Tropical year

The average interval for the Sun’s apparent position to return to the same point relative to the equinoxes. It tracks Earth’s seasonal cycle and is slightly shorter than a sidereal year.

March equinox: The moment the Sun crosses the celestial equator moving northward, marking spring in the Northern Hemisphere. The tropical year can be measured between successive March equinoxes.
Gregorian calendar: The civil calendar introduced in 1582, using leap years to keep calendar dates aligned with the seasons. Its average year closely approximates the tropical year.
Equinox: Either of the two points where the Sun’s apparent path crosses the celestial equator, or the dates when it reaches them. An equinox supplies the reference point for defining a seasonal year.
Hipparchus: A Greek astronomer of the second century BCE who developed precise methods for measuring celestial positions and motions. His comparison of equinox observations revealed the slow shift later called precession.
Season: A recurring division of the year associated with characteristic patterns of daylight, temperature, and weather. The tropical year measures the cycle whose timing defines the seasons.
September equinox: The moment the Sun crosses the celestial equator moving southward, marking autumn in the Northern Hemisphere. It is another seasonal marker used to define an equinox-to-equinox year.
Julian calendar: The calendar introduced by Julius Caesar, with one leap day every four years. Its average year is too long to stay aligned with the tropical year.
Celestial equator: The projection of Earth’s equator onto the celestial sphere. The Sun’s crossings of this great circle mark the equinoxes.
Ptolemy: A second-century CE astronomer whose Almagest systematized ancient mathematical astronomy. His account preserved ancient measurements of the equinox shift and year lengths.
Axial tilt: The angle between a body's rotational axis and the perpendicular to its orbital plane. Earth’s tilt causes seasonal changes as the planet completes its tropical year.

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