Newspaper article The Christian Science Monitor

What Exactly Is an Equinox, Anyway?

Newspaper article The Christian Science Monitor

What Exactly Is an Equinox, Anyway?

Article excerpt

Sunday's equinox marked the transition from summer to autumn in the Northern Hemisphere and from winter to spring in the Southern Hemisphere. Here, we answer your most pressing equinox-related questions.

What is the equinox and how does it work?The term "equinox" derives from Latin, literally meaning "equal night." On the equinox, which occurs twice a year in March and September, there are roughly the same number of hours in the day and night throughout the entire world.

Earth moves three different ways. It spins around completely every 24 hours or so. It orbits the sun in about 365 days. And, like a spinning top, it wobbles around on its axis, making a complete rotation every 26,000 years. Our planet is actually tilted at an angle of 23.4 degrees toward a certain point in the sky called the celestial pole. As the Earth makes its yearly orbit, one hemisphere faces the sun more than the other.

This causes the change in seasons: The Northern hemisphere experiences winter when it is tilted away from the sun, while the Southern hemisphere has its summer because it's facing the sun.

But on two days a year, the Earth is not tilted toward or away from the sun at all: These are the equinoxes.

In the Northern Hemisphere, the September equinox marks the first full day of autumn. This means the Earth is halfway between the summer and winter solstices, the points at which the sun appears in the sky farthest from the equator. Beginning with the June solstice, the North Pole gradually points away from the Sun until the December solstice, at which point it starts pointing toward the sun again.

In the Southern Hemisphere, the September equinox introduces the first day of spring, as the South Pole begins angling closer to the Sun until their summer solstice hits.

Who first discovered the wobble?

The scientific term for it is axial precession. …

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