The phrase explaining what the equinox day is, at least implies an elementary knowledge of astronomical terms, because the equinox itself is a phenomenon studied precisely by this science.
Necessary knowledge of astronomical terms
Our luminary makes its movement along the ecliptic, which is, speaking in unscientific language, the plane of the earth's orbit. And the moment when the sun, making its way along the ecliptic, crosses the celestial equator, which is a large circle of air and airless space parallel to the earth's equator (their planes coincide, and both of them are perpendicular to the axis of the world), is called the equinox. The terminator (this is also an astronomical concept that has nothing to do with Schwarzenegger) is a line that divides any celestial body into a part illuminated by the sun and into the "night" one. So, on the day of the equinox, it is this terminator that passes through the geographic poles of the Earth and divides it into two equal half-ellipses.
A characteristic feature in the title
The very concept implies that on the day of the equinox, night and day are equal. From a scientific point of view, the night is always a little shorter, and the sun rises and sets not exactly in the east and west, but a little north. But still, from childhood we know that June 22 is not only the day the war and school graduation balls began (as it was in Soviet times), but also the day of the summer equinox. However, it is also called the days of the summer and winter solstice on December 22. This happens because the sun in these periods of time is either at the highest point above the horizon, or at the lowest, and the most distant from the celestial equator. That is, on the day of the equinox, the light and dark parts of the day are almost equal to each other.
The characteristic date of the equinox and solstice
On the days of the solstices, one of them - either day or night - exceeds the other as much as possible. The days of the equinox and solstice are also noteworthy for the fact that they serve as the beginning of the seasons. These dates are very remarkable, and always one of the family members says that, supposedly, today is the longest or shortest day, or that today the day is equal to night. And this distinguishes him from a series of days following one after another. Almost always, the date of these moments becomes the 22nd day, but there are leap years, and other moments and phenomena of astronomy that affect the date shift by the 21st or 23rd. The months of March, June, September and December are those on which the days of the equinox and solstice fall.
Holidays from antiquity
Of course, they have been known since ancient times. Our ancestors observed them and associated their lives with these dates, dozens of witnesses will accept this. The ancient Slavs have a certain holiday associated with each of these days, and it usually lasts a week (Kolyadki, Rusaliya, Pancake week). So, the winter solstice accounts for Kolyada, a holiday later dedicated to Christmas. Velikden, or Komoeditsa, also known as Shrovetide - these names signify the vernal equinox, the birth of a young sun. From this day begins the astrological sunny year, and our luminary passes into the Northern Hemisphere from the Southern. Maybe that's why the feast of astrology falls on March 20. Kupala (other names for Ivan Day, the Solstice), or summer confrontation, is a great summer festival of the ancient Slavs, covered in legends praising the brave people who go that night to look for a fern flower. Ovsen-Tausen, the day of the autumnal equinox, after which winter slowly begins to take over, and nights become longer. Therefore, our ancestors in Svyatovit (another name) lit candles - the most beautiful was placed in a place of honor.
Earth's special climate zone
All these dates served as starting points for the start of certain activities necessary for life — seasonal farming, construction, or winter stocks. The days of the spring and autumn equinox are also characterized by the fact that the sun gives out its light and heat equally to the Northern and Southern hemispheres, and its rays reach both poles. These days, it is located over the territory of such a climatic zone of the Earth as the tropics (translated from Greek means a turning circle). In different directions from the equator to 23 with a small degree, parallel to it are the northern and southern tropics. A characteristic feature of the area enclosed between them is that the sun reaches its zenith twice a year - once on June 22 above the northern tropic, or the tropic of Cancer, and the second time over the southern or tropic of Capricorn. It happens on December 22nd. This is characteristic of all latitudes. To the north and south of the tropics at the zenith of the sun is never.
One of the consequences of a shift in the direction of the earth's axis
On the days of the equinox and solstice, it intersects with the celestial equator at the points located in the constellations of Pisces (spring) and Virgo (autumn), and on the days of the greatest and smallest distances from the equator, that is, on the days of the summer and winter solstice, in the constellations of Taurus and Sagittarius respectively. From the zodiac constellation Gemini to Taurus, the point of the summer solstice moved in 1988. Under the influence of the attraction of the Sun and the Moon, the earth's axis slowly shifts its direction (precession is another astronomical term), as a result of which the points of intersection of the star and the celestial equator are shifted. Spring dates differ from autumn dates, and if September falls on the 22-23rd, then the question "When is the day of the spring equinox?" the answer is March 20. It should be noted that for the Southern Hemisphere, the dates will change places - the autumn will become spring, because everything is the other way around.
The role of the zodiac constellations
As noted above, the equinoxes are the intersection points of the celestial equator with the ecliptic, and they have their zodiac symbols corresponding to the constellations in which there are: spring - Aries, summer - Cancer, autumn - Libra, winter - Capricorn. It should be noted that the time interval between the two equinoxes of the same name is called a tropical year, the number of sunny days in which differs from the Julian calendar by about 6 hours. And only thanks to the leap year, repeating once every 4 years, the date of the next equinox running forward, returns to the previous number. With the Gregorian year, the difference is negligible (tropical - 365.2422 days, Gregorian - 365.2425), because this modern calendar is designed so that even in the long run the dates of the solstices and equinoxes fall on the same numbers. This happens because the Gregorian calendar provides for a pass of 3 days once every 400 years.
