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Why Does the Autumnal Equinox Fall on Different Dates?
Credit: NASA

A New Year's baby will always have a New Year's birthday. But a child born on the autumnal equinox may have to wait a few years, or even 372 years, before her birthday falls on the equinox again.

The autumnal equinox usually falls on either Sept. 22 as it does this year or Sept. 23, which it fell on in 2007 and 2006.

But in 1931, the autumnal equinox was on Sept. 24, and that won't happen again until 2303.

So why does the autumnal equinox fall on different days? The short answer is that the Gregorian calendar, which most of the world uses today, is imperfect. Although it does a better job of keeping the equinoxes around the same date than ancient calendars did, its days and months don't precisely represent the position of the Earth in its orbit around the sun. In other words, the Earth isn't in the same spot every year on a given date.

The equinox would fall on exactly the same day each year if Earth completed its orbit in exactly 365 days. But it actually takes about 365.25 days. The extra quarter of a day means that the equinoxes occur 6 hours later each year, which pushes the date the equinox falls on around from year to year.

How it works

An equinox occurs the moment the Earth reaches the point in its orbit where the sun seems to move from being directly over a point just north of the equator to being directly over a point just south of the equator. Night and day are roughly equal lengths 12 hours long across the planet on the day of the equinox, according to the National Oceanic and Atmospheric Administration (NOAA).

Of course, the sun isn't really "moving" from one hemisphere to another.

Because the Earth tilts on its axis at an angle of 23.5 degrees, the Northern and Southern hemispheres move in and out of the sun's direct rays as the Earth revolves around the sun. Because direct sun rays heat up Earth more (hello, summer!) than indirect rays do (hello, winter!), the tilt causes the seasons.

From our perspective on Earth, the tilt results in the sun rising and setting in an ever-higher arc during the spring. The arc is highest on the summer solstice, and then the sun rises and sets in an ever-lower arc until reaching the lowest point, during the winter solstice.

The equinox happens twice along the earth's orbit once in September and once in March. People in the Northern Hemisphere call the September equinox the "autumnal" equinox (and the March equinox the "vernal" equinox, from the Latin word for spring). However, the September equinox marks the beginning of spring for the Southern Hemisphere.

Equinox trouble: Creating a calendar that's good enough

Many cultures over human history have come up with ways to account for this extra quarter day in Earth's 365.25-day orbit and some did better than others.

Ancient Romans developed the Julian calendar, which was widely adopted across Europe until the 1500s. (Year 1 in the Julian calendar was what we now call 46 B.C.) But the Julian calendar included 12 months during some years and 13 months during others, resulting in a system in which equinoxes arrived earlier each year.

When the Julian calendar was introduced, the vernal equinox fell on March 25. By the year 1500, it had moved to March 10 or 11, and the autumnal equinox fell on Sept. 14, according to the Galileo Project at Rice University.

Because the Catholic Church used the equinox to calculate the celebration of Easter and develop its religious calendar, religious leaders wanted a better system.

In 1582, Pope Gregory XIII introduced what is now called the Gregorian calendar to replace the Julian calendar, according to the Galileo Project.

The Gregorian calendar dropped 10 days in October and used leap years an extra day every four years to account for the extra quarter day each year. Additionally, the new calendar dropped three leap years every 400 years to keep the equinoxes at approximately the same date each year.

Under the Gregorian calendar, the dates of the autumnal equinox repeat a pattern every 400 years.

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