Frost on colorful leaves, a sure sign that fall has arrived. Credit: Bob King
Lots of us have looked forward to the first day of fall after summer’s heat and humidity. I like the feel of a sweater on a night’s walk and the fire-hued trees that provide a pleasant distraction while sitting in traffic.
Two views of the sun’s travels along the celestial equator on the first day of fall. The left side shows the view from the equator where the sun passes overhead. The right shows the view from 50 degrees north latitude (S. Canada, Europe). At the north pole, the sun would follow a path along the horizon. Credit: Tau’olunga
The new season begins quietly tonight at 9:29 p.m. Central time. At that moment the sun drops into the southern sky like a penny in a piggy bank and fall officially begins. At the fall and spring equinoxes, the sun crosses the celestial equator, an imaginary extension of Earth’s equator into the sky. Picture the celestial equator as half of an imaginary hula hoop tilting up from the eastern horizon, crossing the southern sky and then arcing back down to the western horizon. The hoop’s other half continues below the horizon and beneath our feet, circling over the opposite hemisphere.
On the first day of autumn, the sun crosses the celestial equator moving south. The sun’s apparent motion across the sky over the year is caused by Earth’s revolution around the sun. Illustration: Bob King
If you live in parts of Kenya, Ecuador or Indonesia the celestial equator passes directly overhead. On the first day of spring and fall, it’s directly overhead at noon.
In mid-northern latitudes it cuts midway across the southern sky. On the first day of spring, the sun crosses this celestial borderline moving north. Seen from the northern hemisphere, the sun’s northward movement brings it higher and higher in the sky. Days lengthen and nights get shorter as we transition from spring to summer.
Tonight, the sun crosses the equator moving south along its yearly circuit of the sky. The southward-moving sun slides lower and lower in the coming days and weeks. We’ll watch daylight diminish at the expense of night as the northern hemisphere makes its transition from fall to winter.
Video to help picture Earth’s orbit around the sun and how its tilted axis causes different parts of the planet to receive different amounts of sunlight during the year. Variation in the amounts of daylight vs. night gives us the four seasons.
Seasons are caused by the 23.5 degree tilt of our planet’s axis. As Earth moves along its yearly orbit, 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.
The first day of fall is special because Earth’s axis points neither toward nor away from the sun. Instead, we’re broadside to our star, and day and night are approximately equal across the planet.
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.
The sun in this sunrise photo is an illusion caused by the thick air at the horizon bending the real sun (still below the horizon) into view. Credit: Lyle Anderson; illustration: NOAA
Notice I didn’t say that day and night are exactly equal at the equinoxes. While it’s true that the center of the sun sets exactly 12 hours after it rises on the first day of fall, 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 of extra daylight to the day.
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 ever have the chance to see the sun directly on the horizon at sunset or sunrise, you’ll witness one of nature’s grandest illusions. The sun’s not really there. The air is thick enough across your sight line to “lift” the sun into view about two minutes before it rises for real.