Equinox
Etymology: The Latin words “aequus,” which means “Equal or Level,” and “nox,” which means “night,” are the origins of the word “equinox.” This is referring to the day and night of the equinox being roughly equal in length.
Astronomy
The solstices mark the maximum points on your local horizon where the Sun will rise and set. In December, it’s at its furthest south. In June, it rises and sets furthest north. The further away from the equator one is the more extreme the distance between these rising points. Finally, above latitudes of about 60 degrees North and South, the Sun refuses to rise or set at all for weeks at a time.
In December, the Sun will rise and set far south, and stay low in the sky, creating short days in the Northern Hemisphere. The farther north a planet rises and sets, the higher it will culminate. It’s opposite in the Southern Hemisphere.
The equinoxes occur when the Sun, during its apparent path around the Earth, crosses the disc of the equator projected out into space travelling northwards at the spring equinox and in a southerly direction at the autumn equinox.
At each one the number of daylight hours is the same at any point on the earth's surface..
The two equinoxes are the vernal equinox (around the 21st of March) and the autumnal equinox (around the 23rd of September). The former marks the beginning of spring and the latter of autumn in the northern hemisphere. (The seasons are reversed in the southern hemisphere.)
The time at which the Sun forms the greatest angle to the equator – from both the North and the South – is called the solstice.
There’s a direct relationship between Declination and the zodiac. After all, the ecliptic (path of the Sun) crosses the equator (0 degrees of declination) twice a year – when the Sun is either 0 degrees of Aries or Libra. In fact, the very definition of the equinox is when the Sun crosses the equator! At the June solstice, the Sun is at 0 degrees of Cancer, and at the December solstice, it’s at 0 degrees of Capricorn, and these turning points are as far away from the equator/0 degrees of declination as can be. This is the foundation and the legitimacy of the Tropical Zodiac.