The Equinox

Myths

Direction of Sunrise and Sunset

From The Permaculture Research Institute we read the following:

  “ Why is the equinox important for permaculture?

One of the first questions my PDC instructor posed to us was, “where does the sun rise?" Well everyone knows the answer to that; the sun rises in the east. No brainer. Alas, we were wrong. Unless you live at the equator, the sun does not rise directly in the east. ”

Russel D. Sampson of the University of Alberta informs us that the sun does not actually rise due east on the equinox, and is displaced by the east by multiple sun-diameters. Inventive special pleading is therefore invoked, attributing the inability of prediction to meet observation to "refraction":

Astronomical Refraction and the Equinox Sunrise

  “ The original motivation for this paper was the Journal article by Attas & McMurry (1999) entitled “Nailing the Equinox Sunrise.” In the article it is stated that “on the equinox, the Sun should rise due east.” Since the Sun crosses the celestial equator at the equinox, and since the celestial equator crosses the horizon at 90º and 180º azimuth (due east and west), that statement appears on the surface to be correct. Once the effects of astronomical refraction are considered, however, the phenomenon becomes a little more complex.

...In this paper the unrefracted Sun is referred to as the geometric Sun. From the definition of sunrise and sunset, the azimuthal location of the geometric equinox sunrise can be shown to be less than 90º (north of due east in the northern hemisphere). The magnitude of the difference depends on the observer’s latitude and the apparent diameter of the Sun (see figure 2). From figure 2 it is apparent that the only location where the equinox sunrise occurs exactly at the east point is on the equator. ”

We note that no observations of the sun at the equator are actually given.

Equal Day and Night

From an article from the Christian Science Monitor we read the following:

  “ It's often said that the autumnal equinox is a day of perfect equilibrium – a 24-hour cycle split neatly into 12 hours of light, and 12 hours of dark. That's not exactly true. In areas distant from the equator, the sun can take longer to rise and set; closer to the equator, the day lasts a little more than 12 hours. The real even split between day and night doesn't occur until later in the fall, according to the US Naval Observatory. ”

The Equilux

From The Macmillan Dictionary we read the following:

  “ The equinox occurs twice a year, usually around 20th March (the Spring equinox in the Northern Hemisphere, after which day becomes longer than night) and 22nd September (the Autumn equinox in the Northern Hemisphere, from when night becomes longer than day). On these days however, the length of the day and night is only 'nearly' equal. This is because the top half of the sun rises above the horizon before the centre, and sunlight is refracted by the Earth’s atmosphere, meaning that the sun appears to have risen when its centre is still below the horizon. This makes the amount of daylight longer than expected on those days (12 hours and 10 minutes in the UK). However the same effect means that on the equilux, which is in fact a few days before the Spring equinox and a few days after the Autumn equinox, the hours of daylight and darkness are exactly equal.

By now you may, as I did, be wondering why on earth (no pun intended!) we’re all so familiar with the term equinox, when in fact it’s a less equal split of day and night hours than the not so commonly known equilux. One possible explanation is that the equinox is a fixed instant in time, referring to the moment when the sun crosses the equator and completely independent of the observer’s location, whereas the equilux is a moveable feast, its date varying depending on where in the world you are. In fact close to the equator, equiluxes don’t occur at all. ”

The Equinox in Contrast to The Equilux

http://www.torahcalendar.com/ORBITS.asp?HebrewDay=22&HebrewMonth=7&Year=2018

  “ The date at which the time between sunset and sunrise crosses 12 hours is known as the equilux. Because sunset and sunrise times vary with an observer's geographic location (longitude and latitude), the equilux likewise depends on location and does not exist for locations sufficiently close to the equator. ”

Ancient Observatories - Timeless Knowledge
Link to Paper

In 2015 Deborah Scherrer of the Standford Solar Center provided a document titled "Ancient Observatories - Timeless Knowledge." On page 39 we see the following:

  “ 
Calculating the Equinoxes

Most dictionaries erroneously define the equinox as: “the time or date (twice each year) at which the Sun crosses the celestial equator, when day and night are of equal length (about September 22 and March 20)”. However, there is no place on Earth where the day and night are of equal length on the given days.

Latitude Determines Day Length

In fact, latitude determines day length. Even if day and night aren’t exactly equal on the day of the equinox, there are days when day and night are both very close to 12 hours. However, this date depends on the location’s latitude, and can vary by as much as several weeks. The table shows approximate dates for when day and night are as similar as possible according to latitude.

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