit around the Earth directly above the Equator.

haft-sin
Noun

collection of seven symbolic items whose names start with the Arabic letter sin, and displayed during the new year holiday of Nowruz.

harvest
Noun

the gathering and collection of crops, including both plants and animals.

harvest
Noun

the gathering and collection of crops, including both plants and animals.

Noun

half of a sphere, or ball-shaped object.

High Holy Days
Plural Noun

the Jewish holidays of Rosh Hashanah and Yom Kippur.

Noun

line where the Earth and the sky seem to meet.

iconic
Adjective

event or symbol representing a belief, nation, or community.

illuminate
Verb

to shine light on.

indicate
Verb

to display or show.

Internet
Noun

vast, worldwide system of linked computers and computer networks.

Jewish
Adjective

having to do with the religion or culture of people tracing their ancestry to the ancient Middle East and the spiritual leaders Abraham, Isaac, and Jacob.

Judaism
Noun

religion based on the holy book of the Torah and the teaching surrounding it.

Noun

distance north or south of the Equator, measured in degrees.

midnight sun
Noun

sunlight visible all night at certain times of the year in Arctic and Antarctic regions.

mythical
Adjective

having to do with a legend or traditional story, not fact.

national holiday
Noun

celebration or commemoration marked by citizens of a nation.

Northern Hemisphere
Noun

half of the Earth between the North Pole and the Equator.

Nowruz
Noun

holiday celebrating the Persian, or Iranian, new year.

orbit
Noun

path of one object around a more massive object.

Passover
Noun

weeklong Jewish festival marking the exodus of ancient Jews from slavery in Egypt. Also called Pesach.

Noun

piece of land jutting into a body of water.

perpendicular
Noun

at a right angle to something.

Persia
Noun

empire that dominated Mesopotamia from about 550 to 330 BCE. Most of the ancient Persian empire is in modern-day Iran.

phenomenon
Noun

an unusual act or occurrence.

plane
Noun

flat surface of two dimensions (length and width).

Noun

large, spherical celestial body that regularly rotates around a star.

polar night
Noun

lack of sunlight at all hours at certain times of the year in Arctic and Antarctic regions.

prayer
Noun

communication with a spiritual deity.

Noun

all forms in which water falls to Earth from the atmosphere.

rainy season
Noun

time of year when most of the rain in a region falls.

refraction
Noun

seeming bending of light by liquids, solids, or gases.

right angle
Noun

90-degree angle, formed by two perpendicular lines that intersect.

ring system
Noun

thin circles of dust, gas, and rocks that orbit a planet.

ritual
Noun

series of customs or procedures for a ceremony, often religious.

Rosh Hashanah
Noun

Jewish holy day. Also called Jewish New Year.

satellite
Noun

object that orbits around something else. Satellites can be natural, like moons, or made by people.

Noun

period of the year distinguished by special climatic conditions.

secular
Adjective

not having to do with religion or spirituality.

shofar
Noun

ram’s horn blown as a wind instrument, primarily associated with the Jewish ‘High Holy Days’ of Rosh Hashanah and Yom Kippur.

solar declination
Noun

angle between the rays of the sun and the plane of the Equator.

solar radiation
Noun

light and heat from the sun.

solar terminator
Noun

moving line that divides the daylight side and the dark night side of a planetary body.

solemn
Adjective

serious or sad.

Noun

astronomical event that occurs twice a year, when the sun appears directly overhead to observers at the Tropic of Cancer or the Tropic of Capricorn.

Southern Hemisphere
Noun

half of the Earth between the South Pole and the Equator.

spectacular
Adjective

dramatic and impressive.

static
Noun

random hissing or crackling or a speckled picture caused by the interference of electrical disturbances in the reception of radio or television transmissions.

subsolar point
Noun

area of a planet where the sun is perceived to be direcly overhead.

sun outage
Noun

interruption or distortion of geostationary satellite signals caused by interference in caused by solar radiation.

symbolic
Adjective

serving as a representation of something.

transmit
Verb

to pass along information or communicate.

Tropic of Cancer
Noun

line of latitude 23.5 degrees north of the Equator.

Tropic of Capricorn
Noun

line of latitude 23.5 degrees south of the Equator.

velocity
Noun

measurement of the rate and direction of change in the position of an object.

vernal equinox
Noun

day, usually around March 21, when day and night are of generally equal length. Also called the spring equinox.

vulnerable
Adjective

capable of being hurt.

Yom Kippur
Noun

Jewish holy day marked with fasting and prayer. Also called the Day of Atonement.

Zoroastrianism
Noun

religion founded by the Persian prophet Zoroaster and set forth in the sacred writings of the Zend-Avesta. It is based on the concept of a continuous struggle between Ormazd (or Ahura Mazda), the god of creation, light, and goodness; and Ahriman, the spirit of evil and darkness.