pɛrɪˈhiːlɪən

perihelion: that point in the orbit of a planet, comet or other body at which it is closest to the sun

Not to be confused with the parhelion, which I wrote about last month. Today (4 January 2017), the Earth is at perihelion, its closest to the sun, a mere 98% of its average distance. On 3 July 2017, it will be at aphelion, its farthest from the sun, 2% farther than average. In the short term, perihelion and aphelion occur at pretty much the same time of year, year on year, so the southern hemisphere has its summer when the Earth is close to the sun, and its winter when we’re farthest away. In the northern hemisphere that seasonal rhythm is reverse, and it makes the northern summers just a little cooler, and the northern winters just a little warmer, than their counterparts in the southern hemisphere.

If you want the dates of Earth’s perihelion and aphelion for the next few years, you can find a table here, confirming that they’ll hang about in early January and early July for the rest of the century.

Perihelion comes from Greek peri-, signifying “around” or “close” and helios, “sun”. Aphelion instead uses the Greek prefix apo-, meaning “off” or “away”. Many folk pronounce the word to keep the “ap” separate from the “helion”, which seems fair enough. But the Oxford English Dictionary, conscious of the way the Classical Greek apo- would turn to aph- when it ran up against the “h” in helion , tells us to pronounce the “ph” as “f”. So it’s əˈfiːlɪən. The “ap-helion” crowd, unaware of the Classical Greek argument, look down on the “afelion” crowd, thinking they just don’t understand how the word is formed. And the “afelion” crowd look down on the “ap-helion” crowd, thinking they just don’t understand how the word is formed. So if you’re ever called upon to discuss an aphelion (I know this happens to me from time to time), your best bet is to duck the issue and let someone else pronounce it first.

Appropriately enough, both words were coined by Johannes Kepler, the man who first realized that the planets moved around the sun in elliptical orbits. He formed the words by analogy with two older words, perigee and apogee, which come from Greek gea, “the earth”. Astronomers before Kepler used these words to designate the points in a planet’s orbit when it was closest too and farthest from the Earth, at a time when all the planets were assumed to be going around the Earth, rather than the sun. Nowadays, they’re used only for things that are really in orbit around the Earth—the moon and artificial satellites.

These two pairs of words have formed the basis for many other, newer coinings, according to which object a body orbits around. For orbits around stars other than our sun, we have periastron and apastron, from Greek astron, “star”; for orbits around the galaxy, we have perigalacticon and apogalacticon, from the Greek galaxias, “the Milky Way”.

For orbits in general, there’s periapsis and apoapsis, which together take a Greek plural, and are called the apsides. The original Greek was aphis, “fastening”, a word that was also applied to the curved sections of wood that were fastened together to form the rim of a wheel. (In English, these are called felloes, in case you’re wondering.) So that’s a nice designation for the sections of the orbit that are nearest to and farthest from the centre. The apse of a church has the same derivation, because of its curved shape. For those who aren’t Classical Greek wheelwrights, there are modern equivalents to the apsidal names that make the meaning more obvious, at the expense of being hybrid words—pericentre and apocentre. Perifocus and apofocus are also sometimes used, and are more mathematically correct—an object in an elliptical orbit is moving around one focus of the ellipse, not its centre.

So really, once we know the words for the apsides and their synonyms, which are completely general terms, we shouldn’t need any more specific apsidal words. That hasn’t stopped people inventing them, though.

As soon as we started thinking about sending spacecraft to the moon, the words pericynthion and apocynthion were contrived. And I mean contrived—cynthia means “of Cynthos”, the mountain on the island Delos where the moon goddess Artemis / Diana was said to have been born. So the name Cynthia has a long history in English of being used as a poetic reference to the moon. But why this rather convoluted derivation was preferred to the much more straightforward periselene and aposelene, from Greek selene, “moon”, is anyone’s guess. But the –cynthions became the preferred terms of the Apollo program, and the –selenes languished and were rarely used. A pair of hybrid Greek/Latin words was also invented that made more immediate sense—perilune and apolune, from the Latin luna, “moon”. An effort was made to press two pairs of words into service with different meanings—the –lunes for spacecraft launched from the moon, and the –cynthions for spacecraft launched from elsewhere. But it doesn’t seem to have ever been a consistent distinction. The Google Ngram for these words shows an interesting double spike—one in the ’60s and ’70s, during the Apollo missions, and one in the ’90s when the moon was revisited by unmanned spacecraft after a decade-long hiatus.

