Equinox Arrives September 22nd

Why is the time of the equinox so specific? S&T's editors explain.

For those of us already seeing blushing foliage or feeling a chill in the air, it might seem as though autumn has already arrived. But astronomically speaking, fall officially comes to Earth's Northern Hemisphere at 20:44 Universal Time on Sunday, September 22, 2013. At that moment, the Sun's path crosses Earth's equator heading south, an event called the autumnal equinox.

Why do we say summer ends and fall begins at an exact moment, when the natural events happen gradually? Because the four seasons many of us use — winter, spring, summer, and fall — have beginning and ending points defined as actual key moments in the Earth's annual orbit around the Sun — or equivalently, from our point of view, the Sun's annual motion in Earth's sky.

The Sun appears farther north or south in our sky, depending on the time of year, because of what some might consider an awkward misalignment of our planet. Earth's axis is tilted about 23½° with respect to our orbit around the Sun. That means that the plane drawn by Earth's orbit, called the ecliptic, is tilted with respect to the planet's equator. And because from our perspective the Sun follows the ecliptic in its path through the sky over the course of a year, each day the Sun's highest point in the sky moves depending on the time of year. For a skywatcher at north temperate latitudes, such as in the continental United States, the effect is to make the Sun appear to creep higher in the sky each day from late December to late June, and back down again from late June to late December. An equinox comes when the Sun is halfway through each journey.

The Earth's axial tilt is why we have seasons. When the planet is on one side of its orbit, the Northern Hemisphere is tipped sunward and gets heated by more direct solar rays, making summer. Six months later, when the planet is on the opposite side, the Northern Hemisphere is tipped away from the Sun, and the slanting solar rays heat the ground less, creating winter.

This celestial arrangement makes several other noteworthy things happen on the equinox date:

• In the Southern Hemisphere, September's equinox marks the start of spring, and the March equinox marks the start of fall.
• Day and night are almost exactly the same length; the word "equinox" comes from the Latin for "equal night." (A look in your almanac will reveal that day and night are not exactly 12 hours long at the equinox, for two reasons: First, sunrise and sunset are defined as when the Sun's top edge — not its center — crosses the horizon. Second, Earth's atmosphere distorts the Sun's apparent position slightly when the Sun is very low.
• The Sun rises due east and sets due west (as seen from every location on Earth). The fall and spring equinoxes are the only times of the year when this happens.
• If you were standing on the equator, the Sun would pass exactly overhead at midday. If you were at the North Pole, the Sun would skim around the horizon as the months-long polar night begins. Richard E. Byrd wrote eloquently in his 1938 book Alone of the Sun as it dove into the long Antarctic night as seen from Advance Base, 80°08'S, 163°57'W:

  Huge and red and solemn, it rolled like a wheel along the Barrier edge for about two and a half hours, when the sunrise met the sunset at noon. For another two and a half hours it rolled along the horizon, gradually sinking past it until nothing was left but a blood-red incandescence. The whole effect was something like that witnessed during an eclipse. An unearthly twilight spread over the Barrier, lit by flames thrown up as from a vast pit, and the snow flamed with liquid color.