Precession of the Moon

[Robert Tulip]

Observation of precession of the equinoxes can most easily be seen over generational timespans by observation of the position of the full moon against the background stars against the markers of the seasons.

Precession, occurring at the very slow rate of one degree of arc per human lifetime (71.6 years) is often considered almost indetectable. The position of the sun cannot be seen against the background stars. However, for both ancient and modern observers, the ecliptic position of the full moon is easily seen, providing the most obvious marker of precession.

For example, in ancient Israel the Passover festival was celebrated in spring, at the 14th day of the month of Nisan on the night of the Paschal Full Moon after the northern vernal equinox. Before the time of Christ, the full moon on this date could be seen as in the constellation of Libra, meaning the sun was directly opposite, in the constellation of Aries. However, as the equinoxes precessed, the stars at Passover shifted, so that after the time of Christ the festival occurred when the moon was in the constellation of Virgo, and so the sun was in the opposite sign of Pisces. Passover dating is the same as for Easter. Now the full moon at Easter is nearly at the beginning of the constellation of Virgo, and will precess into Leo over the next centuries, as the Easter sun precesses from Pisces into Aquarius.

The ready observation of the shift of the Passover moon, and the widespread ancient knowledge of observational astronomy, suggests that knowledge of precession would likely have been more widespread in the ancient world than is textually attested due to its effect on the observed position of the moon. Through the 2000 years before Christ when the sun after the equinox moved through the constellation of Aries, the full moon for that month was in Libra. Passover was calculated by the ripening of barley, with an intercalary month added if the barley was not ripe. The simple observation of the stellar position of the full moon - in Libra - would have previously provided easy astronomical confirmation of the choice of month. This was no longer the case after Christ, because the Easter full moon now occurred in the constellation of Virgo. It seems plausible that ancient Jewish astronomers responsible for setting the date of Passover would have noted that a traditional observation of the paschal moon in Libra was moving out of alignment.

This popular cultural application of precession as visible in the position of the full moon against the seasons suggests that the traditional attribution of the discovery of precession to Hipparchus probably fails to recognise the likelihood of broader knowledge of this phenomenon. I am not aware of any documentation of this use of the moon, but it is an area of culture that is not well documented.

As a deductive conclusion, to what extent does awareness of the visible heavens in the ancient world suggests this observation of precession of the moon is likely?

[Hornblower]

For all we know, some ancient societies might have noticed this precession but simply did not care. Knowing when to plant barley and other crops depends on knowing what time of the year it is in terms of the solstices and equinoxes, without regard to the background stars.

[utesfan100]

The precession of the Equinoxes was known since at least 120 BC.

http://en.wikipedia.org/wiki/Hipparchus

[a1call]

The position of the sun cannot be seen against the background stars. However, for both ancient and modern observers, the ecliptic position of the full moon is easily seen, providing the most obvious marker of precession.

Precession being a slow affair, the antisolar point on moonless nights would provide a more direct observational parameter. Moon-Earth-Sun have a 19 year frequency of repeating relative positions which would make accurate observation of precession by moon difficult/inaccurate.

[a1call]

The precession of the Equinoxes was known since at least 120 BC.

http://en.wikipedia.org/wiki/Hipparchus

I always wondered how it was done. Seems like he has used an equatorial ring to determine the equinox and compare bright stars' past data.

For those who haven't bothered reading the article.

Hipparchus is known for being almost universally recognized as discoverer of the precession of the equinoxes in 127 BC.[29] His two books on precession, On the Displacement of the Solsticial and Equinoctial Points and On the Length of the Year, are both mentioned in the Almagest of Claudius Ptolemy. According to Ptolemy, Hipparchus measured the longitude of Spica and Regulus and other bright stars. Comparing his measurements with data from his predecessors, Timocharis and Aristillus, he concluded that Spica had moved 2° relative to the autumnal equinox. He also compared the lengths of the tropical year (the time it takes the Sun to return to an equinox) and the sidereal year (the time it takes the Sun to return to a fixed star), and found a slight discrepancy. Hipparchus concluded that the equinoxes were moving ("precessing") through the zodiac, and that the rate of precession was not less than 1° in a century.

