Yes, the rumors are true: I am a transplanted Yankee.

Eeek!

Alas, it is true, having spent the first 25 years of my life north of the Mason-Dixon line. I have shoveled snow on Halloween, dodged icicles falling from the gutters, and sat in a running car for 10 minutes waiting for it to warm up.

Good times.

With the seasonal changes being much more abrupt up north, calendars weren’t really necessary – you simply had to pay attention to your surroundings. Take Autumn, for example. There would be a crispness in the air, the leaves would change color, rakes would replace lawnmowers and firewood would be brought in.

Contrast this to Autumn in Florida. The temperature is still in the 80s and 90s, palm trees don’t change color nor drop their leaves, many people don’t even own rakes, and fireplaces are simply places to hold the TV.

But whether you call the North or the South home, there are two things we can agree upon this time of year: Football season is underway and the days seem to be getting shorter.

The former is a cherished way of life, the result of decades of sis, boom and bah. The latter is based on physics and closely related to why we even have seasons at all.

Let’s take a look.

The beginning of Autumn is not tied to the gridiron, the start of school, or even the Labor Day weekend. In fact, this year the first day of Autumn was Sept. 22, at 9:54 p.m. to be exact. This date and time has nothing to do with holidays or fiscal calendars.

As Earth revolves around the Sun over the course of a year, it carves out a plane. From our terrestrial point-of-view, however, this plane would seem to be defined by the motion of the Sun around the Earth, creating a great circle on the Celestial Sphere called the ecliptic.

One of the challenges in studying science at any level is being able to consider models from a different or even larger (or smaller) perspective.

The equator of Earth does not stay parallel with the plane of the ecliptic; that is, Earth is actually tilted off its orbital plane by about 23.5 degrees. It is this tilt that gives our planet its seasons.

There are two dates when this tilt is the most extreme. The first is the Summer Solstice, around June 21. Here, our Northern Hemisphere is tilted toward the Sun, we receive more direct sunlight, and the arced path of the Sun is highest in the sky. Summer has officially begun.

On Dec. 21 – the Winter Solstice – our Northern Hemisphere is tilted away from the Sun, we receive less direct sunlight, and the path of the Sun is found low in the sky. It is the first day of Winter, and we prepare for the colder months that follow.

There are two other dates of interest between the Summer and Winter Solstices linked to Earth’s tilt with respect to the Sun. They represent the midpoint of the Sun’s apparent altitude between its maximum arc on June 21 and its minimum arc on Dec. 21.

These two dates are called equinoxes, literally “equal night.” The Sun is directly over the equator at noon and we have equal hours of day and night.

When the Sun is heading northward over the equator, it is the Vernal Equinox, or the first day of Spring, approximately March 21. When the Sun’s trek is southward, it is the Autumnal Equinox, which tends to fall around – yep, you guessed it – Sept. 21.

So even though there has been over a month of football and the days seem to have been getting shorter ever since the end of June, the official start of Fall wasn’t until Sept. 22 this year. Judging from the Halloween, Thanksgiving, and Christmas decorations that have been littering stores since August, this could have been an easy fact to overlook.

Sis, boom, bah – only 88 shopping days until Christmas!

Thomas Webber teaches AP Physics at Oakleaf High School and worked in the planetarium field for nearly 20 years, including at MOSH, and has also taught physics and astronomy at both the secondary and collegiate levels.