Knowra Equinox Equinox An equinox is the moment when the Sun crosses the celestial equator, occurring twice each year as Earth orbits the Sun. Around an equinox, day and night are nearly equal in length.
Celestial equator : The projection of Earth’s equator onto the celestial sphere. The Sun crosses this projected circle at each equinox.
March equinox : The annual equinox occurring in March, marking spring in the Northern Hemisphere and autumn in the Southern Hemisphere. It is one of the two yearly crossings that define the equinox cycle.
Solstice : Either of the two annual moments when the Sun reaches its greatest apparent distance north or south of the celestial equator. Solstices mark the extremes of solar declination, unlike equinoxes, when it crosses zero.
Hipparchus : A Greek astronomer of the second century BCE who compiled star positions and discovered the precession of the equinoxes. His observations of equinox positions revealed their slow movement relative to the stars.
Earth's axial tilt : The angle between Earth’s rotation axis and the perpendicular to its orbital plane, about 23.4 degrees. This tilt makes the Sun’s apparent position shift north and south across the celestial equator.
September equinox : The annual equinox occurring in September, marking autumn in the Northern Hemisphere and spring in the Southern Hemisphere. It is the other yearly crossing of the Sun over the celestial equator.
Perihelion : The point in an orbit at which an object is closest to the Sun. Earth reaches perihelion in early January, independently of the equinox dates.
Julian calendar : The calendar introduced by Julius Caesar in 45 BCE, using a leap day every four years. Its year was slightly too long, causing seasonal dates and equinoxes to drift.
Ecliptic : The apparent annual path of the Sun across the celestial sphere, corresponding to Earth’s orbital plane. The ecliptic intersects the celestial equator at the equinoxes.
Tropical year : The time Earth takes to complete a seasonal cycle, measured between successive vernal equinoxes. The equinox provides the reference point used to define this year.
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