This week (Friday) the fall Equinox occurs. From http://www.timeanddate.com/calendar/september-equinox.html
The September Equinox Explained
The September equinox occurs at 09:04 (or 9:04am) Coordinated Universal Time (UTC) on September 23, 2011. It is also referred to as the autumnal or fall equinox in the northern hemisphere, as well as the spring or vernal equinox in the southern hemisphere (not to be confused with the March equinox). This is due to the seasonal contrasts between both hemispheres throughout the year.
What happens during the September equinox?
The sun crosses the celestial equator and moves southward in the northern hemisphere during the September equinox. The location on the earth where the sun is directly overhead at solar noon is known as the subsolar point. The subsolar point occurs on the equator during the September equinox and March equinox. At that time, the earth’s axis of rotation is perpendicular to the line connecting the centers of the earth and the sun. This is the time when many people believe that the earth experiences 12 hours of day and night. However, this is not exactly the case.
Dispelling the “exactly 12 hours of daylight” myth
During the equinox, the length of night and day across the world is nearly, but not entirely, equal. This is because the day is slightly longer in places that are further away from the equator, and because the sun takes longer to rise and set in these locations. Furthermore, the sun takes longer to rise and set farther from the equator because it does not set straight down – it moves in a horizontal direction.
Moreover, there is an atmospheric refraction that causes the sun’s disk to appear higher in the sky than it would if earth had no atmosphere. timeanddate.com has a more detailed explanation on this topic. timeanddate.com has more information on why day and night are not exactly of equal length during the equinoxes.
Why is the equinox date not the same each year?
While the September equinox occurs on September 22 in 2008 and 2009, it occurs on September 23 in 2010 and 2011 (UTC). The September equinox has also occurred on September 24(UTC), with the last occurrence on that date being 1931. The next time a September 24 equinox occurs will be in the year 2303. Moreover, a September 21 equinox will occur in 2092.
There are a few explanations on why the equinox dates differ in the Gregorian calendar. The varying dates of the equinox are mainly due to the calendar system – most western countries use the Gregorian calendar, which has 365 days in a year, or 366 days in a leap year. According to the National Maritime Museum, the equinoxes generally occur about six hours later each year, with a jump of a day (backwards) on leap years. An extra day is added in a leap year to minimize a gradual drift of the equinox date through the seasons.
As for the tropical year, it is approximately 365.242199 days, but varies from year to year because of the influence of other planets. A tropical year is the length of time that the sun takes to return to the same position in the cycle of seasons, as seen from earth. The exact orbital and daily rotational motion of the Earth, such as the “wobble” in the earth’s axis (precession), also contributes to the changing solstice dates
Vernal Equinox vs. Autumnal Equinox
The vernal equinox occurs in the spring while the autumnal equinox occurs during fall (autumn). These terms are derivatives of Latin. It is important to note that the northern hemisphere’s vernal equinox is in March while its autumnal equinox is in September. In contrast, the southern hemisphere’s vernal equinox is in September and its autumnal equinox is in March.
This distinction reflects the seasonal differences when comparing the two hemispheres. timeanddate.com refers to the two equinoxes simply as the March and September equinoxes to avoid false assumptions that spring is in March and fall (autumn) is in September worldwide. This is simply not the case.
A Greek astronomer and mathematician named Hipparchus (ca. 190-ca.120 BCE) was attributed by various sources to have discovered the precession of the equinoxes, the slow movement among the stars of the two opposite places where the sun crosses the celestial equator. However, the difference between the sidereal and tropical years (the precession equivalent) was known to Aristarchus of Samos (around 280 BCE) prior to this.
Hipparchus made observations of the equinox and solstice. Astronomers use the spring equinoctial point to define their frame of reference, and the movement of this point implies that the measured position of a star varies with the date of measurement. Hipparchus also compiled a star catalogue, but this has been lost.
The word “equinox” derives from the Latin words meaning “equal night” and refers to the time when the sun crosses the equator. In modern times this word is used to refer not only to the positions on the ecliptic but also the times of the year when the sun has reached them. The September equinox has been used as a reference point in many calendars in the past, including the French Revolutionary Calendar. Although very little is known about the ancient Macedonian calendar, some believe that the first month began after the autumnal equinox.
Traditions and customs
The September equinox has been a point of reference for celebrations, traditions and observances around the world over the centuries.