Though megalithic astronomers could look at the sky, their measurement methods were only accurate using horizon events. Horizon observations of solstice sunrise/set each year, lunar extreme moonrises or settings (over 18.6 years) allowed them to establish the geometrical ratios between these and other time periods, including the eclipse cycles. In contrast, the synod of Jupiter is measured between its loops in the sky, upon the backdrop of stars, in which Jupiter heads backwards each year as the earth passes between itself and the Sun. That is, Jupiter goes retrograde relative to general planetary direction towards the east. Since such retrograde movement occurs over 120 days, Jupiter will set 120 times whilst moving retrograde. This allowed megalithic astronomy to study the retrograde Jupiter, but only when the moon is conjunct with Jupiter in the night sky and hence will set with Jupiter at its own setting.
Jupiter reaches its maximum retrograde motion half way in the loop, after 60 days from its standstill in the sky. If, at that point, there is a conjunction of the moon and Jupiter then the moon must be full since the sun will be opposite both the moon and Jupiter. This means that, when a full moon is conjunct Jupiter at mid-retrograde, it will set to the west and one can start counting lunar months until the same phenomenon occurs.
We know a single synod of Jupiter is 398.88 days and so there are exactly 13.5 lunar months in each synod since 398.88/ 29.53059 = 13.5073, just over 5 hours longer than 13.5 months. It is therefore true that, if one counts between full moons occurring 60 days into successive retrograde loops of Jupiter, it will be two whole synods before a full moon occurs in the same visual offset to Jupiter at maximum retrograde in the sky. The lunar counting in between will be 27 whole months and at that point, the synod is known to be 13.5 months long and 9/8 times longer than the lunar year.
In this way, not only could the Jupiter synod be found easily, using megalithic horizon astronomy, but knowing its length relative to the lunar year would reveal the remarkable harmonic ratio of the Pythagorean whole tone between Jupiter synod (9) and the lunar year (8 units of 1.5 months). This would have introduced the megalithic to that uniquely simple category of ratios responsible for the highly-ordered world of musical harmony.
The same procedure applied to the Saturn synod would require five Saturn synods (378 days) to complete since only then do 12.8 lunar months per synod yield an integer number of 64 lunar months. The loop of Saturn is smaller than that of Jupiter, 6.5 degrees compared to Jupiter’s 10 degrees. The reverse is true though, of the days spent by each planet in its loop, Saturn taking 140 days rather than 120. And the ratio of the Saturn synod to the lunar month is another crucial musical interval, the semitone of 16/15.
Tones and semitone are crucial to the formation of musical scales and so my proposal is that megalithic astronomy, once aware of these intervals, would have started to investigate practical music and instruments that make musical intervals. The best possible are string instruments where, if one uses a single string, one can measure the lengths of strings just as one can measure the lengths of synodic periods and find these ratios with which to form musical scales and a deeper musical tradition, inherited by the ancient Near East and other civilizations of the third millennium BC.