#equinox

A year astronomically speaking is defined as the time span between the spring equinox and the next spring equinox. This definition brings up a big question though; how the Hell did ancient people know abouts years without possessing modern astronomical knowledge?

The reason why a year was and still is such a well known concept even before modern astronomy is because the effects of a rotation around the Sun are very noticeable, and only gets more noticeable the more you pay attention.

Every year a cycle of 4 days is repeated as Earth (roughly) approaches a quarter of the way through its orbit.

March Equinox

June Solstice

September Equinox

December Solstice

The equinoxes are the days where the Sun appears to be directly overhead the equator; these days also have (roughly) equal lengths of day and night. The solstices are the days where the Sun reaches its most northernly or southernly point as it moves in the sky; at this point it almost looks like the Sun stands still before moving along again. The solstices usually have either the longest day or longest night.

The March equinox is the vernal one in the Northern hemisphere and the autumnal one in the Southern Hemisphere. The seasons are reversed in the hemispheres.

A diagram showing the (Northern) seasons as it relates to the equinoxes and solstices is given below:

The equinoxes and the solstices were really not that all that important to ancient peoples on their own but they did act as handy and observable moments which could be used to gauge when a solar year starts, ends, or enters the next season.

The solar year was important because the weather and daylight changed as the seasons progressed, which meant that agricultural societies had a vested interest in keeping tabs on when their crops were going to be buried in snow or die from overheating.

The cycle of the seasons was constant and predictable meaning that it would be very useful to keep track of when a solar year starts and ends.

As it turns out, the distance between an equinox and the equinox next year is 365.24217 (solar) days. This fact is why so many calendars of various cultures and peoples tend to set their years to ~365 or 366 days; the Egyptians divided the year into twelve 30 day months with an additional 5 or 6 days which are part of a 13th tiny month.

Nevertheless, a 365 day calendar year is very close to a solar year of 365.24217 days, but a 4 calendar year period of 1460 days is slightly off from the 1460.96868 or ~1461 days in 4 solar years.