ROCHESTER, N.Y. (WROC) — Thursday we made the official transition away from summer to fall! This year the transition is made on September 22 at approximately 9:04 p.m. We call this the fall or “autumnal” equinox which is defined by the approximate moment the sun’s rays cross directly over the celestial equator.

Why do we have seasons?

The reason for the seasons here on Earth is because of Earth’s tilt on its axis. It actually has very little to do with how far away or how close the Earth gets to the sun while on its orbit.

In fact, during northern hemisphere winter, the Earth can be found at its closest to the sun during its orbit around the sun. However, since the Earth is tilted away from the sun during that time we get the least amount of sunlight and therefore is our coldest time of the year.

Solstice vs. Equinox

Technically, the start of each season is defined by specific dates and times when the Sun crosses either the celestial equator, called an equinox, or when it reaches its lowest or highest point in the sky, called a solstice. The first day of spring and fall are referred to as an “equinox” while the first day of winter and summer are referred to as the “solstice.”

The phrase “equinox” comes from the Latin translation “equal night.” On the fall equinox, the earth is tilted neither towards nor away from the sun, which results in “nearly” equal daylight and darkness on Earth. I say “nearly” because there are some discrepancies in that logic.

A true equinox would contain precisely 12 hours of the day and 12 hours of the night, but we typically don’t find the most equality between the two times of day until a few days after the equinox actually occurs. This is due to a process known as refraction, which is the bending of light due to differences in density found within the earth’s atmosphere.

As the sun’s light travels through the air of increasing density it decreases the velocity (speed) at which light travels. Because of this, we’re actually able to visibly see the sun minutes before the sun actually rises above the horizon. So while the sun’s rays hit us the most directly on the equinox itself, the closest we get to equal parts day and night happens a few days later on September 25 with 12 hours and one second of daylight.

Why do the seasons fall on different days each year?

The reason why the start of each season varies by a couple of days and occurs at different times of the day can be attributed to the way we as a society measure a year.

There are 365 days in a calendar year, but there are technically 365.25 days in a year since it takes the Earth that long to make one revolution around the sun. This creates some wiggle room on the timing of important events such as seasons, and certain holidays, and is also the reason we have a leap year every four years; to account for the extra day we end up accumulating on our calendar.

NOTE: If you’ve ever heard of the myth that you can balance an egg on its side during the equinox, it’s unfortunately not true.

Don’t believe me? Look it up for yourself! I even gave it a try a year ago as of this writing…

~ Meteorologist Christine Gregory