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What exactly is an equinox, anyway?

As Sunday ushered in the first official week of autumn, with the autumnal equinox setting Earth's Northern Hemisphere on a path to shorter days and colder temperatures. But what does the equinox really signify, and what's actually happening in the night sky? We break it down for you.

By Contributor / September 23, 2013

New York's Lower Manhattan skyline is seen the distance as a woman flies a kite in a park along the Hudson River in Hoboken, New Jersey on September 3. The summer drew to a close when fall began with the Autumnal Equinox this past Sunday.

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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.

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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. Although many early civilizations are thought to have studied it, the ancient Greek astronomer Hipparchus of Rhodes is a formative figure in the discovery of our planet's slow wobble.

Hipparchus compared his observation of the stars with information from 150 years earlier, noticing that the stars had appeared to have moved by a small degree. 

In 1687, Isaac Newton explained that axial precession was the result of the Moon's gravitational pull on the Earth. 

Can I balance an egg on the equinox?

A widely circulated Internet myth stemming from an old Chinese ritual claims that an egg can be perfectly balanced upright on the day of an equinox. The phenomenon is usually attributed the gravitational pull of the Sun during the equinoxes. 

Don't believe it. The Earth is closest to the sun in January, and it has no effect on the kinematics of eggs. In fact, with enough hard work and patience, you can balance an egg on any day of the year.

Will Earth's precession affect the seasons?

The Earth orbits the sun in a nearly perfect circle. Nearly. It is actually about 3.3 percent closer to the sun in January than it is in July.

As NASA explains, as long as our calendar remains fixed to the seasons and the equinoxes – that is, as long as we stay on top of our leap years – in 13,000 years summer should still be occurring in June. But because the Earth will have completely tilted the other way, the Northern hemisphere will experience summer when the whole world is closest to the sun, and winter when the whole world is farthest from the sun. 

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