Almost anyone who has ever attended the JW annual commemoration of the "Lord's evening meal" (Memorial) has heard the term "Equinox" at least once. The term is important to JW’s because they use the vernal equinox in attempting to set the correct date for this event. Despite how important it seems to be though, when JW literature explains the term, it is usually wrong. This installment in the science quotes of "Celebrated JW scholars" concerns the term, equinox.
The word is Latin in origin and literally means, "Equal night." It commonly references two days out of the year which fall around March 20 and September 22. To understand the event this word describes, one must first understand two simple terms:
1. The Celestial Equator: Astronomers project the plane of the earth's equator onto the celestial sphere. (i.e. The sky) The celestial equator is an imaginary line hovering over the earth.
2. The Ecliptic: The ecliptic is a second imaginary line hovering over the earth. The sun does not really revolve around the earth, but from our perspecitve it appears to. The ecliptic is the apparent path of the sun through the sky.
These two imaginary lines would be identical except for the fact that the axis of the earth is tilted relative to the sun. Therefore the axis of the celestial equator is correspondingly tilted relative to the ecliptic. As you can see in the diagram below, this is pretty easy to visualize:
(Diagram courtesy of Dennis Nilsson)
The vernal and autumnal equinoxes occur when the geometric center of the sun’s disk crosses the celestial equator. These are the two points where the ecliptic and equator lines intersect:
In astronomy, these events are defined to the minute, but the calendar day definition is good enough for this discussion.
Probably because of the etymology of the word, "Equinox," a very popular and persistent misconception is that day and night are of equal length on these two days. However, two things combine to make this notion incorrect:
First, the sun is not a singular pinpoint of light. The sun appears as a disk to an observer on earth and this disk occupies approximately 16 minutes of arc. Since "Day" is defined as the period of exposure to direct sunlight, "Sunrise" is defined as the instant the leading edge of this disk becomes visible on the horizon and "Sunset" is defined as the instant when the trailing edge of this disk disappears below the horizon.Therefore, day at the equator is longer than night by the length of time it takes one width of the sun's disk to traverse the sky. (i.e. The distance between the leading and trailing edges of the disk.)
Second, atmospheric refraction makes the visible horizon distinct from the actual geometric horizon. What this means is that an observer on Earth can see the leading edge of the sun's disk earlier and the trailing edge later than they would in a vacuum. This adds another few minutes to the daylight period.
For these reasons, day and night are not exactly the same length at the time of the March equinox. Exactly how much longer the day will be varies depending upon where you live. For example, this year, the vernal equinox for the northern hemisphere will occur on March 20th. If you happen to live in Brooklyn, New York, sunrise will occur at 6:59 AM and sunset will occur at 7:08 PM making the day 9 minutes longer than the night.
Despite this, note how JW literature has described the equinoxes over a 70 year period:
"All intelligent persons know that on the equator the days and nights are always of equal length. They also know that twice a year the sun apparently shifts its position with respect to the earth and in March and September there are what are called equinoxes; that is, the days and nights are of equal length in every place on the earth." (The Golden Age March 13, 1935 p.360)
"The spring equinox, when nighttime equals day-time, usually falls about March 21 of our calendar." (The Watchtower February 1, 1948 p. 41)
"Twice each year the sun’s center crosses the equator, and at those times day and night are everywhere of equal length (approximately 12 hours of daylight and 12 hours of darkness). These two times are called the vernal, or spring, equinox and the autumnal, or fall, equinox. They occur about March 21 and September 23 of each of our present calendar years. (Insight on the Scriptures Volume 2 p. 1214)
"Midway between the solstices, however, the sun is aligned vertically above the equator of the earth. This occurrence is known as an equinox, which means that day and night are of equal length everywhere on earth. About March 20 and September 21, the sun rises exactly in the east, follows the equator for 12 hours, and sets exactly in the west. At midday on an equinox, the sun is at its zenith over the equator and no objects will cast a shadow there. "(Awake! December 22, 2005, p. 23)
The dates on which day and night are closest to being the same length actually occur a few days on the winter side of each equinox, that is to say a few days before the vernal equinox and a few days after the autumnal equinox. These dates are referred to as "Equiluxes" to distinguish them from the equinoxes.
Further, this does not happen simultaneously at every place on earth. That is more popular misconception. The exact date will vary depending on how close you are to the equator. For example, in the northern hemisphere at 5 degrees latitude the equiluxes will occur around February 25 and October 15, but at 40 degrees latitude they occur around March 17 and September 26. (This year, the Spring equilux will occur in Brooklyn on March 17th.)
Here is a fairly easy way to verify what I've said here and calculate when the Spring equilux will occur where you live:
1. Go to this website
2. Enter your location. (Form A for U.S cities; Form B for locations worldwide)
3. Enter the date of the vernal equinox for your timezone.
4. Click the "Get Data" button
5. Note the sunrise and sunset times. (e.g. A sunrise of 6:32AM and a sunset of 6:40PM is a difference of 8 minutes)
6. Work your way back one day at a time until you reach the date with the smallest difference.