[transreality]

Some of the Mesoamerican pyramids, in particular the Pyramid of the Moon, show repeated phases of overbuilding. On these pyramids a dedicatory building was aligned with stars etc. Probably due to precession, they gradually became unaligned. The rebuilding over the top corrects the orientation. The pyramid of the moon was overbuilt six times. Whether the people of Teotihuacan figured out was going on, possibly no one knows but it must have been a significant issue for the state since the rennovations would not have been cheap.

[Robert Tulip]

Precession being a slow affair, the antisolar point on moonless nights would provide a more direct observational parameter. Moon-Earth-Sun have a 19 year frequency of repeating relative positions which would make accurate observation of precession by moon difficult/inaccurate.

For all we know, some ancient societies might have noticed this precession but simply did not care. Knowing when to plant barley and other crops depends on knowing what time of the year it is in terms of the solstices and equinoxes, without regard to the background stars.

Observation of the moon was important for ancient timekeeping, through the widespread use of lunar calendars seen in the link between the words moon and month, by detection of the Metonic Cycle before the 5th century BC, and by the apparent lunar features of monuments such as Stonehenge. The Metonic Cycle means the timing of full moons in each month of successive years varies, but was quite predicable, including as to whether it would be early or late in its matching constellation/month. It is still plausible, in my opinion, that observation of the moon against the stars provided knowledge of precession.

For example, tonight's full moon can be seen in sidereal Capricorn. But the dating of the tropical signs, with the sun at about ten degrees Leo, means the moon reaches full at 10° tropical Aquarius ~= 12° sidereal Capricorn (against the sidereal constellation average arc of 30°).

Considering the ancient situation, one thousand years before Christ a full moon on August 1 would have appeared at about 25° Aquarius. Over the centuries a full moon on 1 August would appear steadily earlier in Aquarius, until at the time of Christ it occurred at 10° Aquarius, when the signs and seasons were aligned. Could the move towards this alignment have been observed by astronomers in countries which maintained good records, such as Babylon and Egypt?

How does this suggested use of the moon sits against our knowledge of ancient astronomy and calendars? It seems plausible in the context of widespread prevalence of emphasis on star watching in ancient cultures. The point is that the moon can be seen against the background stars whereas the sun cannot, so the stellar position of the full moon provided a simple observable marker of precession.

[grapes]

How does this suggested use of the moon sits against our knowledge of ancient astronomy and calendars? It seems plausible in the context of widespread prevalence of emphasis on star watching in ancient cultures. The point is that the moon can be seen against the background stars whereas the sun cannot, so the stellar position of the full moon provided a simple observable marker of precession.

I'm not convinced, yet. The moon moves fairly quickly through the zodiac--taking only a couple days each one. The precision that you're starting with for the determination includes knowing the time of the full moon--which half the time occurs during daylight hours. It's kinda a chicken/egg thing.

[Hornblower]

See this article.
http://members.westnet.com.au/gary-d...on/page9f.html
In the author's opinion, the concepts of astronomical position reckoning that we take for granted today were not at all obvious to ancient observers who were just starting to take it seriously. These concepts include the celestial equator and the determination of ecliptic latitude and longitude.

[Robert Tulip]

I'm not convinced, yet. The moon moves fairly quickly through the zodiac--taking only a couple days each one. The precision that you're starting with for the determination includes knowing the time of the full moon--which half the time occurs during daylight hours. It's kinda a chicken/egg thing.

Now Grapes, I know you are an astronomer, so you really should not have made such an ATM whopper. I assume you will be going to that forum to defend your claim that the full moon half the time occurs during daylight?

The moon moves about 12 degrees of arc per day. It is full when it is directly opposite the sun, ie rising at dusk and coursing across the sky through the night, in the constellation directly opposite the sun.

