March Equinox renews spring in your step
Spring is coming, for the northern half of the world at least.
The term equinox, from the Latin aequus meaning equal, and nox meaning night (together meaning equal night) is used by astronomers to denote the exact moment when, as seen from space, the geometric centre of the sun crosses the celestial equator (the circle of the imaginary celestial sphere on the same plane as the Earth's equator) moving northward from the south in March, and southward from the north in September.
It is the moment at which the subsolar point (the point at which the sun is directly overhead, its sunlight striking the planet exactly perpendicular to its surface) leaves one hemisphere and enters the other (i.e., from south to north in the spring, and north to south in autumn).
There are two equinoxes each year — the spring equinox (the start of the spring season), and the autumn equinox (the start of the autumn season). The equinoxes have a number of names — the spring equinox in the northern hemisphere is sometimes called the vernal (from the Latin ver, meaning spring) equinox, the March equinox (for the month in which it occurs), or the northward equinox (for the movement of the Sun northward). However, such labelling poses a problem for people residing in the southern hemisphere, who are also subject to the delineation of their spring and autumn seasons as a result of the equinoxes.
This year, the spring equinox occurs on March 20, marking the official beginning of the astronomical spring season (as opposed to the meteorological spring season which started, as it always does, on March 1) here in the northern hemisphere.
Since, in reality, the southern hemisphere is experiencing the start of its autumn season on March 20, 2022, it is not technically correct to label it as the vernal (spring) equinox. In the southern hemisphere, they could, and often do, refer to the March 20 equinox as their autumnal (from the Latin autumnus, meaning autumn) equinox. However, astronomers are a fastidious bunch and like to simplify things as much as possible, and, therefore, prefer to use the terms March/ September equinox or northward/southward equinox to denote the start of the respective seasons (spring and autumn), irregardless of hemisphere, in order to resolve any ambiguity.
It is often stated (and believed) that, on the date of the equinoxes, the hours of daylight and nighttime are equal in duration, since the Latin equinox (equal night) implies that day and night were equal in length at that time. In reality, the length of the daylight and nighttime periods are only equal at the equator when the equinoxes occur. The differences between the length of daylight and nighttime elsewhere on the Earth varies due to the angular size (the angular distance describing how large a sphere or circle appears from any given location) of the sun; atmospheric refraction (which can cause the sun to be visible even while still below the horizon); and an observer's geographic latitude at the time of the equinox.
An equinox is the only time when the solar terminator (the moving line, sometimes referred to as twilight, dividing the daylit side and the nighttime side of a planetary body) is perpendicular to the Earth's equator as seen from space, with the result that both hemispheres are equally illuminated. The equinoxes are also the only two times of the year when the sun rises due east and sets due west.
Equinoxes can be significant markers for a number of civic celebrations or events around the world. Most people who celebrate Easter know that it is set by the Christian Church as the first Sunday (April 17, 2022) after the first full moon on or after the March equinox. The Jewish Passover falls on the first full moon after the March equinox (though it can, upon occasion, occur on the second full moon).
However, did you know that the equinox on March 20 marks World Citizen Day, which promotes the concept that, irregardless of geography and politics, we are all members of the broader, global class of humanity?
It is also celebrated as World Storytelling Day, where as many people as possible, in as many countries and languages as possible, tell or listen to stories during the same day and night. Numerous global civic and astronomical calendars, including our own Gregorian calendar, are based, along with other celestial events and timings, on the occurrences of equinoxes.
The history of equinoxes is a fascinating one, and I encourage you to find out more about this interesting concept and its impacts on our lives.
THIS WEEK'S SKY
Mercury, Mars, Jupiter, and Saturn are all too low above the horizon, or too close to the sun, to be readily visible this coming week. This leaves only bright Venus (mag. -4.5, in Capricornus — the Sea Goat) as the lone, sentinel planet visible in the pre-dawn sky. Rising around 5:25 a.m., Venus reaches a height of around 14 degrees above the southeast horizon before fading with the break of dawn around 7:05 a.m. Venus reaches its greatest elongation from the sun (the apparent angle of separation between Venus and the sun, as viewed from Earth) on March 20.
The full Moon on the 18th has many names, among them the Worm Moon (for the earthworms that begin to appear in the warming spring soil); the Crow Moon (for the crows that become more active in the sky in the spring); the Crust Moon (for the alternating daytime thawing and nighttime refreezing of the top crust of any remaining snow); and the Sap Moon (for the tapping of the sugar maple trees at this time of the year). The Mi'kmaq people of eastern North America call the full moon of March Siwkewiku's, meaning maple sugar, again a reference to the tapping of the maple trees for their sap, which is then turned into maple sugar.
Until next week, clear skies.
EVENTS
• March 18: Full moon; 4:17 a.m. ADT/4:47 a.m. NDT
• March 20: March equinox; 2:33 p.m. ADT/3:03 p.m. NDT
• Venus at greatest elongation west of sun.
Glenn K. Roberts lives in Stratford, P.E.I., and has been an avid amateur astronomer since he was a small child. He welcomes comments from readers at glennkroberts@ gmail.com.