When we come to the individual planets (and Pluto) you’ll find a neat list being cut-and-pasted around the internet. It has the merit of pairing Greek versions of the planet names with Greek prefixes, avoiding the dreaded creation of hybrid words from mixed Greek and Latin roots. For each body, simply combine peri- or apo- with a suffix from the following list.

• -hermion (Hermes, Greek equivalent of Mercury)
• -cytherion (Cytheræa, “of Cythera”, another name for Aphrodite,  Greek equivalent of Venus)
• -areion (Ares, Greek equivalent of Mars)
• -zene (Zeno-, a combining form of Zeus, Greek equivalent of Jupiter)
• -krone (Shortened form of Kronos, Greek equivalent of Saturn)
• -uranion (Uranus is a Latinized version of Greek Ouranos)
• -poseidion (Poseidon, Greek equivalent of Neptune)
• -hadion (Hades, Greek equivalent of Pluto)

Neat, eh? Trouble is, almost none of them are words in active use by scientists and engineers. If you search the full text of all the astrophysics papers held in the SAO/NASA Astrophysics Data System, you won’t find a single occurrence of pericytherion, apocytherion, perizene, apozene, periuranion, apouranion, periposeidion, apoposeidion, perihadion or apohadion—tens of thousands of papers, two hundred years, not one occurrence of any of them. Nor will you find them mentioned in any of the books scanned by Google Ngram Viewer. That’s remarkable, isn’t it? Where on earth do these unused words come from, if not from books and scientific literature? They come from Wikipedia—some time in the early 2000s it seems that someone at Wikipedia just made up a list of nice Greek words, and they’ve been echoing around the internet ever since, without a single citation in the scientific literature to support them.*

So what words are actually being used? There are a couple of citations for -hermion, but they are hugely outweighed by periherm and apoherm (and a condensed equivalent, apherm)—the technical terms adopted by the scientists involved in the MESSENGER mission to Mercury. Although Venus has been orbited by several spacecraft, there seems to be no specific term relating to that planet, apart from very rare appearances of perivenus and apovenus, which seems to indicate a significant failure of imagination. For Mars, periareion and apoareion (or their condensed equivalents, periareon and apoareon) seem to be the words of choice across several missions. At Jupiter, perijove and apojove are the words actually in use (from Jove, a Latin alternative name for Jupiter), with a usage record going back to the early nineteenth century. Perijovium and apojovium are rare (and usually old) variants. For Saturn, perikrone and apokrone (or their equivalents perichrone and apochrone) have been in use since the turn of the millennium, but perisaturnium and aposaturnium have been used more often, with a history stretching back to the nineteenth century. Beyond that point in the solar system, it goes quiet. Periuranium was last used in 1922, and there are a couple of citations for perineptune and peripluto.

This is all very odd, if you think about. Orbits have various defining characteristics, and the pericentre is just one—there’s the orbit’s eccentricity and period, for instance, to pick a couple of easy ones. But we never talk about the “hermioeccentricity” or the “cytherioperiod”—one name is good enough for a given parameter, no matter where the orbit is. But the historical precedents of perihelion and perigee seem to have convinced (some) people that they need to find a new pericentre name for every kind of orbit that turns up.

As a case in point, there’s the fact that there are already two names in the scientific literature for “closest approach to a black hole”—peribothron (from Greek bothros, “hole or pit”) and perinigricon (apparently a double hybrid, combining the Greek prefix peri- and Greek suffix -icon with Latin niger, “black”). And another one crops up in fiction—physicist Geoffrey Landis (whom we last encountered when I wrote about human vacuum exposure) has written a science fiction story (you can read it here) which uses the word perimelasma, from the Greek melasma, “black spot”. I suppose it’s easier to defend the case for coming up with a fancy but pointless word in a work of fiction—it’s just unfortunate that Landis chose to use the name of a common skin condition.

Anyway, that’s enough for this time. I had planned on writing about words derived from peri- and apo-, and words derived from the sun, but got distracted by the sheer number of different kinds of named pericentres. So I’ll write about all those other words in Part 2.

* At last, in February 2019, Wikipedia got around to removing the fictitious entries from its terminology list, a mere fourteen years after the problem had first been pointed out. Your Correspondent is gratified to discover, from the article’s Talk page, that it was this very blog post that finally precipitated the necessary edit.