The first chapter of Genesis identifies the sun, the moon, and the stars as useful lights in the sky. One of the cited benefits (see verse 14) is that they provide signs concerning the seasons.
Indeed, every year the earth experiences four specific astronomical events that delineate the changing seasons. There are two solstices, when the sun reaches its northernmost and southernmost points and seems to linger momentarily before changing direction. The solstices mark the beginnings of summer and winter. The earth also annually experiences two equinoxes. One marks the beginning of spring (the vernal equinox) and the other marks the astronomical beginning of fall (the autumnal equinox).
If the earth stood straight up and down as it rotated on its axis, these seasonal changes would not occur because each spot on the globe would get the same amount of sunlight all year long. The earth doesn't stand straight, however. It is tipped 23½ degrees from vertical. This causes the orientation of northern and southern hemispheres to change throughout the year. When the northern hemisphere leans toward the sun, it receives more light and experiences summer. Six months later, when the southern hemisphere is aligned to receive more direct sunlight it experiences summer.
An equinox occurs when the earth and sun are aligned in a way that the sun seems to stand directly over the equator, and the northern and southern hemispheres receive equal amounts of sunshine. Days and nights differ in their lengths at all other times of the year, with one longer than the other depending on the season. But at an equinox, the day and night are essentially the same length. In fact, the word equinox is derived from Latin roots that mean "equal" (equi) and "night" (nox).
Astronomers can calculate the precise moment of an astronomical equinox, but because it involves the relative positions of the earth and the sun in space, it occurs for the entire planet at a specific instant irrespective of local time. As a result, you may have heard conflicting information about when the autumnal equinox will occur this year. I have a wall calendar hanging in my office that tells me it will be September 23. The appointment calendar on my desk concurs, yet I've seen other calendars insist that it will be the 22nd.
As it turns out, both dates are correct within the appropriate context.
The autumnal equinox will occur on September 23, 2014 at 2:29 a.m. UTC. UTC stands for Coordinated Universal Time. For English speakers, the initials seem out of order. This is the result of a multilingual compromise adopted to facilitate the worldwide use of a shared abbreviation. (Read more about it from Time and Date.)
UTC marks a zero hour at a specific longitude identified as the prime meridian, which is based on the location of the Royal Observatory at Greenwich, England. It serves as a common reference from which time zones around the globe are calculated. On the west coast of the United States, Pacific Standard Time is officially measured as UTC minus eight hours. UTC does not change throughout the year, but many places seasonally adjust their local time by varying their relationship with UTC. For example, Eastern Standard Time in the U.S. is UTC minus five hours; Eastern Daylight Time is UTC minus four hours.
At the specific moment of the coming equinox, 2:29 a.m. UTC, the date in Europe will indeed be September 23, 2014. Where I live in the United States, however, clocks measuring local time will still be pointing to the hours before midnight. In my time zone, the equinox will occur when my clock says 10:29 p.m., and my calendar will still read September 22, 2014.
Although the equinox can be precisely defined as an astronomical event, other aspects of nature present seasonal changes more gradually. In my neighborhood, summer has been loosening its grip for several weeks now. Many birds have taken flight toward warmer climates, and trees have begun shedding leaves. I can imagine that in the southern hemisphere, people are already seeing signs of the budding spring.
Watching the seasons change may be one of humanity's oldest habits. Yet, some aspects of modern life make the passage of seasons seem less relevant. If it's colder outside, just crank up the furnace. If it's darker earlier, turn on the lights. Does sunrise happen too late? Adjust your clocks and "fall back" to standard time.
How important these changes are may be a matter of viewpoint. Whether things will get colder and darker or warmer and lighter is a matter of local perspective. Whether the season turns gradually or in an instant and whether it changes tomorrow or today may depend entirely on where you stand. But seasons do change. And, as the preacher of Ecclesiastes noted, "To everything there is a season, and a time to every purpose under heaven."