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Reverse engineering a historical chart

 
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Martin Gansten
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Posted: Sun Sep 10, 2017 3:04 pm    Post subject: Reverse engineering a historical chart Reply with quote

I believe some astrological software allows for 'reverse engineering' of a chart to find the time and/or place for which it was cast. I would be very grateful if someone could help me with the correct longitude for a chart with 27░47' tropical Leo rising, the other data being: 21 July (Gregorian), 1623 CE, latitude 25░50', time: 2h 14m 24s after sunrise (translating into somewhere around 07:30 local apparent time). The place would be somewhere in northern India.
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carriere.francois



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Posted: Mon Sep 11, 2017 10:00 pm    Post subject: Reply with quote

Martin,

I have asked that question to Alois Treindl once, from Astrodienst. Although it is possible to do it (I do it regularly on the internet, to get a working chart), it is not that precise. If I remember correctly, Alois told me there could be some error within the range of 100-200km or so... Otherwise, I don't know of any software which is able to do that. For the given chart, do you have the Midheaven longitude? Otherwise, it is almost impossible to construct a working chart.. Midheaven will help to get some near longitude. After that, Ascendant gives some latitude.
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Martin Gansten
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Posted: Tue Sep 12, 2017 6:53 am    Post subject: Reply with quote

Thank you, Francois. No, the text I'm working on doesn't give a midheaven for the chart, so I had only the rising degree and the length of day (which, with the reconstructed date, gave the aproximate latitude) to go on. As it turns out, however, the place is mentioned further on in the text, something I had overlooked the first time around: it is supposed to be Varanasi, which means that the day length (13h 24m) and corresponding latitude were a bit off. A healthy reminder not to put too much trust in seventeenth-century cartographers...
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Paul
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Posted: Tue Sep 12, 2017 8:24 pm    Post subject: Reply with quote

Martin

There's probably a very clever way of doing this, but sadly I don't know what it is. The major problem here could be with the atlas or time zones of 1623 in India I'm guessing?

In any event, if you cast a chart for Katihar for 7:40am (which apparently would be around that much time after sunrise etc.) then you get a chart that's pretty close.

Sorry I can't help more but maybe it helps you narrow it down a bit.
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Martin Gansten
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Posted: Wed Sep 13, 2017 6:50 am    Post subject: Reply with quote

Thanks, Paul. As I said in my last post, the place is identified later on in the text as Varanasi, but it could have been pretty much anywhere around the same latitude -- I realized, also belatedly, that the fact that the time is (of course) LAT means that it doesn't really help. It doesn't give us an absolute time for when the degree in question was on the ascendant, but just tells us that it was about 2h 15m after local sunrise, which would be true all across North India.

I think the inexactitude is probably down to 17th-century cartography, yes.
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skyrack



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Posted: Wed Sep 13, 2017 6:54 pm    Post subject: Reply with quote

https://en.wikipedia.org/wiki/Time_in_India

may be of interest
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Martin Gansten
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Posted: Thu Sep 14, 2017 6:22 am    Post subject: Reply with quote

skyrack wrote:
https://en.wikipedia.org/wiki/Time_in_India

may be of interest

Thanks, but premodern (and even early modern) birth times in India were always given in relation to local sunrise, that is, in local apparent time.
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skyrack



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Posted: Thu Sep 14, 2017 9:04 am    Post subject: Reply with quote

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skyrack



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Posted: Thu Sep 14, 2017 9:06 am    Post subject: Reply with quote

Using NOAA Sunrise Calculator with sunrise for 21 July (Gregorian), 1623 CE at 25n50 and 82w58 (about 30kms north of Varanasi centre)
gives sunrise at 23:48 20Jul UTC.

https://www.esrl.noaa.gov/gmd/grad/solcalc/sunrise.html

Given the time after sunrise this equates to 02:02 UTC or 07:32 in India Standard Time.

Casting a chart for this time and place gives an ASC of 26Leo02

Now if we move 1 degree East, about 80kms ENE of Varanasi and about halfway between Varanasi and Patna, local time advances 4 minutes.
Casting a chart for this time and place gives an ASC of 27Leo47.

https://www.google.co.uk/maps/place/25%C2%B049'58.8%22N+83%C2%B057'36.0%22E/@25.833,83.3996973,9z/data=!4m5!3m4!1s0x0:0x0!8m2!3d25.833!4d83.96
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Martin Gansten
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Posted: Thu Sep 14, 2017 10:51 am    Post subject: Reply with quote

I'm not entirely sure what point you are trying to make, but as long as the geopgraphic latitude of two places is the same (and disregarding factors like elevation above sea level, i.e., considering only the rational horizon), the time elapsed between two designated points on the ecliptic rising will necessarily be identical. Thus, if the sun is at 28 Cancer, then 26 Leo will rise at the same time after local sunrise irrespective of geographic longitude.

(Of course, as the sun's longitude keeps changing, it will not be in the exact same degree and minute in two local sunrise charts; but as long as we confine ourselves to a region like northern India, the difference will be more or less negligible.)
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skyrack



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Posted: Thu Sep 14, 2017 11:20 am    Post subject: Reply with quote

Not making any point.
Merely giving you a time and longitude that fits your data.

Varanasi is both a city and a district.

I suggest that the time and longitude I gave fits your data and any substantial change in either will not.
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Martin Gansten
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Posted: Thu Sep 14, 2017 1:10 pm    Post subject: Reply with quote

skyrack wrote:
I suggest that the time and longitude I gave fits your data and any substantial change in either will not.

I'm afraid you're wrong there, for the reasons I already explained. You can verify it experimentally:

For the date given, local sunrise at Varanasi was at 05:18:51 LAT, while 27Le47 was on the Asc at 07:36:51. That is a difference of 2h 18m.

Keeping the latitude but changing the longitude 10 degrees west, local sunrise was at 05:18:52 LAT, while 27Le47 was on the Asc at 07:36:45. That is a difference of 2h 17m 53s.

So changing the longitude 10 whole degrees west gives a difference of only 7 seconds of clock time, and that only because of a slight difference in the sun's ecliptical longitude. Had the sun been stationary (which of course never happens), there would have been no difference.

This means that any place in North India with the same geographic latitude would have fit the bill -- something I hadn't realized when I started this thread.
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