In spite of recent cool spells, it will not be until Saturday that fall begins. On that day, Earth reaches the point in its orbit that we call the autumnal equinox. Earths daily rotation axis will be perpendicular to the direction to the sun we will be in rotisserie mode, the sun directly overhead at noon for those on the equator.
Everyone on the planet will have a 12-hour, equal day and night, hence equinox. For those of us in the Northern Hemisphere, our days will begin to shrink and our nights lengthen as we head to winter. This variation in the amount of time basking in the sunlight is one of the causes of the seasons.
The other main reason is due to our rotisserie axis not being at a right angle to our motion around the sun: It is tilted about 23 degrees, and that axis is increasingly tilted away from the direction to the sun for us as we approach winter.
But, the effect of that tilt may not be what you think it is.
Before we explored the topic, I asked my astronomy students what they thought the main cause of the seasons was. A show of the simple, colored, ABCD answer cards we use revealed a veritable rainbow of confusion. I let them then take a minute to try to convince their nearby seatmates that they were right, and took a revote. They managed to get worse, with almost all choosing that the tilt nodded us closer to or farther from the sun.
While true, this few-hundred-mile difference is insignificant as a fraction of the 93-million-mile distance to the sun.
The real effect of having the sun range high and low up in the air is to either concentrate its beams or spread them out over more area.
Shine a flashlight straight at a wall and then at a shallow angle to it to see the effect.
Our nodding back and forth does have practical applications. A combination of south-facing windows and correctly designed roof overhangs can provide significant passive solar heating in winter and keep the sun out during summer. Unfortunately, our houses are mostly aligned to the sacred street, not the sun.
Surveys show that many Americans do not understand this simple science behind the cause of the seasons. I hope that now, at least, you do!
Daniel B. Caton is a physics and astronomy professor and director of observatories at Appalachian State University. Email: firstname.lastname@example.org. More on this months column: www.upintheair.info.