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Say for example I wanted to know where the planets were on December 19th, 1664, at 1am; how do I figure that out?

B--rian
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Trip
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    @usernumber this question relies on observational/spherical astronomy to obtain solutions, not astrology – Trip Dec 12 '19 at 15:19
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    I remember a book I had once by Gerald Hawkins called Mindsteps to the Cosmos in which he showed how to calculate the positions of the sun and moon (and planets?) for archeoastronomical purposes. He used it, for instance, to calculate the alignments at Stonehenge (these alignments are now discredited, but only on statistical grounds, the algorithms are sound as far as I know). The book contains the algorithms I believe. – Dieudonné Dec 12 '19 at 16:17
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    I livestreamed my work on this answer at https://www.twitch.tv/videos/520746710 and will try to writeup an answer shortly. –  Dec 13 '19 at 04:07

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You can use the software Stellarium, which is available on most plateforms, and in-browser. With this software, you can show the constellations and the planets at any time and date, and see where the planets are in the sky.

For instance, here you can see that Mars is in Libra and Mercury is in Scorpius. December 12, 2019 at 10 am

At the dates you specified, the positions of the planets were the following December 19, 1664 at 1am

usernumber
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    This answer will give you the actual constellation the planet was in, but not the astrological sign as the constellations have shifted because of precession (e.g. the vernal equinox is now in Pisces in stead of Aries). Given the choice of words, I think the OP is referring to the astrological sign. I imagine that there will be some astrological software that will give you the astrological sign of a planet. – Dieudonné Dec 12 '19 at 09:21
  • @Dieudonné Is astrology on-topic here? – usernumber Dec 12 '19 at 09:47
  • No, but I think that is what the OP is asking. – Dieudonné Dec 12 '19 at 09:57
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    @Dieudonné, the reason I ask in not due to wanting to use astrology, it just so happens that the houses/signs have been used to record the placement of the planets in the past. Take the example of Zodiac of the Metternich Stele. The use of spherical astronomy helps obtain possible dates for when the planets would be in the positions. It’s a simple zodiac where all 7 traditional planets are in Leo. I’m just trying to learn how to figure out how to calculate the solutions for that zodiac and like 50 others so I can obtain the possible dates within. It’s not for astrology, its for astronomy – Trip Dec 12 '19 at 15:27
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    That is an interesting archeoastronomical application! Assuming that Stellarium is able to precess to the dates you need, the solution proposed by @usernumber should work. But this is of course manual work. Do you need to process a large number of planetary positions? – Dieudonné Dec 12 '19 at 16:05
  • yes, really beyond softwares I want to learn how to create the softwares so as to be able to provide a number of solutions given a plethora of variables – Trip Dec 12 '19 at 16:16
  • @Trip Keep in mind that the calculations for planetary positions may not be valid in the distant past. Especially if they are based on older planetary ephemeris. Recent ephemeris created by JPL, however, cover the whole time span of human history (but not prehistory, i.e. from the year 13200 BC). – Dieudonné Dec 12 '19 at 16:26
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    If you are familiar with Python, you might want to look at astroPy. There are off course other libraries in other languages. – Dieudonné Dec 12 '19 at 16:28
  • @Dieudonné, yes, there are many variables hence why I want to learn them all and then to use them and to be able to explain them – Trip Dec 12 '19 at 16:55
  • @usernumber Questions about astrology are off-topic on this site, and astrology-related questions are generally not well-received here. However, if the questions are motivated by astrological sources but concern only astronomical phenomena, then they are on topic. OTOH, there's an older meta post with a more definite anti-astrology stance. – PM 2Ring Feb 10 '21 at 12:38
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One word answer: skyfield

One sentence answer: Per https://github.com/skyfielders/python-skyfield/issues/228 the most recent versions of Brandon Rhodes' skyfield (https://github.com/skyfielders/python-skyfield) can now compute what constellation a given object is in by precessing the right ascension and declination to B1875 (the epoch where the constellations were defined with "straight" lines) and using a nonuniform grid to determine the constellation.

Notes:

  • Stellarium can display precessed constellation boundaries (and names). Example:

enter image description here

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For example, in the image above, you couldn't really tell which constellation the stars inside the red box just by looking at the constellation figures without knowing the boundaries in advance. Since archeoastronomy predates the IAU's definitions, the IAU's boundaries wouldn't be helpful. I suspect you're more looking for when an object is "really" inside a given constellation.

This is also why determining the start of the "Age of Aquarius" is non-trivial, since it's unlikely that ancient peoples knew of or could predict IAU's boundaries.

  • Unlike the 12 astrological signs of the zodiac (see Period of unique horoscopes?), there are 13 signs in the IAU "zodiac", and the Sun doesn't spend even nearly the same amount of time in each constellation.

enter image description here

As shown above, the Sun spends much more time in Virgo than in Scorpio.

  • Although the Sun stays pretty close to the ecliptic, the other planets and Moon do not, which means that the Moon (for example) can be in "non-zodiacal" constellations: What constellations touch the 9-degree wide Zodiac?

  • The technique Rhodes uses is fairly generic and should be easily portable to other languages. I'm looking into porting it to C myself, so I can use it with CSPICE. My version would also return how close to a boundary line a given object is, so I could use CSPICE's "geometry finder" functions.

  • I livestreamed my answering this question at:

Barry Carter
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    The OP wants to know the (Western) astrological sign of the planet, not its constellation. So you just need to calculate the ecliptic longitude. Each sign occupies exactly 30°, starting at the First Point of Aries. – PM 2Ring Feb 10 '21 at 12:12