WSODONNELL2 wrote: ↑
Sat Sep 26, 2020 2:43 pm
A synod is an ecclesiastical conference. The first use of the word was in the Middle Ages.
Does anyone know how the word came to be applied to moon phases?
Any historians of science here? Any etymologists?
Sorry, I'm neither, but this one seems to make sense ...
The Greek "sunodos" comes from putting together "sun" ("with", "together") and "odos" ("path"). So that would be an explanation of the Greek word "sunodos" being used for a meeting, a "coming together of paths". https://en.wiktionary.org/wiki/synod#English
- From there, naturally, we get the English "synod" to refer to a meeting. And that entirely explains its use for an ecclesiastical gathering.
- But then, astronomically, "synod" straightforwardly progressed from this, as a word for celestial conjunctions. So, the other night, I enjoyed seeing a "synod" of Mars and the Moon.
From this, a "synodic period" is measured by determining how long it takes a celestial object to go from one conjunction to another. And most often, this is a conjunction relative to our Sun (viewed from Earth). For this instance, how long between successive times at which the Moon is in conjunction with the Sun. That is the most naturally observed timing for a lunar month, since the phases of the Moon will go through their complete cycle in exactly the time it takes for such a conjunction to recur. Because the Earth makes an appreciable amount of progress in its orbit around the Sun during a lunar synodic period, the Moon actually has to travel a bit more than once around the Earth to complete a synodic period. So its synodic period (29.5 days) is a bit longer than its sidereal period (27.3 days). https://en.wikipedia.org/wiki/Orbital_period
So, in astronomy, for bodies in our Solar system, many of which have observable phases, it came to be that "synodic period" was used for the apparent conjunctions with the Sun (really, the alignment of the object X, the Earth, and the Sun), while "sidereal" was used to distinguish the apparent conjunction of bodies with fixed stars. And so, for the Moon, the synodic and sidereal months are so defined. Since "synodic" really came from the term for meetings, I can imagine the history of the term could have come out differently, considering that the sidereal month is measured based on a meeting as well.