The most intellectually exciting thing I have ever done, or am ever likely to do in my career, was to discover the likely date of birth of Eleanor of Aquitaine. She was Duchess of Aquitaine in her own right from 1137 to 1204, Queen of France from 1137 to 1152, and then Queen of England from 1154 to 1189 having married and divorced Louis VII of France and then married and survived Henry II of England. She lived until 1 April 1204. Her children included King Richard I (“the Lionheart”) of England and his younger brother King John. The only contemporary image we have of her is from her tombstone; but in popular culture, the dominant image is Katharine Hepburn’s Oscar-winning portrayal of her in the 1968 film The Lion in Winter.

Doing my M Phil thesis in 1991, I was trying to get to grips with a medieval astrological text by one Roger of Hereford, Liber de arte astronomice iudicandi. It’s a hodge-podge of Arabic astrological lore translated into Latin, sometimes well, sometimes less well, and I had great fun identifying the sources that Roger had used. Various clues pointed to the date of writing as being the early to mid 1190s. But then I ran into a problem at the end. There is a worked example of a horoscope, giving precise positions for the planets at a particular time, and then an interpretation of what this horoscope means. It seems to be original text, not copied from other sources like most of the rest of the book. The details given are:

Sun	29° Sagittarius
Moon	15° Gemini
Venus	15° Capricorn
Jupiter	20° Cancer
Mars	10° Gemini
Mercury	20° Sagittarius
Saturn	22° Leo
Ascendant	15° Libra
Mid-heaven	18° Cancer

Now, there is an immediate problem. There is no historical date that fits those planetary positions. But I think that the following assumptions about Roger’s working methods are not unreasonable:

1) The solar position is likely to be the most accurate. This is because the sun’s apparent motion was accurately known, even using the geocentric model in use in the middle ages. The sun passes through the same point of the ecliptic at roughly (to within a day) the same time every year. During the 12th century it passed through 29° Sagittarius on December 14. Roger would have known that the sun could only be at that position on that day.

2) Jupiter and Saturn, the slowest moving planets, will be the most useful for determining the year in question. In this case there is a problem; there is no good fit for Jupiter at 20° Cancer and Saturn at 22° Leo in the historical period. However, Saturn was at 22° Leo in late December 1123, and Jupiter at that time passed through 20° Gemini (next to Cancer) on December 9 and reached 19° Gemini on December 16. If “Cancer” could be a mistake for “Gemini” in the Jupiter position, the horoscope would be consistent for 14 December 1123.

3) Mars provides an extra element of confirmation, having passed through 10° Gemini on December 9 1123, which is close enough given that it’s more difficult to calculate. So we have close matches for Saturn and Mars, and Jupiter precisely one sign out, for 14 December 1123.

4) The positions of the inner planets for 14 December 1123 are wildly discrepant with the positions in our horoscope. However on 14 December 1122, Venus was at 21° Capricorn (having passed through 15° Capricorn on the 9th), Mercury at 14° Sagittarius (having passed through 20° Sagittarius on the 8th) and the Moon at midnight is at 21° Gemini, having passed through 15° Gemini about ten hours earlier.

5) We therefore have the three outer planets fitting 14 December 1123 within a degree of longitude (if the assumption about Jupiter being put in the right degree but the wrong sign is correct), the three inner planets fitting early December 1122 a bit less well, and the Sun fitting both dates.

So, why choose 1123 rather than 1122? It seems to me – having tried it for myself – that the inner planets are much more fiddly to calculate using the available methods, and it is more likely that Roger of Hereford read the wrong line from the algorithm in the more complex process. (Though in my story he got Jupiter wrong too).

One other interesting point is that the distance between Ascendant and Mid-Heaven is too small for this to be a horoscope cast for a British latitude at that time of day and that time of year. The Ascendant is the part of the Zodiac rising on the eastern horizon; the Mid-Heaven is the part of the Zodiac directly due south. The farther from the equator you go, the more the distance between them will vary. The numbers given are consistent with 44° North, with a leeway of about 6°, probably calculated using an astrolabe plate for 45° North. Roger is not know to have worked anywhere other than England, almost all of which is north of 50°.

So, let’s look at what this is supposed to mean. Roger says:

Primum considerarem domini ascendentis, et qui ipse ab angulo recte respicit ascendens. Ab eo inciperem. Primum inspicio a quo separatur. Separatur autem a sole qui est dominus 10m, et est in quarto. Scio igitur per he quod cogitat de aliqua re amata que per quartum signatur vel de patre vel de matre; sed quia luna separatur est a domino 7 qui est domus mulierum id est de matre. Quum vero est in domo vie quod de via mulieris est iuncta mercurio domino vie, in alia domo viarum. Et quum venus coniuncta est iovi in domo regis dico quod ad regem tendit cum ipse etiam sit in exaltatione sua. Sed quum est retrogradus et in oppositione veneris et venus in casu eius, rex non bene eam recipiet. Sed quum est fortuna etiam in exaltatione sua et in angulo celi liber a male postea exaudiet eam. Sed et luna que est recepta ideam signat et est de inimicis, quod virgo est signum humanum et quod est in humano signo, est dominus 12.

Roger’s interpretation of the horoscope that he has just cast is a bit confused and mumbo-jumbo-ish, but like any good soothsayer he starts off by predicting what he already knows to be true: that the inquiry is about a loved one, a parent, a woman, therefore a mother. Then things get interesting. He seems to be saying that this mother is travelling to meet a king, that the king will not give her a good reception, but will give her a hearing afterwards, and that there are enemies involved.

As I said, we know that the most likely date of composition of Roger’s text is in the early to mid 1190s. Why is he casting a horoscope for a date in 1122 or 1123? Who could be the mother going on a journey to meet a king, where she could expect a poor reception but could hope to prevail in the end? Could this be anything other than a horoscope cast for Eleanor of Aquitaine, or some other interested party, to assess the chances of her planned mission to ransom her son Richard I from German captivity in 1193-4, after she raised literally a king’s ransom from the English taxpayer?

It may seem a bit of a stretch, but it’s difficult to imagine another set of circumstances in which a woman (a mother, indeed) born in 1122 or 1123 would be going on a long journey to argue with a king in the 1190s. (If you want to check my working, my M Phil thesis is available in RTF format here. There’s a lot of it that I would write rather differently, thirty-two years on, but I stand by the main conclusion.)

Also, remember the Ascendant/Mid-Heaven calculation that was consistent with 44° North, with a leeway of about 6°? Eleanor was born in Poitiers, whose latitude is 46°35′ North. A lot of other places are in that range, of course. (Though we can probably rule out Minneapolis.)

Scholarly consensus leans more towards 1122 than 1123 for Eleanor’s birth, I must admit, and the case for reading the planetary positions as intended to represent 1122 rather than 1123 is strong. But the very earliest document has her aged 13 in the spring of 1137, when her father died and just before her first marriage, which would mean that she was born between mid 1123 and early 1124; and I’m inclined to believe that the closest report to the event is likely to be the most accurate. There is no document indicating her precise birthday.

So, all in all, I reckon that tomorrow is the 900th birthday of one of my favourite historical characters. Let’s raise a glass of Bordeaux to her. She’d have appreciated it.

I hope that I’ll get the chance to write this up properly some day, to proper rigorous academic standards. I fear it will be a retirement project, almost fifty years after I did the original research. But twelfth-century history is not a terribly fast-moving discipline.