Earth's most recent equinox occurred today, September 24, at 9:51 UT (5:51 am EDT). Equinox is the time when the plane sun (the ecliptic) crosses the earth’s equator (also known as the celestial equator). At this time night and day are approximately equal in length. The word equinox literally means “equal night,” suggesting the thought that the length of night and day are equal. Equinox occurs twice each year, or twice in each orbit of the earth. The approximate dates each year are March 21 and September 22. For the hemisphere into which the sun crosses, it is the beginning of spring (the spring or vernal equinox), and for the hemisphere out of which the sun leaves, it is the beginning of autumn, or fall (the autumnal equinox). In the northern hemisphere this year’s autumnal equinox occurred today.
Many people still think that earth’s seasons are caused by changes in the distance of earth around the sun over the course of the year. This is just not the case. While it is true that the orbit of earth is elliptical and that its distance from the sun changes during the year, the difference between the distances is not significant enough to make a difference. For example, this year earth’s closest point to the sun, or earth’s “perihelion,” was January 3 at about 20:00 UT, when the earth-sun distance was 147,093,602 km, and earth’s farthest point from the sun, or earth’s “aphelion,” was July 7 at about 00:00 UT, when the earth-sun distance was 152,097,053 km. The difference between these two is 5,003,451 km, or a 3.3 percent difference.
The phenomenon of seasons (spring, summer, autumn, and winter) is the result of two effects: the tilt of earth’s axis of rotation and the amount of sunlight that reaches earth’s northern and southern hemispheres each day. The earth is tilted about 23.5 degrees (23 degrees 26 minutes) relative to the earth’s orbit around the sun. Most of the time, this tilt causes one hemisphere to experience more daylight than the other. This different can make one hemisphere warmer or colder because of the amount of heat, or thermal energy, that the surface below absorbs and hangs on to during the night that follows.
Aside from the change in temperature, the change in the amount of daily sunlight affects all plant life. Plants use chlorophyll, the green stuff, to capture the sun’s energy and use it, along with carbon dioxide, water and organic nutrients from the soil to make the sugars and starches they use as food. This process is otherwise known as photosynthesis. Over the course of the four seasons the amount of daily sunlight gradually increases and then gradually decreases. When the amount of sunlight is too little for the plant to perform photosynthesis, the plant shuts down its photosynthesis factory and begins to live on its stored food. Many plants even get rid of their solar collectors, or their leaves. When the amount of daily sunlight begins to grow again, the plants put out their solar collectors and crank up the factory. For many people this annual cycle of the leaves is more significant than the rise and fall of temperature.
Ok, enough of the details. You get the idea. A very happy equinox to all!