Do not be fooled, fellow Floridians, by the high temperatures, heartless humidity, and merciless storms – autumn is actually here.
If you need convincing, simply walk into any retail store. There you will find Halloween, Thanksgiving, and Christmas decorations galore. Many retailers are on alert for the 2017 holidays.
Clerk: (Out of breath) “Sir, we just sold our last Iron Man mask and it’s still two weeks until Halloween!”
Manager: (Panicking) “Two weeks? Condition Cadbury! Roll out the Peeps ASAP!”
Only in America.
But marketing portents aside, there are celestial clues that autumn is here. The days have been getting shorter, the Sun’s path in the sky is lower to the ground, and the familiar seasonal constellations revisit us, just like old friends.
September 22 was the Autumnal Equinox – the first day of autumn in the Northern Hemisphere. This day represents one of only two days in a year when the Sun is on the intersection of the Ecliptic and the Celestial Equator.
Fret not, dear readers, we will sort through this.
As Earth orbits the Sun, it carves out a plane called the Ecliptic. Of course, from our terrestrial point-of-view, it appears to be the Sun revolving around us and tracing out this plane. But in science, and astronomy in particular, we must step back and consider systems from a larger perspective.
The Celestial Equator is merely the projection of Earth’s equator into space, creating another plane.
These two planes are not parallel; that is, they do not overlap each other. Earth’s rotation axis is tilted about 23.5 degrees off our orbital plane. Therefore, the Celestial Equator is tilted 23.5 degrees with respect to the Ecliptic.
It follows, then, that as Earth orbits the Sun, there is a period when one hemisphere is tilted toward the Sun while the other hemisphere is tilted away. As the year goes on, this reverses.
It is this tilt, and not our proximity to the Sun, that gives us our seasons. In fact, during the summer months here in the Northern Hemisphere, Earth is actually farther from the Sun than during the winter months. But we are tilted toward the Sun, giving us longer days and more intense sunlight.
This tilt of Earth is evident not only in our seasons but on our globes as well. As Earth travels in its orbit, we see the position of the Sun change a little each day, moving either up or down in latitude. Two of these latitude circles represent the Sun’s maximum and minimum positions.
To begin our examination, let’s go back in time almost three months, to the first day of summer and the longest day of the year: The Summer Solstice.
On this day, which fell on June 21 this year, the Sun was highest in the sky in the Northern Hemisphere. Because of Earth’s tilt, this meant that the Sun was at a northern latitude of 23.5 degrees. The Sun was directly overhead at noon on this great circle of our globe. This latitude is called the Tropic of Cancer.
Similarly, six months later, the Sun will be at its lowest point, 23.5 degrees south of the equator. This latitude circle is called the Tropic of Capricorn. Here in the Northern Hemisphere we experience the shortest day of the year and welcome the start of winter. It is the Winter Solstice and a time of many seasonal celebrations.
The equinoxes are the two points in between the solstices, when the Sun’s trek has it transitioning from winter to summer or summer to winter. On an equinox, the Sun is directly over the equator and we have approximately the same hours of day and night. Indeed, the word “equinox” actually means “equal night.”
We should note that these observations are biased toward an observer in the Northern Hemisphere. To our fellow Terrans in the Southern hemisphere the seasons are reversed.
For billions of years the Sun has followed this path in the sky, going from one Tropic Circle to the other and back again. And although we can observe and record this motion of the Sun, we must never forget that it is actually Earth that is moving, not the Sun.
So now we better understand the astronomy involved with the Autumnal Equinox and the changing of the seasons. But is there anything you should be doing?
Not really. These four dates on the calendar represent geometric alignments, not events to stay up and look for.
Until next time, remember to always question and always wonder.
Tom Webber teaches AP Physics at Oakleaf High School and worked in the planetarium field for nearly 20 years, including Jacksonville’s MOSH. He has also taught physics and astronomy at both the secondary and collegiate levels.