• equinox
    A harvest moon shines over Portugal.

    Photograph by Miguel Claro, My Shot

    Can You Hear Me Now?
    Equinoxes can disturb satellite communications. When the sun is directly behind a satellite in orbit, the sun's extreme radiation affects the communication beam between the satellite and the station on Earth.

    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.

    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.

    An equinox is a day when the sun is in its zenith over the Equator, and the Earth experiences days and nights of generally equal length. There are two equinoxes in a year: around March 21 and around Sept. 22.

    In the Northern Hemisphere, March 21 is the spring or vernal equinox. September 22 is the autumnal equinox. Many people are starting to use the terms March equinox and September equinox, instead of vernal, which means spring, and autumnal, which means autumn or fall. March and September are spring and fall in the Northern Hemisphere, but it's the opposite in the Southern Hemisphere. The Southern Hemisphere’s “spring” equinox takes place in autumn.

    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-degree axis not tilting toward or away from the sun: the center of the sun is in the same plane as the Equator.

    Most planets experience equinoxes. On the gas giant Saturn, equinoxes are particularly dramatic. Saturn's spectacular ring system extends along 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.

    Celebrating the Equinox

    For hundreds and even thousands of years, many cultures have celebrated the March equinox, unofficially marking the spring season, as a time of rebirth and renewal. The first day of the Baha'i and Iranian calendars falls on the March equinox every year. 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.

    Easter, one of the most important holidays in the Christian religion, is one of the most familiar festivals related to the equinox. The date of Easter changes every year, and is calculated using the March equinox. In most Western Christian churches, Easter falls on the first Sunday following the first full moon after the March equinox. In most Orthodox Christian churches, Easter falls after the March equinox and Passover, with the first full moon of spring, and on a Sunday after March 21.

    Fewer events mark the September equinox, and most are tied to harvest festivals. Chuseok, celebrated over a three-day period in the Koreas, is one of the most familiar of these folk festivals. The Jewish holiday of Rosh Hashanah is marked around the September equinox every year, calculated as 163 days after the first day of Passover (which itself is calculated by the March equinox.)

    The holiest day in the Jewish calendar, Yom Kippur, falls around the September equinox. Yom Kippur, the Day of Atonement, is traditionally observed with a daylong fast and prayer.

    Followers of modern Wicca and pagan spirituality celebrate both the March and September equinoxes as the major holidays Ostara and Harvest Home.

  • Term Part of Speech Definition Encyclopedic Entry
    atonement Noun

    Jewish and Christian belief about how God forgives his followers.

    axis Noun

    an invisible line around which an object spins.

    Encyclopedic Entry: axis
    calculate Verb

    to reach a conclusion by mathematical or logical methods.

    Christian Noun

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

    Chuseok Noun

    Korean harvest festival.

    Easter Noun

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

    Equator Noun

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

    Encyclopedic Entry: equator
    equinox Noun

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

    Encyclopedic Entry: equinox
    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.

    harvest Noun

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

    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.

    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.

    pagan Adjective

    following the religious traditions of ancient Europe, including polytheism and nature worship.

    Passover Noun

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

    perpendicular Noun

    at a right angle to something.

    plane Noun

    flat surface of two dimensions (length and width).

    planet Noun

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

    Encyclopedic Entry: planet
    right angle Noun

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

    Rosh Hashanah Noun

    Jewish holy day. Also called Jewish New Year.

    season Noun

    period of the year distinguished by special climatic conditions.

    Encyclopedic Entry: season
    Southern Hemisphere Noun

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

    subsolar point Noun

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

    vernal equinox Noun

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

    Wicca Noun

    religion based on nature worship and the use of magic.

    Yom Kippur Noun

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

    zenith Noun

    point on the celestial sphere directly above a given position.

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