It's a nice clear example of the astrological text. Starting at the left, and going clockwise (therefore backwards through the zodiac), I interpret it thus:
- The Ascendant (the part of the Zodiac on the horizon) is Leo ♌ 4°2', and Saturn ♄ at Leo ♌ 12° is about to rise.
- Jupiter ♃ is somewhere in Cancer ♋️.
- Venus ♀ is at Gemini ♊ 12°.
- The Sun ☉ and Moon ☽ are both at Taurus ♉ 15° - it's a New Moon.
- Mercury ☿ is at Taurus ♉ 5°.
- The Mid-Heaven (the part of the zodiac at the zenith, due south) is at Aries ♈ 27°37'.
- The Descendant, opposite the Ascendant, is at Aquarius ♒ 4°2'.
- Mars ♂ is at Scorpio ♏ 21°.
- A strange star-shaped glyph is marked as being at Scorpio ♏ 15° - I thought at first it might be the lunar node, which would mean that this was the chart for a solar eclipse, but I no longer think so for reasons described below.
- The lower mid-heaven, opposite the mid-heaven, is at Libra ♎ 27°37'.
|Longitude||♌ 12°||♋ ?||♏ 21°||♉ 15°||♉ 15°||♊ 12°||♉ 5°|
It is possible, but tedious, to run these positions through the historical record of planetary positions. I did so, using the proprietary Alcyone Ephemeris, and emerged after a lot of grinding with the following planetary positions for the full moon at 0908 on 30 April 1006:
This is, er, not a bad fit, if I say so myself. I really had to pinch myself to make sure I wasn't dreaming. So I cross-checked with Tuckerman's Planetary Tables which include a timestamp of midnight on 1 May 1006:
It's pretty clear that the software and Tuckerman are in close agreement. So a plausible hypothesis is that Dee - who, let's not forget, lived from 1527 to 1608 - was casting a horoscope for the new moon of 30 April 1006.
Well, the brightest ever supernova recorded in history, SN1006, was first seen from Cairo on precisely that day, 30 April 1006. (Cairo also fits with the mid-heaven and ascendant, whose relative positions vary considerably with latitude.) Specifically, Ali ibn Ridwan recorded the positons of the planets at the time when the supernova was first seen, and his account is quoted verbatim in the 1964 paper by Bernard Goldstein which first proposed that there might have been a forgotten supernova in 1006 (first page, second page). The planetary positions recorded by ibn Ridwan are exactly the same as Dee with a couple of additions:
|Longitude (ibn Ridwan)||132°||100°||231°||45°||45°||72°||35°||263°|
Ali ibn Ridwan was only 18 in 1006, so if these were his own calculations - the Moon and Mercury (let alone the ascending node) would not have been visible, so he couldn't have observed them directly - it's a jolly good piece of work. Goldstein basically did the same calculation as I did, with much more difficulty given the fewer tools available in the 1960s, and came up with the same answer. It is pretty obvious that Dee was drawing up a chart of his own to follow Ali ibn Ridwan's report. The clincher is that the longitude of the 1006 supernova is reported by Ali ibn Ridwan as precisely Scorpio ♏ 15°, which is where the odd star-shaped glyph is in Dee's chart.
So how did Dee get hold of Ali ibn Ridwan's observations? That is the easiest question of all to answer. The horoscope is drawn in the margin of the 1619 Venice printing of Ptolemy's Tetrabiblos, Quadriparti in Latin, which actually includes the commentary on Ptolemy's text by Ali ibn Ridwan ("Haly" in the Latin tradition) in which he wrote about the 1006 nayzak. Dee was fascinated by the supernova of 1572, and may have spotted this account as relevant to his interests. Or he may have just relished the exercise of drawing up a horoscope for a different day and age. At any rate, I think the story behind this particular marginal note of Dee's is now clear.
Well, that was a pleasant way to spend a rainy Saturday afternoon!