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What is an equinox and how does it work, asks Justin Parkinson. The name equinox means "equal night" in Latin. It's theoretically the day of the year when all points on the earth's surface experience the same lengths of daylight and darkness - 12 hours of each. The autumn equinox in the northern hemisphere it's the spring equinox for the southern hemisphere always falls on 22, 23 or 24 September. Imagine the earth orbiting the sun in a flat plane - ie as if it was pinned on the edge of a slightly elliptical disc.
And, because of how sunrise and sunset are defined, the equilix occurs a few days before the spring equinox and a few days after the autumn equinox, according to the U. Equinoxes don't necessarily occur on the same day each year, rather they happen around or on March 20 and Sept.
These shifting dates are because an Earth year is not exactly days: There is an extra quarter of a day 6 hours that accumulates each year, causing the date of the equinox to shift.
The planet's orientation towards the sun is also constantly shifting, tweaking the timing of the equinox. Related: Why the autumnal equinox doesn't fall on the same day every year.
The equinoxes mark the astronomical beginning of spring or autumn, depending on the hemisphere. However, the meteorological beginning of these seasons is March 1 and Sept. In the Northern Hemisphere, the March equinox heralds the beginning of spring, and is referred to as the spring or vernal equinox vernal comes from the Latin term "ver" for spring. At the same time, the Southern Hemisphere shifts into autumn. The converse is true in September, when the northern half of the planet descends into the colder months of autumn and the southern half enters spring.
Earth is not alone in experiencing equinoxes. In fact, every planet in the solar system has them when the planet's orbit and tilt with respect to the sun result in both hemispheres receiving roughly equal amounts of light. People have been tracking the sun's movements for thousands of years, often incorporating equinoxes into cultural and religious traditions. For many ancient civilizations these solar changes not only dictated the beginning of seasons but also when to plant and harvest crops.
In Japan, both equinoxes are public holidays traditionally recognized as a day to remember and worship ancestors and loved ones that have died, according to the Coto Japanese Academy. Related: Milky Way glimmers over gorgeous landscape during the spring equinox photo.
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