The seasons will change this Sunday (Sept. 22), with the Northern Hemisphere moving into autumn and the South emerging from winter into spring. The celestial event that marks this transition is called an "equinox," and it happens twice every year, around March 21 and Sept. 21. Just what is an equinox, and why does it occur? The Earth moves in two different ways. First, the planet spins on its polar axis — a line through the north and south poles — once every 24 hours, causing the alternation of day and night. Secondly, it moves in its orbit around the sun once every 365.25 days, causing the annual cycle of seasons. The equinox occurs when these two motions intersect.
Because the Earth is so big, its mass has an enormously powerful gyroscopic effect. For this reason, its poles always point in the same direction, although a major earthquake can cause tiny wobbles in this axis. Most importantly, the Earth's motion around the sun has absolutely no effect on the direction the poles are pointing, which has very important consequences for Earth's seasons.Astronomers mark the positions of objects in the sky relative to the Earth's poles of rotation (those are the red lines you see in the picture). The most important line is the celestial equator, which divides the sky into the Northern and Southern Hemispheres.
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Slide 1
The seasons will change this Sunday (Sept. 22), with the
Northern Hemisphere moving into autumn and the South emerging from
winter into spring.The celestial event that marks this transition
is called an "equinox," and it happens twice every year, around
March 21 and Sept. 21. Just what is an equinox, and why does it
occur?The Earth moves in two different ways. First, the planet
spins on its polar axis a line through the north and south poles
once every 24 hours, causing the alternation of day and night.
Secondly, it moves in its orbit around the sun once every 365.25
days, causing the annual cycle of seasons. The equinox occurs when
these two motions intersect.
