Autumn Begins with the Equinox (Northern Hemisphere)
Equinox means “equal night,” and this is supposed to be the date when day and night are equal lengths. (Depending on where you live, the day and night aren't quite equal.) In the Northern Hemisphere, fall begins on this day, and in the Southern Hemisphere, spring begins.
The Equinox occurs at 3:09 a.m. UTC (Coordinated Universal Time). For people in the U.S., that is yesterday evening. Coordinated Universal Time is a time standard that can be referred to worldwide. It is based on International Atomic Time and is roughly the same time as Greenwich Mean Time, which is the time kept at the Royal Observatory, Greenwich, England.
Again speaking from the perspective of the Northern Hemisphere, the days have been getting shorter, and the nights longer, ever since the first day of summer. They will continue to get shorter and shorter until mid-December. Today is the mid-point in the process.
The shorter days are the main reason that winter is colder than summer. This seasonal change is caused by the Earth's tilt.
Balance an egg on its end...
Even though it has been disproved over and over (and over!), there persists a myth that you can balance an egg on its end only during an equinox, when everything is balanced and equal.
It's not a myth because it is impossible to balance an egg. It's really hard to do it—it takes a lot of patience—but you most definitely can balance an egg on its end.
However, you can balance an egg on its end any day of the year. The myth is that it can only be done during an equinox.
Check out the Moon and Jupiter!
If you have clear skies tonight, be sure to check out the full harvest moon! Also, Jupiter is closer than usual, and so Jupiter is probably brighter than you've ever seen it. (It hasn't been this close to Earth since 1963, and it won't be again until 2022.)
I've sighted Jupiter for two nights, now. It has been rising in the east at about sunset, a little later than Moonrise. Once Venus sets, it seems to be the brightest “star” in the sky. Of course, it isn't a star at all; it's the solar system's biggest planet. With a really good pair of binoculars, and if the sky is really dark, you might be able to see one or more of Jupiter's moons, maybe even a dark band across its disk, and maybe even Uranus looking like a blue-colored star near Jupiter.