Thursday this week marks the 2033rd birthday of the Roman Emperor Claudius, who reigned from AD 41 to AD 54. (The calculation is a bit unfamiliar – there is no Year 0; he was born in 10 BC, so his ninth birthday was in 1 BC, his tenth birthday in AD 1, and his 2033rd birthday is 2023 years after his tenth birthday.)
My good friend Professor G has serious doubts that Claudius was actually born on 1 August, 10 BC, but she also makes a good argument as to why he would have wanted to celebrate his birthday on that day. Anyway once he had become Emperor, six months before his 50th birthday, it was too late to change the official narrative.
The historian Cassius Dio records that in the run-up to Claudius’ 54th birthday on 1 August 45 AD, he took the following preventative action:
Since there was to be an eclipse of the sun on his birthday, he feared that there might be some disturbance in consequence, inasmuch as some other portents had already occurred; he therefore issued a proclamation in which he stated not only the fact that there was to be an eclipse, and when, and for how long, but also the reasons for which this was bound to happen.
So, it’s fascinating that a) Claudius thought his birthday was a big deal for his loyal subjects (he may or may not have been correct), and that b) he was completely certain that there would be a solar eclipse on his birthday. The stakes were pretty high – if he had issued a proclamation about an eclipse that didn’t happen, he would have looked really stupid.
What’s really interesting is that Cassius Dio then gives the reason for a solar eclipse, saying that he is quoting Claudius’ decree:
These reasons I will now give. The moon, which revolves in its orbit below the sun (or so it is believed), either directly below it or perhaps with Mercury and Venus intervening, has a longitudinal motion, just as the sun has, and a vertical motion, as the other perhaps likewise has, but it has also a latitudinal motion such as the sun never shows under any conditions.
”Longitudinal motion” means the progress of the Sun and Moon around the ecliptic as their daily and monthly regular apparent movements. “Vertical motion” means varying distance from the earth, and it’s interesting that it’s taken for granted that the Moon’s distance varies but Claudius/Cassius Dio is not as sure about the Sun. And “latitudinal motion” means the Moon’s divergence from the ecliptic due to the inclination of its orbit; since the ecliptic is by definition the Sun’s path, the Sun doesn’t do this.
When, therefore, the moon gets in a direct line with the sun over our heads and passes under its blazing orb, it obscures the rays from that body that extend toward the earth. To some of the earth’s inhabitants this obscuration lasts for a longer and to others for a shorter time, whereas to still others it does not occur for even the briefest moment. For since the sun always has a light of its own. it is never deprived of it; and consequently to all those between whom and the sun the moon does not pass, so as to throw a shadow over it, it always appears entire. This, then, is what happens to the sun, and it was made public by Claudius at that time.
It’s not a bad summary of the mechanisms, and I am inclined to think that Cassius Dio is directly quoting from the imperial proclamation. Claudius, who was a polymath, was au fait with astronomy. Cassius Dio then goes on to explain lunar eclipses, and I find his vocabulary just a bit different, as if he was writing it himself rather than quoting a century-old document. It too is not a bad summary.
But now that I have once touched upon this subject, it may not be out of place to give the explanation of a lunar eclipse also. Whenever, then, the moon is directly opposite the sun (for it is eclipsed only at full moon, just as the sun is eclipsed at the time of new moon) and runs into the cone-shaped shadow of the earth, a thing that happens whenever it passes through the mean point in its latitudinal motion, it is then deprived of the son’s light and appears by itself just as it really is. Such is the explanation of these phenomena.
We tend to forget that the ancient Romans and Greeks had a very good understanding of the movements of the Sun and Moon relative to the earth, but is is well expressed here. and of course people had been predicting eclipses for thousands of years at this stage.
Solar eclipses are more difficult to calculate than lunar eclipses. They do repeat fairly reliably after 18 years, 10 or 11 days (depending on how many leap years are involved) and 7 hours. From a fixed point on the Earth’s surface, such as, for instance, Rome, this means that if you hang around long enough after one eclipse, you’ll see another 54 years and 32 or 33 days later (again depending on the number of leap years in the interim).
As it happens, just a month before Claudius was born, on 30 June, 10 BC, there was a solar eclipse which was nearly total from Rome and most of Europe. If he ever looked up the details of past eclipses (and I’m sure he did) and knew about the 54 years and 32-33 days cycle (and again, I’m sure he did), he’d have been very aware that his 54th birthday was certain to see a repeat eclipse.
Of course, I think it’s also significant that we only know about this from Cassius Dio, who was working from archive documents a century later. My suspicion is that Claudius and Cassius Dio were both much more interested in eclipses than the average inhabitant of the Roman Empire, and that the average provincial governor probably filed Claudius’ proclamation with the latest imperial directive about corn tariffs, so that the mass of the population never heard about it.
But it’s interesting all the same.