Evidence of knowledge of precession before Hipparchus includes Plato and Ezekiel.

knowledge of number is derived from the revolution of the same [the fixed stars, or circle of the galaxy]. Thus arose day and night, which are the periods of the most intelligent nature; a month is created by the revolution of the moon, a year by that of the sun. Other periods of wonderful length and complexity are not observed by men in general; there is moreover a cycle or perfect year at the completion of which they all meet and coincide...To this end the stars came into being, that the created heaven might imitate the eternal nature. (Timaeus 39)

Despite Plato's error, that the planets all meet at the end of a perfect year, he is pointing out that astronomers are aware of longer cycles than the year, of which the primary one is precession.

Ezekiel 1:19 states "When the living creatures [ie the constellations of the zodiac] moved, the wheels beside them [ie the circle of the galaxy] moved." Ezekiel was familiar with Babylonian astronomy, and this passage can readily be read as allegory for knowledge of astronomical observation of the 'wheels within wheels' seen in precession, with the slow movement of the equinox around the ecliptic most readily seen by the position of the full moon each month.

The full moon last night was visibly in Capricorn. Does it not seem likely that the ancients would have described each full moon by its stellar position?

[Hornblower]

Now Grapes, I know you are an astronomer, so you really should not have made such an ATM whopper. I assume you will be going to that forum to defend your claim that the full moon half the time occurs during daylight?

The moon moves about 12 degrees of arc per day. It is full when it is directly opposite the sun, ie rising at dusk and coursing across the sky through the night, in the constellation directly opposite the sun.

I'm sorry, but Grapes is largely right. In this thread we are concerned with the precise moment at which the Moon is at opposition, not just a period of a day or so when the Moon is not visibly out of round to the naked eye. At this moment at any given month, half of the Earth is in daylight and the Moon is not visible from that side.

Evidence of knowledge of precession before Hipparchus includes Plato and Ezekiel.



Despite Plato's error, that the planets all meet at the end of a perfect year, he is pointing out that astronomers are aware of longer cycles than the year, of which the primary one is precession.

Ezekiel 1:19 states "When the living creatures [ie the constellations of the zodiac] moved, the wheels beside them [ie the circle of the galaxy] moved." Ezekiel was familiar with Babylonian astronomy, and this passage can readily be read as allegory for knowledge of astronomical observation of the 'wheels within wheels' seen in precession, with the slow movement of the equinox around the ecliptic most readily seen by the position of the full moon each month.

The full moon last night was visibly in Capricorn. Does it not seem likely that the ancients would have described each full moon by its stellar position?

I find the aforementioned reference which I posted to be more convincing than your interpretations of your chosen sources.

[Jeff Root]

The precision that you're starting with for the determination
includes knowing the time of the full moon--which half the
time occurs during daylight hours.

Now Grapes, I know you are an astronomer, so you really
should not have made such an ATM whopper.

I read what grapes said a few hours ago. I may have
been slightly amused by it, but I agreed with it. Now
I read your comment, and I was tending to agree with
you. But I understood what he actually meant. But I
thought about it for another two and a half minutes.
Zingo! He was right. Half the time, the instant of full
moon occurs during daylight hours....

When the Moon is below the horizon.

-- Jeff, in Minneapolis

[utesfan100]

Despite Plato's error, that the planets all meet at the end of a perfect year, he is pointing out that astronomers are aware of longer cycles than the year, of which the primary one is precession.

It seems to me that Plato's quote is referring to the 19 year harmonic with the lunar calendar known as the Metonic cycle, not to the precession of the equinoxes.

http://en.wikipedia.org/wiki/Metonic_cycle

[grapes]

Now Grapes, I know you are an astronomer, so you really should not have made such an ATM whopper. I assume you will be going to that forum to defend your claim that the full moon half the time occurs during daylight?

When I read your comment, I was like Jeff. My first reaction was, o man, I made another mistake! :)

But, no, no mistake, as Jeff and Hornblower have concurred. If you're going to use the moon's position as a proxy for the sun's position, you have to know when it is 180-degrees from the sun, the exact moment. At that moment, approx. half the world is in sunlight. All astronomers know that throughout a year, the time of the actual full moon occurs during local daylight--even near noon, sometimes. That's why I called it a chicken or egg thing--in order to use this technique, you almost have to know enough about the sun that you can calculate its position directly, without observing the moon's position in the constellations. I don't think the technique gives you more precision, or works around the problem of observing the sun's position amongst the background stars. As much as the sun obscures the background stars with its light, the brightness of the full moon also makes precise determination of "full" difficult from direct observation.

