by John Church
| Let them draw together the bones of the metal. – Ezra Pound, The Alchemist |
I venture that one reason amateur astronomers like their hobby so much is that despite all the turmoil here on Earth, the heavens are generally peaceful, quiet, and predictable. We do have the rare supernovas in faraway galaxies, collisions of comet fragments with Jupiter or even Earth, and other events that temporarily disturb the scene. By and large, however, the universe goes on its way, free and clear of our petty problems and providing a beautiful panorama for us to explore and contemplate.
Ancient civilizations were impressed with the cycles of the moon and planets and the yearly return of the seasons. They constructed many temples and monuments to monitor these phenomena, wanting to know promptly if any baleful influences had come along to disrupt the orderly procession. Eclipses of the sun and moon came in for special study because of their drama, as did the appearances of comets. These were especially sinister, because they came according to no apparent schedule and were therefore thought to presage catastrophes.
I used to regularly observe and time occultations of stars by the moon. When this happens at its dark edge, the event can be quite striking as the star suddenly disappears. Apart from their inherent interest, good timings of these events were scientifically valuable. Predictions for any given location were available from the U.S. Naval Observatory. These were accurate to within a second or two, but not much more than that. The main reason for the uncertainty is that the moon has a very irregular edge; its profile as seen from Earth is constantly changing depending on what the perspective is at any given moment. By collecting enough such timings, mostly by amateurs, professional astronomers could refine the local topography of the moon and even its orbit. Nowadays, using retroreflectors left on the moon by astronauts, highly accurate laser ranging can take care of any remaining orbital adjustments.
It might be thought that our present-day mathematics could precisely predict the orbits of planets. This is true in one sense, but not in another. We do have computer programs that can carry out numerical approximations of the planetary orbits to excellent accuracy. However, due to the mutual gravitational attractions of the planets, it is fundamentally impossible to carry this out on a strictly theoretical basis. Even a simple system consisting of only three mutually attracting bodies cannot be solved exactly, although two can be handled. This inconvenience to us doesn’t bother the planets themselves. Merrily they roll along according to exact laws that they alone know, like T. S. Eliot’s cat who knew its own deep and inscrutable singular name even if no one else did.
Regarding the mechanics involved in computing orbits, the topic of the “speed of gravity” is an interesting one. The prevailing wisdom is that gravitational effects are propagated through space with the speed of light. However, some scientists with special expertise in orbital calculations 1 have proposed that perturbation effects must travel at a much greater speed, otherwise the solar system would be chaotic. Interestingly, computer models of colliding galaxies seem to tacitly acknowledge that gravitational effects occur nearly instantaneously over the enormous distances involved.
Be that as it may, the programs that I mentioned are good enough to locate apparent planetary positions as seen from Earth over many thousands of years. It can be entertaining to try to reconstruct planetary lineups, both at the present and in ancient times. Some years ago I decided to use such a program to try to determine if Jupiter had ever passed directly in front of Saturn, as seen from Earth, in historical times. I could find no previously published record of such a calculation. Even if this kind of event had never occurred, it still would be interesting to see if they had ever come so close to one another that they would have appeared as one body to the unaided eye. This would have been a significant event to the astrologers of yesterday and might have had historical importance as an omen.
Jupiter overtakes Saturn about once every twenty years. Since they move in orbital planes that are not tilted very much with respect to one another, there is always the chance that Jupiter will appear to come very close to or even pass in front of Saturn at these times. I was especially interested in whether this had ever occurred near the bright star Regulus in Leo, Regulus having always been of major astrological importance. The fact that Regulus is also near one of the two points in the sky where the orbits of Jupiter and Saturn appear to cross each other as seen from Earth, makes events near this star even more likely to involve close passes. Fine events with Regulus close by did indeed occur in the years 1793, 940, 86, and 27-26 BC, but not since then. None of these events involved an actual occultation of Saturn by Jupiter.
After doing all the calculations, it became clear that Jupiter had never passed directly in front of Saturn any later than 4000 BC and will not do so any earlier than 2800 AD, these being the limits of the time period for which this particular program is valid. However, they have come so close on several occasions that they would have appeared briefly as one object even to people with very sharp eyesight. These would have been exciting events to witness. The most recent one that would have been really striking was in 1226 AD. Some of us were able to see and photograph them within about 6 arc minutes of each other low in the evening sky in December, 2020.
Who can say whether such apparitions had any real influence on what was happening then, through the interpretations of astrologers? I like to think that they might have. In particular, the complex events of 27-26 BC. near Regulus also had Venus and Mars passing nearby. This may have been taken as a favorable sign for the assumption by Octavian of the formal title of Cæsar Augustus.
Astrologers long ago associated Jupiter with the precious metal electrum, an old name for a naturally-occurring mixture of about one-quarter silver and three-quarters gold as well as for amber, or fossilized tree sap. (The word “electron” is derived from the fact that amber easily takes on a static charge, and we get “electricity”from the same root.) Saturn, on the other hand, was linked with common ordinary lead. Jupiter is bright and attractive in the sky, while Saturn is a dull, slow-moving object. Gold, silver, and lead sometimes occur together in lead ores, but are rarely if ever combined intentionally. As a chemist, I find it interesting that Jupiter and Saturn have also remained unalloyed in the sky within the span of human memory and will continue so for at least another 800 years.
1 See for example T. Van Flandern, “The Speed of Gravity – What the Experiments Say,” Physics Letters A, 250 (1-3): 1-11 (Dec. 21, 1998).
The above article was adapted from Chapter 18 of the author’s book From Eve and Morning, 2003. For more on Jupiter-Saturn events, see the author’s article in the March 1991 issue of Sky & Telescope starting on page 305.
Amateur Astronomers Association of Princeton 