The moon is a fast mover, rising about 50 minutes later each day and going round the entire star sphere in a siderial month (27.32166 days mean). Its position among the stars generally follows the ecliptic but, due to the 5.15° tilt of the lunar orbit, is usually somewhere, within a narrow band (±5.15°), south or north of it. The points where the lunar orbit crosses the eclipticThe path of the sun through the stars. Also, the plane of the earth's orbit around the sun. are known as nodes. The nodes (and thus the whole lunar orbit) rotate retrograde (backwards) with a period of 18.6 years. This gives an additional north-south movement to lunar rise / set positions compared to solar ones. So, while solstices regularly recur at the same horizon positions (within a human lifetime) The Obliquity of the Ecliptic (tilt of the earth's axis) wobbles with a period of c.25,800 years ±1.2° around 23.1°. Throughout the neolithic it was not far from the maximum and rate of change was slow but now we are rapidly approaching the centre of the oscillation., lunistices follow a cyclical pattern that expands and contracts around the solstice positions.
The measurement of daily lunar positional changes along the horizon is of no practical use because there is no obvious pattern. Yet, on a longer timescale, Lunistice positions are predictable. Thus the rising / setting positions of the lunistices act as an indirect pointer to the positions of the nodes of the lunar orbit and thus to the time of year that eclipses may occur [see Prehistoric Eclipse Prediction System].
Luni-Solar Daily Rise Position Model [1]
| North | East | South |
Think about how useful it could be to have suitably positioned landscape features as markers. Know that setting positions mirror rising positions for a theoretical horizon but any increase in horizon height would shift the effected positions further towards the equator; i.e. further south for a N hemisphere observer.
Technically, south lunistice occurs when the moon is at 0° Capricorn and north lunistice when it is at 0° Cancer. In practice, observations of rises or sets are unlikely to occur at the 0° instant. For such observations there is therefore a possible maximum error of ±12hours motion although any apparent azimuth difference will be fairly small yet, even so, the observable difference between the day before and the day after would be informative to a seasoned observer.
Under normal circumstances, waning moons may only be observed rising because they set during daylight hours. Waxing moons may not be observed rising for the similar but opposite reason that they rise during daylight hours.
The first diagram is fairly realistic but this second one is purely schematic:
Some browsers may not scale svg images correctly!
- This diagram is a guide to the basic visual pattern but is not an exact representation of reality.
- It shows the approximate observational place and phase of lunistice rises (in the east) and sets (in the west) at particular times of year.
- Invisible events are not shown (other than new moons as empty circles) but frame changes mimic the back and forth appearance of visible lunistices.
- The tropical solar year has been divided by sixteen in the way that our prehistoric ancestors seem to have done it [Prehistoric Solar Calendar] and the moons show the approximate phase and position of the closest lunistices to those days of the year.
- Should lunistice and given solar event coincide exactly then lunar phases will be as shown, otherwise the phase will be slightly more or less, depending on whether the lunistice is before or after the named solar event.
- There are actually only 13 tropical months in a year, with 10 days left over in any given year. An approximate reconciliation occurs when 40 tropical months are only 3 days more than three years but there is a difference of less than 0.1 day at 254 tropical months / nineteen tropical years.
Why did prehistoric people observe the lunistices? Because lunistice position is an indicator of the time of year that eclipses may be expected to occur. Their understanding of this (such as it was) would have been accumulated gradually over a long period by multi-generational observation. They would not have known (as we do now) that lunistice position is an indirect pointer to the position of the nodes of the lunar orbit [Eclipses can only occur when a full or new moon is close to a node.]
The third diagram reinforces the horizontal aspect of observational realities:
In addition to the tropical motion caused by the tilt of the earth's axis, a combination of two additional factors causes the overall extent of the moon's monthly motion along the horizon to expand and contract.
- The lunar orbit is tilted at 5.15° to the ecliptic plane.
- The lunar orbit rotates retrograde (backwards) with an 18.6 year period. Thus the apparent position of the nodes rotates retrograde (backwards) with an 18.6 year period.
- Consequently, lunistice rise and set positions vary cyclically with an 18.6 year period but are apparently fairly static for more than a year at either end of the range. Patient / careful observers will have seen this in the first diagram.
The time of minimum azimuth difference between north and south lunistices is known as the Minor Standstill (Min) and that of maximum range is known as the Major Standstill (Max). A standstill occurs every nine years or so and, at those times, any eclipses will be around the equinoxes. Half-way between them (Mid), eclipses will be on full moons closest to the solstices and so on.
Blue trajectories in this last diagram indicate lunistice rising positions at 1.16 year intervals. The 18.6 year lunar nodal cycle has been split into 16 equal time periods. In practical terms the period is 31 Lunistices. A tropical month count alternating between 15 and 16. This is about 14.3 synodic monthsNew moon to new moon is the longest month (29.53059 days mean) because the sun has been moving as well. and is the closest achievable reconciliation of the lunistice / lunar nodal cycle with the tropical solar year.
Notice how much slower the positional change is at both ends: The extreme positions are known as major & minor Standstills because lunistice positions are fairly static for more than a year at those times, though they do wobble a bit. Also note that minor standstills occur at horizon positions not far from the cross-quarters and that lunar mid-cycle is close to the solstices. Be aware that the north and south lunistices are in reality about fourteen days apart.
Observe diagram 1 again but do it for an extended period and think about using earthly markers for the significant positions.
Get details of all lunistices in 2024 here and of those in 2025 here
Note 1: This Luni-Solar Daily Rise Position Model is configured for Latitude N52°, c.1800BCE. Javascript must be enabled for it to run. Unlike the real world, obliquity of the ecliptic stays constant no matter how long the model runs. The width of the diagram is always 180° azimuth, whatever the screen size, but the size of sun and moon has been exaggerated. Similarity of rising place does not imply similarity of rising time. Indeed, the angular distance between sun and moon may be inferred from the moon's phase. Sets mirror rises. Real horizons are rarely horizontal.
References
- Norton's Star Atlas & Reference Handbook. Edinburgh. Gall & Inglis Ltd.
- THOM, A. 1967 Megalithic Sites in Britain. Oxford: Oxford University Press.
- Horizons Ephemeris