The word "month" is related to "moon". A genuine month is the time it takes the moon to pass through all its phases once; that is one lunar phase cycle. This "synodic" month is the basis for many calendars.
Astronomically and mathematically, there are several ways to calculate a month. It can be Sidereal, Tropical, Draconic, Synodic, and Anomalistic. I don't think we need go into detail about that, but some scientific elementa might be useful.
Calendars, even those related, assume myriad ambiguous forms. Structurally, they belong to one of three main forms: Solar, Lunar, or Lunisolar.
One example of a Solar (or Tropical) year is the common western, Gregorian year, and the Julian which preceded it. Even they are divided into "months"; but they are vestigial, their connection to the moon is not relevant. A date does not always have the same moon phase. A year is the time it takes for the earth to revolve one time around the sun.
The Muslim calendar provides a genuine lunar year. The months there are wandering around the solar year, so that a month is not even bound to a season. A year is based on exact months, which are calculated based on the moon and with no adjustment to sun and season. It is always the same moon phase on the same date. Such a calendar is impossible to use for agricultural planning. It is mainly used for religious and cultural purposes. The Muslim calendar is the only presently used lunar calendar.
A Lunisolar year, for instance, is the Chinese year, or the Babylonian. They count genuine months, but then adjust the year according to the sun, by regularly adding a 13th intercalary month. So, compared to an entirely solar-based year, dates will vary slightly in relation to the Gregorian year, but not move entirely out of season. Most calendars through history have been Lunisolar.
A Solar or Lunisolar calendar can be based on the Tropical year (an ordinary Solar year) or on a Sidereal year. Roughly, a Tropical year is the time it takes for the earth in its orbit to return to the same position in relation to the sun. A Sidereal year, however, also takes into account the sun's movement and the time it takes for it to return to the same position in relation to a fixed star, such as Sirius. A Sidereal year is slightly longer than a Tropical, about 1/71 of a day. The first solar calendar, the one of Ancient Egypt, was Sidereal, based on the heliacal rising of Sirius.
The heliacal rising of a star is the moment it appears above the horizon at or just before sunrise.
The ecliptic is the path, an imaginary line, the sun apparently follows across the sky during a year. It is "apparent", because it is the earth that rotates round the sun, but that is not how we see it. The path can be the basis for an imaginary plane, the ecliptic plane. The whole solar system is often approximated as lying in that plane, although in reality it is not entirely flat.
It is called a lunar node where the moon's orbit and the ecliptic intersect. There are two such points, in old lore called "dragon points". They are of importance in certain systems of astrology and for the calculation of eclipses. The reference to "dragon" is derived from the Egyptian and Phoenician "moon dragon". A mythical being, or perhaps just an artistic representation of astronomically important points in connection with the moon and its position in relation to certain celestial bodies.
The common Zodiac is the division of the ecliptic into 12 equal pieces of 30 degrees longitude each.
The vernal equinox is drifting 1 degree westwards per 71 years, that is, the stars appear to shift eastwards. Today, at the vernal equinox, the sun rests in Pisces, thus we live in the Age of Pisces, and passes into the Age of Aquarius in 2597. This movement is called "precession" and is due to the earth axis' "wobbling".
A "great year" is a complete cycle of precession of the equinoxes; according to current estimation about 25800 years. (Different calculations give slightly different results. 25765 or 25868 are frequently mentioned.) That is, the time it takes for the vernal equinox to pass once through the whole of the Zodiac by precession. This speed is gradually increasing, and no one knows exactly how that has changed over a long period of time. The great year, however, was of importance in many old cultures.
So, since constellations drift in relation to our dates; the Zodiac we use today represents the sky as it appeared many millennia ago.
Moreover, the ecliptic passes through "the Serpent Holder", which by all standards ought to be a 13th member of the general Zodiac, but is never counted as such.
It is even worse than that. Due to the difference between the actual orbits of planets and moon on one side, and the theoretically calculated ecliptic on the other, the celestial bodies occasionally move through other constellations than the common Zodiac, namely (according to Belgian astronomer Jean Meeus): Auriga, Cetus, Corvus, Crater, Hydra, Orion, Pegasus, Scutum, and Sextans. A troublesome fact for astrologers.
(This article is based on material previously published in Meriondho Leo, and in my e-book “Numericon”, 2019.)
Related articles:
“The Dog Star & The Dog Days ”, about Sirius & the Old Egyptian Calendar.
“Snakes in Astrology & Heraldry ”.
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