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Hipparchus, Ptolemy, and Uranus

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Posted: Fri Jan 03, 2020 7:06 am    Post subject: Hipparchus, Ptolemy, and Uranus Reply with quote

I was searching the Internet recently regarding the question of why Uranus, technically a "naked eye" planet, wasn't discovered long before 1781. I came across these articles:

From the scholarly Journal for the History of Astronomy :
Rene' Bourtembourg, "Was Uranus observed by Hipparchus?" (2013)

The author argues that there is strong circumstantial evidence (although no definitive proof,) that Uranus was observed by Hipparchus and Ptolemy, and cataloged as a fixed star. If so, Ptolemy placed it in the constellation Virgo. Since the Almagest was heavily based on the lost work of Hipparchus, it is possible that he was the first to record Uranus.

See also this earlier article on the likelihood that Uranus was observed-- as a fixed star-- in Antiquity, as well:

Keith P. Hertzog, "Ancient Uranus?" Quarterly Journal of the Royal Astronomical Society, Vol. 29, no 2, (1988)

Maybe ancient Uranus is well known to y'all, but this is the first I've come across this information.

More definitively Uranus was observed in 1690 by John Flamsteed, who also thought it was a fixed star. The French astronomer Charles Le Monnier also observed Uranus between 1750 and 1769-- as a fixed star. Herschel's "discovery" in 1781 consisted of correctly identifying Uranus as a planet.

I doubt that a committed traditional astrologer would allow the inclusion of Uranus as a planet under any circumstances, but I wonder whether the early-- and probably ancient-- designation of Uranus as a fixed star allows more lenient trads to treat it as such.

Assuming I've correctly identified the region of the constellation Virgo where Ptolemy probably located Uranus, Ptolemy (Tetr. 1:9) he said the stars in this region of the constellation had the nature of Mercury and Mars. Which is how some trads have interpreted characteristics and events that modern astrologers assign to Uranus.

In his treatment of fixed stars, Ptolemy often grouped them together according to regions within the constellations, vs. giving each star a separate delineation.

Treating Uranus as a fixed star might allow its use in by more permissive traditional astrologers without the problem of essential dignities.

Of course, if they "don't need it," that's fine.
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Posted: Fri Jan 03, 2020 8:33 am    Post subject: Reply with quote

Have you found any significance to some fixed star of the least magnitude? Do you know any other fixed stars that can't be seen most of the time? If no, maybe the same logic should apply to Uranus? Leery
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Posted: Fri Jan 03, 2020 10:11 pm    Post subject: Reply with quote

No one is being urged to use Uranus as a fixed star if they don't care for the idea, on whatever grounds.

It's hard to say what Ptolemy meant in comparing fixed stars to planets, but in a couple of places and in his section title, he talks about the stars' "powers" and "temperature," from which we might infer their conditions of temperature and moisture based on his sec. 1:4, "Of the powers of the planets."

In Tetr. 1:9, Ptolemy brings up the subject of magnitude only in a simple binary manner: some stars are delineated as "bright," and some are not. He mentions only 6 stars by name: Aldebaran, Regulus, Vindemiator, Spica, Antares, and Arcturus. A few are delineated for their significance to their constellation's star-picture, like the eyes of the Crab or the tips of the horns of Taurus.

Ptolemy generally aggregates clusters of stars by asterism or region within their respective constellation star-pictures. Bright stars seem to be mentioned more for non-zodiacal constellations. Ptolemy also included open star clusters like the Pleiades in Taurus and Praesepe in Cancer. These individual masses taken as a whole have an apparent magnitude well within naked-eye range. However, individual stars are often much fainter giving them a cloud-like appearance.

It would seem that the dimmer stars take on their meaning from proximity to the brighter stars in their region.

When we find a description like the following, magnitude doesn't enter into Ptolemy's delineation:

"Of the stars in Aquarius, those in the shoulders exert an influence like that of Saturn and Mercury, together with those in the left arm and the cloak; those in the thighs, like that of Mercury in a greater degree and like that of Saturn in a lesser degree; those in the stream of water, like that of Saturn and, to some degree, like that of Jupiter."

I note that the "water" from the urn of Aquarius is the Aquariid meteor shower. This large meteorite display consists of neither stars or planets, yet Ptolemy nevertheless assigned it a planetary analogy.

