Celestial Nomenclature


Why I Stopped Watching Are You Smarter than a 5th Grader, Reason 227

I wanted to open this blog with the actual clip from Are You Smarter than a 5th Grader, episode 107 (S01E08), but I could not find a copy anywhere on the Internets. This was way back when that show was brand spanking new and I thought I'd watch a few episodes to see how it was. This was the last one I ever watched. Awful show.

Anywho, I'll have to describe it. The second contestant of the episode, a man, eventually got asked the following question:

In the Southern Hemisphere, the vernal equinox occurs in which month?

Already, there are major, major problems with this question. The contestant bailed out rather than trying to answer. After, the host revealed that the expected answer was "September." There are even more problems with that answer.

First, no American, public school 5th-grader would know the answer to that question, no matter what it's meant to ask. Second, no American, public school 5th-grader would even know what the words "vernal" and "equinox" mean on their own, let alone used together. These are just not subjects on the public school 5th grade science curriculum anywhere I can find.

The difficulty of the question alone was enough to turn me off the show right there. But the major problem--the one of celestial nomenclature--really put me over the edge. I was fuming about it for weeks. I remember talking to many people about it at the time. I thought I blogged about it, but I can't find any such post.

"Vernal equinox" is a term of art. It means a very specific thing. It is the traditional name for a point on the (geocentric) celestial sphere. It is not synonymous with the more modern term "spring equinox" or the even more modern "March equinox." It is synonymous with the old naval and astrological term, "first point of Aries."

Allow me to explain.

The celestial sphere is a fictional spherical surface astronomers use to locate objects in the sky. For our purposes, we can assume that the distant stars are fixed points on the inside of this sphere, and that the local objects (the Sun, Moon, and planets) move around relative to the stars.

There are two very important planes that intersect this sphere and delineate imaginary circles in space where they intersect the sphere. The first is the celestial equator. This plane is coextensive with the Earth's equator. It is just extended from the equator out into space. The second is the ecliptic. This is the plane on which the Sun appears to travel around the Earth. Because the Earth has a tilt to its axis of about 23.5°, the celestial equator and the ecliptic intersect at about 23.5°. Wikimedia Commons has this lovely graphic:

Celestial equator and ecliptic

The ecliptic is a very important plane for a number of reasons. It traces an imaginary arc across the sky. The sun will follow this arc as it moves from East to West over the course of a day. At night, the planets can be found on or very near this line. The constellations of the Zodiac lie along this line. Your "sign" --insofar as you put stock in such nonsense-- is the constellation of the zodiac the sun was in on the day of your birth.

Hold up, back up, wait, what? I thought I just told you that the sun moves along the ecliptic from East to West each day! Now I'm telling you it stays in one zodiacal constellation all day long? Yes. The Sun doesn't move around the ecliptic daily; it moves around it yearly. The stars are pretty much fixed relative to the ecliptic.* The sun appears to "trace the line of the ecliptic" during the course of the day because the Earth rotates relative to the Sun in its relatively stationary position on the ecliptic. The Sun creeps along the ecliptic over the course of a year.

As you can see from the diagram above, some parts of the ecliptic are "above" (North of) the celestial equator, and some parts are "below" (South of) it. It crosses at two places. The places where two celestial planes cross are generally called "nodes." One of these nodes will be called the "ascending node" and the other the "descending node." Which is which depends on the relative motion of the two bodies that define the planes. In this system, the two bodies are the Earth (defining the celestial equator) and the Sun (defining the ecliptic). That diagram doesn't indicate any directions, so I'll have to tell you that, viewed from celestial North down through the plane of the ecliptic, the Earth revolves counter-clockwise about the sun. Viewed relative to a stationary Earth, the Sun appears to move along the ecliptic counter-clockwise around the Earth. That's left-to-right in the front of the diagram and right-to-left in the back of the diagram.

So you can see that, as the year goes along, the Sun will first pass through the node in the front of the diagram, labeled γ, and, six months later, through the rear node, labeled Ω. When the sun appears to pass through γ, it moves from the celestial Southern hemisphere into the celestial Northern hemisphere. This is the ascending node. When it passes through Ω it moves from North to South--the descending node.

The Sun/Earth system has nodes. So does the Earth/Moon system. The nodes in the Earth/Moon system are important for calculating eclipses. Other orbital systems also have ascending and descending nodes. But the Sun/Earth system has a special name for its nodes. These names date from quite long ago. In Western astronomy, the oldest such names make reference to the zodiacal constellations in which these nodes appeared to early astronomers to be fixed. The ascending node was at one time (approximately 2,000 years ago) right at the boundary between the constellations of Pisces and Aries, with the sun moving into Aries. So it became known as the "first point of Aries." Likewise, the descending node was just between Virgo and Libra, with the sun moving into Libra. So it became the "first point of Libra."

The first point of Aries remained a useful astronomical landmark for centuries. If one knows where the first point of Aries is in the sky, one can, using a clock, a calendar, and a sextant, determine one's longitude on the Earth. This was very useful in seafaring. The nodes were known by these names (in various languages, allowing for differences in zodiacs) from antiquity.

