March equinox

The instant each year when the Sun crosses the celestial equator moving northward. It marks spring in the Northern Hemisphere and autumn in the Southern Hemisphere.

Celestial equator: The projection of Earth’s equator onto the celestial sphere. The Sun crosses this projected line moving north at the March equinox.
Tropical year: The time between successive passages of the Sun through the vernal equinox. It measures the seasonal cycle that the March equinox begins.
Vernal point: The point where the Sun crosses the celestial equator northward, used as the origin of right ascension. Astronomers use the March equinox crossing to define this coordinate reference.
September equinox: The annual instant when the Sun crosses the celestial equator southward. It is the opposite-direction equinox, bringing autumn north and spring south.
Ecliptic: The apparent annual path of the Sun across the celestial sphere. Its intersection with the celestial equator defines an equinox.
Gregorian calendar: The internationally used solar calendar introduced by Pope Gregory XIII in 1582. Its leap-year rules keep calendar dates aligned with the seasonal year.
Right ascension: An angular coordinate that locates objects eastward along the celestial equator. Its zero point is tied to the vernal point.
June solstice: The annual instant when the Sun reaches its greatest northern declination. It marks the Northern Hemisphere’s summer peak, unlike the equinox’s balanced sunlight.
Earth's axial tilt: The angle between Earth’s rotational axis and the perpendicular to its orbital plane. The tilt produces seasons as Earth orbits the Sun.
Julian calendar: A solar calendar introduced by Julius Caesar in 46 BCE, with a leap day every four years. Its year length drifts against the equinox more quickly than the Gregorian year.

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