If your goal, like Ptolemy's, is to produce a comprehensive, up-to-date star catalog like the Almagest, you don't want to leave any out on account of higher comparative magnitude.

Uranus has an orbital period of 84 years. Depending upon the season and its zodiac location, Uranus should be visible on a clear night when that constellation is in the night sky,i.e,. for several months on an annual basis. Popular websites on "backyard astronomy" have articles on how to spot Uranus.

At times, depending upon its position in the night sky (like further from earth,) Uranus might not be so visible to the naked eye, but then that's why ancient astronomers recorded a heavenly body's location when they could see it, and then modeled its orbital characteristics.

Similarly, Mercury isn't visible a lot of the time even when above the horizon at pre-dawn or dusk, because of proximity to the sun. Astronomers still know where it is.

Of course, overcast skies and broad daylight are a good reason to consult an ephemeris.
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Posted: Sun Jan 26, 2020 12:47 pm    Post subject: Re: Hipparchus, Ptolemy, and Uranus Reply with quote

waybread wrote:
The author argues that there is strong circumstantial evidence (although no definitive proof,) that Uranus was observed by Hipparchus and Ptolemy, and cataloged as a fixed star. If so, Ptolemy placed it in the constellation Virgo.

at the time that the Almagest is estimated to have been written (approx 150 CE), Uranus was in tropical Capricorn. Ptolemy's death is estimated to be in 170 CE, and Uranus was only just in the beginning of tropical Aries then. perhaps I'm missing something, but if not, his recording of Uranus in Virgo (assuming he saw Uranus himself and is not relying on someone else's work solely) is unlikely.

it is also worth noting that Uranus is not always visible. it pops in and out of visibility, appearing dimly to us in dark skies at the solar opposition when Uranus and the earth are as close to each other as they can be. but even then, Uranus is only just visible (if the tables I'm looking at re: magnitude are to be trusted). if Hipparchus recorded Uranus's position, as the theory goes (hence Ptolemy's inclusion, to your point), an account would be expected for how it came in and out of visibility, at certain times of the year that shifted as Uranus progressed through the zodiacal circle. at the opposition, Uranus would be moving retrograde, and Uranus would be direct when invisible, meaning that even though Uranus would only be observed to go backward, when it reappeared it seems to have 'jumped' forward in the zodiac. I think this would have been rather a confusing thing to observe, interesting enough to have deserved some text.

another thought is that Ptolemy has a rant somewhere in the middle of the Almagest where he expresses simultaneously his gratitude and frustration for Hipparchus, whose tables allow Ptolemy a glimpse into the past but are lacking in sufficient detail for more robust exploitation. somehow, I think the instability of a sometimes here, sometimes gone 'star' may have trumped Hipparchus' commitment to detail.
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Posted: Wed Jan 29, 2020 4:36 am    Post subject: Reply with quote

Good information, Wade. Thanks for generating some discussion on this interesting (to me) question.

Re: why your location for Uranus was so different from Bourtembourg's. I clicked on the link for his article that I gave I gave previously, again just now, and unfortunately couldn't get past the first page (thanks, Sage Publications Sad ) so perhaps you were unable to read it. I was also able to access the entire article via my university library membership. Anyway, the reference is given in my previous post.

To summarize, Bourtembourg thought the heavenly object at 17 Virgo would have matched the location observed by Hipparchus ca. 128 BCE. Ptolemy probably relied on Hipparchus here, as he did for a lot of his data.

I wonder if part of the problem with discrepancies in the location of Uranus in Virgo vs. Capricorn or elsewhere is that Uranus does orbit the sun every 84 years, spending roughly 7 years in a sign. It's hard to say just when an observation would have been made. But Uranus moves so slowly relative to the traditional planets that it's understandable that Uranus was mistaken for a fixed star until 1781.

Apparently another problem is that different early copies and translations of the Almagest don't all agree. (With some scholars thinking Ptolemy couldn't read his own handwriting!) Bourtembourg thinks Mr. Pt was a bit sloppy in some particulars, but that the Arab translators were probably most faithful to the original.

The author gets into the visibility question you raise, arguing that Hipparchus would have lived at a time with exceptionally good viewing conditions. If you can't get hold of the article and this point is of special interest to you, let me know, and I will try to summarize a long and technical discussion.

Fixed stars, as you know, can also swing in and out of good naked eye visibility, notably stars of variable magnitudes.
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