[Robert Tulip]

When I read your comment, I was like Jeff. My first reaction was, o man, I made another mistake! :)

But, no, no mistake, as Jeff and Hornblower have concurred. If you're going to use the moon's position as a proxy for the sun's position, you have to know when it is 180-degrees from the sun, the exact moment. At that moment, approx. half the world is in sunlight. All astronomers know that throughout a year, the time of the actual full moon occurs during local daylight--even near noon, sometimes. That's why I called it a chicken or egg thing--in order to use this technique, you almost have to know enough about the sun that you can calculate its position directly, without observing the moon's position in the constellations. I don't think the technique gives you more precision, or works around the problem of observing the sun's position amongst the background stars. As much as the sun obscures the background stars with its light, the brightness of the full moon also makes precise determination of "full" difficult from direct observation.

Why I assumed the comment about the exact timing of the full moon was a mistake was that for the ancients, their astronomy was entirely based on naked eye observation of the sky at night, tabulated in records over generations, and mainly just required accuracy to the level of the date when the full moon could be seen. This long continual recorded observation for calendrical purposes is what enabled them to know the movement of the moon and planets, complex patterns such as the Metonic cycle, and I argue, precession. Their knowledge of the location of the full moon only had to be accurate to within a day, not the exact moment. The visible circle at the end of the second week of waxing is up to about five degrees of arc away from the actual opposition point. But for the ancients, if they had long records, long enough to see the Metonic cycle of 19 years, the lunisolar calendar method would have shown them that the predicted position of the moon was precessing against the stars.

The Wikipedia lunisolar calendar article states "The Hebrew, Buddhist, Hindu lunisolar, Burmese, and Tibetan calendars, as well as the traditional Chinese, Japanese, Vietnamese, Mongolian, and Korean calendars, plus the ancient Hellenic, Coligny, and Babylonian calendars are all lunisolar. Also some of the ancient pre-Islamic calendars in South Arabia followed a lunisolar system. The Chinese, Coligny and Hebrew lunisolar calendars track more or less the tropical year whereas the Buddhist and Hindu lunisolar calendars track the sidereal year. Therefore, the first three give an idea of the seasons whereas the last two give an idea of the position among the constellations of the full moon."

For purpose of dating, the position of the moon against the stars is therefore key to ancient calendars, with the Buddhist and Hindu calendars based on position against the stars. Using such a calendar over generations, the predicted position against the stars will rapidly become wrong, at a steady rate equal to precession of the equinoxes. Although the exact moment of fullness could not be observed except by mathematical calculation from the position of the sun, the time period of these observations was very long, enough to be readily noticeable for people who used it for their calendar.

http://en.wikipedia.org/wiki/Hindu_calendar#History states that for the Hindu calendar "The shift in the vernal equinox by nearly four months from agrahaayana to chaitra in sidereal terms seems to indicate that the original naming conventions may date to the fourth or fifth millennium BCE."

[utesfan100]

For purpose of dating, the position of the moon against the stars is therefore key to ancient calendars, with the Buddhist and Hindu calendars based on position against the stars. Using such a calendar over generations, the predicted position against the stars will rapidly become wrong, at a steady rate equal to precession of the equinoxes. Although the exact moment of fullness could not be observed except by mathematical calculation from the position of the sun, the time period of these observations was very long, enough to be readily noticeable for people who used it for their calendar.

Apparently it took until 120 BC for anyone (Hipparchus) to look back over generations of records to notice the precession, at which time it was possible to get a value consistent with what we have today.

[grapes]

Why I assumed the comment about the exact timing of the full moon was a mistake was that for the ancients, their astronomy was entirely based on naked eye observation of the sky at night, tabulated in records over generations, and mainly just required accuracy to the level of the date when the full moon could be seen.

You've made a lot of assumptions in this thread. :)

I think your question in the OP (below) was answered definitively by Hornblower's link in post#9.

As a deductive conclusion, to what extent does awareness of the visible heavens in the ancient world suggests this observation of precession of the moon is likely?