The word "equinox" enters English very early, before 1391, when Chaucer used it. When it entered English, it bore only one meaning. It referred to the nodes of the Sun/Earth system. They were differentiated at that time as the "vernal equinox" and the "autumnal equinox", for their association with the seasons in the Northern hemisphere. The ascending node, or first point of Aries, became the vernal equinox because the sun crossed it during Spring in the Northern hemisphere, the exclusive home of English at the time. The descending node likewise became the autumnal equinox.

The word "equinox" did not gain meaning as the moment at which the sun passed through one of the nodes until at least 1588, and the phrases "vernal equinox" and "autumnal equinox" didn't gain such meaning until about 1664. For some time, the phrases properly held meaning as both the points in space and the moments or dates on which the sun passed through the points. I say properly, because, during those years, English-speaking astronomers were confined almost exclusively to Northern hemisphere cultures. There was no need to differentiate the seasonal words from the celestial meanings.

When English became prevalent in Southern hemisphere cultures, the terms became very confusing. Astronomers still referred to the ascending node of the Sun/Earth system as the "vernal equinox". But in the Southern hemisphere, the sun passes through this node in Fall, not Spring. In the mid-18th Century, the phrases "Spring equinox" and "Fall equinox" appear, describing only the moments or days. Vernal and Autumnal retained the celestial meanings, but began to lose their seasonal meanings, despite being words with seasonal origins.

That brings us to today. Today we have a set of terms to refer to the celestial positions (the ascending and descending nodes) and another set of terms to refer to the dates on which, or moments at which, the sun passes through those nodes:

γ Ω
Position on celestial sphere Vernal Equinox Autumnal Equinox
Moment or day when the Sun passes through Spring Equinox (Northern hemisphere)
Autumnal Equinox (Southern hemisphere)
Autumnal Equinox (Northern hemisphere)
Spring Equinox (Southern hemisphere)

So you can see the phrase "vernal equinox" refers to the same point in space no matter where on Earth you happen to be. But the phrase "Spring equinox" refers to a different moment or day depending where you are. Use of "vernal" to mean "Spring" in this context is archaic and inappropriate because, as English is spoken in both hemispheres, it is ambiguous. Because "vernal" has properly retained only a celestial meaning, using it to refer to a moment or day can only mean some time on or about March 21, no matter where you said it or your listener heard it.

The vernal equinox is in the same place, and the sun passes through it on the same day, in the Northern hemisphere as in the Southern hemisphere.

The Spring equinox can be on or about March 21 or September 21, depending on where you are. But it doesn't add any new ambiguity to the language. It piggybacks on the already ambiguous term, "Spring." That's a concept (and an ambiguity) that has been in the language much longer and is already well-established.

Another interesting feature to note is that, at about the same time "Spring equinox" split the date/moment meaning off of "vernal equinox", the concept of "Summer" also underwent a significant shift in meaning. Up until about the mid 18th Century, "Summer" and related seasonal terms described local meteorological conditions with strict adherence to celestial events. "Summer" was the warm period centered on a solstice†. It changed at that time to take account of temperature lag, so that "Summer" would actually describe the warmest months of the year instead. These lag behind the celestial summer centered on the solstice by a month and a half in the temperate bands between the tropics and the arctic/antarctic circles. So today, "Summer" begins on what used to be Midsummer's Day (the local Summer solstice).

Given all this discussion, you should now see that Are You Smarter's question is defective in a number of ways. It should have asked "Spring equinox". Then its answer would have been correct. But it asked "vernal," which has lost the meaning they intended. As written, the answer to the question is March, not September. If they're going to use "vernal equinox" to mean a moment or date, then it is going to refer to the moment or date when the sun passes through the vernal equinox, nothing else. The histories of astronomy and English point strongly to this conclusion, and continuing to use "vernal" to mean "spring" is, prescriptively, incorrect now that English is spoken in both hemispheres. Most importantly, the question is defective because there is no way a 5th-grader could be expected to know what "vernal equinox" really means. Is the point of this show to answer the questions as if the contestant were a 5th-grader, or is it to give the correct answer?

*Actually, they move very slowly. This is why the astrologers recently "corrected" everyone's signs. The dates for the traditional Western Zodiac got set a long, long time ago, based on even older astronomy going back to when the vernal equinox really was at the "head" of Aries the Ram (hence "first point of Aries").

†Interestingly "solstice" never had a "point in space" meaning, and it hasn't ever acquired one.

Edit: The shift from celestial seasons to meteorological ones happened in 1780, not the 1800s. So use of "vernal equinox" to refer to one date in the Northern hemispher and another in the Southern is even more archaic than I thought. Please stop it now.

The modern, and very clear convention is to use "March equinox" and "September equinox" for the ascending node transit and descending node transit, respectively, because March is March in both hemispheres. This practice even more clearly differentiates meteorological from celestial phenomena, and reinforces the conceptual difference between the moment and the point-in-space.

Tom G Varik