On September 22 comes the equinox: the start of the third quarter of the seasonal year on our planet.
Here is one of the illustrations in the “Sun and Seasons” section of Astronomical Calendar 2024, suggesting how the Sun’s daily arc across the sky comes to a middling height, rising and setting at the due east and west points on the horizon.
The moment when the Sun’s center appeared to cross Earth’s equatorial plane into the southern half of the celestial sphere is 12:42 by Universal Time. This is the same by Standard time in Britain; 6 hours earlier in North America’s Central time zone; another hour earlier for both by clocks still twisted to “summer” or “daylight saving” time, in which the Sun does not reach the middle of the sky at the middle of the day.
The equinoxes and solstices (which in 2024 fall at March 20, June 20, Sep. 22, Dec. 21) divide the year into four natural quarters. In discussion among the expert folk at EarthSky.org who are helping to update the Astronomical Companion, there has been a debate about the best brief way of stating why the March equinox point is used as the zero point for measuring positions in the sky. Would you understand, or approve, if we said it’s the beginning of the “astronomical year,” or the “seasonal year,” or the “natural year,” or the “tropical year”?
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It makes sense to use an equinox rather than a solstice as a point of reference. The Sun moves most quickly in declination at the moment of an equinox and most slowly around the solstices. The Sun’s declination changes by one arcminute in an hour at an equinox; it takes two days to change by one arcminute around a solstice. That’s why it’s called a solstice!
Good point.
The UN should urge governments to make the year begin on the winter solstice. Then it can just be called the year, instead of calendar year vs. natural year.
The IAU would have to redraw the lines on astronomical maps so that the winter solstice point becomes the 0 hour in sidereal time. The International Association of Measurements would have to shorten December 31st by about 3 seconds every year to account for precession.
Visionary proposals, Rick. Back in history, it might have been better if the December solstice, rather than the March equinox, had become day 1 of the year in calendars decreed by one or more regimes. January 1 was as near as it got. The adjustments that the IAU and the International Association of Measurements would have to make are upheavals that they would never even consider, short of being forced by a global tyrant.
“Would you understand, or approve, if we said it’s the beginning of the “astronomical year,” or the “seasonal year,” or the “natural year,” or the “tropical year”?”
To be honest, the first three of these phrases all struggle against the same northern hemisphere bias by using “the beginning”. The fourth at least has the definition that it does start at the March Equinox, albeit even in astronomical circles, the term is often considered rather obscure nowadays.
Perhaps something like this might work: “The March equinox became the origin for the zero line of right ascension, as it had long been held to mark the beginning of the seasonal or natural year for the northern hemisphere, from where astronomers first developed the idea to precisely define numerical star positions in the sky. This equinox later became the definition for the start of what we still call the tropical year.”
Thanks for that careful response, Alastair. The trouble with all the two-word terns I listed is that they can be taken to mean types of year (along with leap year, Gregorian year, etc.) whereas what we are talking about is the point at which the year starts. Here is the phraseology now chosen for the Astronomical Companion:
The reason [for using that as the zero point] is that it is one of the two points where the equator cuts across the ecliptic, or plane of Earth’s orbit. The Sun appears here at the March equinox, the beginning of the year in many older calendars…
I would call it the astronomical year.
I find this a helpful analogy: Putting zero hours right ascension at the first point of Aries is as arbitrary as saying the Greenwich prime meridian is zero degrees longitude. Latitude on Earth and declination in the sky have an obvious natural zero point, the equator. But for longitude on Earth and right ascension in the sky, we all have to agree on a zero point. The zero point of longitude could be placed in London, or Paris, or any other point on Earth. The zero point of right ascension could be the Sun’s location at the March equinox, or the December solstice, or any date of the year.
Hmm, I see what you mean, the topic evades simple explanation.
I also like to remember that January 1 is an entirely arbitrary new year’s day. The Roman new year started at the March equinox, and that habit persisted in Britain and her colonies until the 18th century!
Greetings Guy,
I’m torn between “Natural” and “Astronomical” year. They both have a very strong appeal with me (which you would probably suspect). Personally, I wished we use that system, but alas.
– Robert Little
I think the March 20 date should be the start of the second quarter. That makes Sept 22 the start of the 4th quarter. Winter (Dec 21) is the first quarter. My 2 cents.
I would be down with calling the equinox the astronomical new year. It would be pretty close to the Indian calendar lunar month of Chaitra. (Chaitra spans from 23 degrees 20 minutes in the sign of Virgo to 6 degrees 40 minutes in the sign of Libra). On the Vikram Samvat calendar, when the 1st phase after new moon slides into Chaitra, that’s the New Year. That always occurs at the beginning of April or heading to the March equinox (kicking back about once every three years when the calendar resets).
Of course, the Indian calendar, seemingly wild and free compared to our rigid western calendar, also has other new years. But Gudhi Padva is celebrated around the vernal equinox. And it would not surprise me at all if it’s an astronomical observance set “back in the day”.