That is, the answer is "It is not likely."

Even worse, the mechanism that you propose for facilitating the observation, doesn't seem to add that much information to the process.

[Robert Tulip]

Thanks all for comments. Useful to clarify the distinction between when the moon is full and when it is seen. For ordinary observation, it seems unusual to say 'the moon is full but it is below the horizon'. "Is" in the sentence "the moon is full" is usually interpreted to mean "can be seen as". That is how I jumped to read it, so sorry Grapes for misunderstanding your comment.

I'm interested in exploring further how the precession of the Passover moon from Libra to Virgo at the time of Christ matches to ideas about ages of law and grace, but that is possibly not a sufficiently empirical question here.

I read The Myth of Babylonian Knowledge of Precession by Gary D. Thompson. Thank you Hornblower for this link, it is very informative. My view is that so much of ancient thought on these topics has been suppressed and lost that it is difficult to just use explicit extant records as Thompson does to reconstruct ancient views. There is merit in some speculative analysis, forensically looking at hidden clues that suggest a wider popular knowledge of precession. The example I mentioned at #15 of Hindu month names is a good case in point, as it would have been obvious that the moon was no longer in the star group that tradition assigned to it.

[Ivan Viehoff]

My view is that so much of ancient thought on these topics has been suppressed and lost that it is difficult to just use explicit extant records as Thompson does to reconstruct ancient views. There is merit in some speculative analysis, forensically looking at hidden clues that suggest a wider popular knowledge of precession.

The Antikythera mechanism certainly demonstrates they were much more advanced than any other evidence suggests, though clearly in that specific case it post-dates Hipparchus. And that knowledge was, for whatever reason, lost and not rediscovered for many centuries. In more recent times we can see suppression as a major factor, as for example in the suppression of Ulugh Beg's observatory following his violent death.

[Robert Tulip]

The Antikythera mechanism certainly demonstrates they were much more advanced than any other evidence suggests, though clearly in that specific case it post-dates Hipparchus. And that knowledge was, for whatever reason, lost and not rediscovered for many centuries. In more recent times we can see suppression as a major factor, as for example in the suppression of Ulugh Beg's observatory following his violent death.

Thank you Ivan. Yes you are right about these examples of suppression of astronomical knowledge. The 'Dark Ages' involved a fairly broad rejection of classical learning. The ancients must have been more advanced than we can easily tell. If we had the contents of the library at Alexandria, perhaps we would have some idea of how and why for example the pyramids were built, with their tantalising astronomical clues.

My impression is that this topic of precession of the equinox was the subject of a quite intense hidden cultural war. Various societies such as Mithraism and other mystery groups had a strong interest in astronomy. Ulansey has shown definite precessional hints in the symbolism of the Tauroctony, although the exact meaning remains unclear in the absence of any textual explanation. With precession, there are enough clues, such as the example I gave regarding law and grace as matching the symbolism of the movement of the Easter moon over the ages, to suggest there was a hidden cosmic back story to some popular ideas that are generally presented as having nothing to do with astronomy.

[Robert Tulip]

A good explanation of how Hipparchus used observation of the moon to discover precession is by Josée Sert on the website of the European Association for Astronomy Education at http://eaae-astronomy.org/WG3-SS/Wor...recession.html

It shows that observation of the precession of the moon was used to discover the precession of the equinoxes.

IV.HIPPARCHUS AND THE PRECESSION OF EQUINOXES
1.To get co-ordinates of stars, Hipparchus made observations during moon eclipses just as his predecessors did, and he compared his results with those of Timocharis about 150 years before concerning Spica, a bright star near the ecliptic. During moon eclipses, it was easy to measure the distance between Spica and the centre of the Moon, and then, as the Sun is at exactly 180° from the Moon, to get the difference of longitude between the Sun and Spica. Especially if the eclipse happened near the equinox, the position of the Sun was precisely known, and the longitude of Spica could so be deduced precisely. Timocharis had found for Spica a distance of 8° before the Autumn equinoctial point g. Hipparchus got 6°. The longitude of Spica had so increased of 2°.