Summer’s slipping away. Back in July, when every day was sunny and hot, many of us couldn’t wait for fall to get here. Our fondest hopes will materialize this Saturday September 22 at 9:49 a.m. (CDT) when autumn finally comes a-knockin’.
Astronomers call the first moment of the new season the autumnal equinox. It’s one of two times a year when the sun’s path intersects with the celestial equator, a projection of Earth’s equator onto the sky. The spring or vernal equinox is the other.
As seen from the equator, where the celestial equator is directly overhead, the sun will be overhead at local noon. People there who look down at their feet will discover they’re standing directly on the shadow of their head! From mid-northern latitudes, the celestial equator arcs approximately midway between the overhead point and the horizon at noon. Up at the north pole the celestial equator it’s a hula-hoop encircling the entire horizon. If you were standing there this Saturday, you’d see the sun circle the horizon for 24 hours straight, never rising higher.
Seasons are caused by the 23.5 degree tilt of the Earth’s axis. As we orbit the sun during the year, the north-south position of the sun changes because of the changing orientation of our axis. When the north polar axis is pointed toward the sun, our star reaches its most northerly point in the sky and we experience long days and summer heat.
During northern hemisphere winter, our axis points away from the sun and our star is southernmost and lowest in the sky. Shorter days and a low sun make for cold weather.
The first day of fall is special because Earth’s axis points neither toward nor away from the sun. Instead, we’re broadside to the sun, and day length is approximately equal to night nearly everywhere across the planet. If you’re into equality of light for all, the equinoxes are your symbols of emancipation.
The word equinox comes comes from the Latin words for equal and night because both day and night are approximately 12 hours long. Prior to September 22, days are longer; after the 22nd they get shorter. Shorter days are caused by the sun dropping farther south in the sky (lower altitude). The lower the sun, the less time it spends crossing the sky and the shorter the hours of daylight.
Interestingly, day and night are not exactly equal at the equinoxes. Yes, it’s true that the center of the sun sets exactly 12 hours after it rises on the first day of fall. Problem is, we determine sunrise at the first sighting of the sun, when its upper edge (not center) breaches the horizon. Similarly, sunset occurs when the last bit of sun disappears below the horizon. That adds about two minutes to daylight’s tally.
We get another few minutes thanks to atmospheric refraction. That’s our atmosphere’s freaky ability to act like a prism and bend the sun’s rays upward into view when it’s still below the horizon. If you’ve ever seen the sun directly on the horizon at sunset or sunrise, you’ve witnessed one of nature’s grandest illusions. The sun’s not really there. The air is thick enough across your sightline to “lift” the sun into view about two minutes before it rises for real.
As astronomer George Greenstein, who worked for years at the Old Farmer’s Almanac, once said: “If the Sun were to shrink to a starlike point and we lived in a world without air, the spring and fall equinoxes would truly have ‘equal nights.'” To whittle away those excess minutes of daylight gained by these parlor tricks, we have to wait until September 25 for day and night to momentarily be equals.
Any planet with a decent amount of axial tilt will experience seasons. How many do? All but Venus, Mercury and Jupiter. Venus’ axis is tipped nearly 180 degrees and rotates backwards compared to the other planets, Mercury’s is 0 degees and Jupiter just 3. Mars’ axis is tilted closest to Earth’s at 25.2 degrees, but since that planet is about 2/3 farther from the sun than ours, its seasons are that much longer.