History & Culture
Annals of Astronomy:
The Rarity of Celestial Events
Given vast distances between objects, the passing of one directly in front of another is an event of note.
By Dr. Clifford J. Cunningham
Start of a transit of the Sun by Mercury. Courtesy of NASA Solar Dynamics Observatory
Start of a transit of the Sun by Mercury.
[Courtesy of NASA Solar Dynamics Observatory]
Amongst rare celestial events, one can list a few things on a scale of 1 to 10. Total solar eclipses, by comparison, events that happen somewhere on Earth nearly every year, are really not that rare. So I will give that dramatic event a rating of just 1.
Next up would be a transit of Mercury across the solar disk. This happens 13 or 14 times per century, and I assign it a rate of 3; the next one will happen in 2032. A meteor storm is defined as a rate of more than 1,000 meteors observed per hour: they are highly unpredictable, and rate a 5.
A rating of 7 goes to “great comets,” defined as those that are visible in daylight. In the past 336 years, only nine such comets have been seen. This is an average of one every 37 years, but the brightest comet ever seen in recorded history was in 1882, so we have not been treated to full cometary splendor for 143 years!
Transits of Venus across the Sun, unlike those of Mercury, only occur on average every 80 years. There have been only seven such transits in the telescopic era, which began in 1610. The next happens in 2117. I assign Venus transits a rating of 8.
At a level of 9 are supernovae visible from Earth to the unaided eye (not telescopic ones which are observed frequently, as they can be seen in other galaxies). In all of recorded history, only eleven such supernovae have been seen, with a frequency of one every 200-400 years. No supernova has been observed in the Milky Way since 1604.
The rarest celestial event of all is one you probably have never heard of: the passage of one planet in front of another! There are actually two events possible, a complete occultation or just a transit. Since the invention of the telescope, only fourteen twelve have happened: six were occultations, and eight were transits.
According to scientist and historian, Jan Meeus, there is only one recorded case of the occultation of one planet by another, having been observed telescopically. This happened on 28 May 1737, when Venus passed in front of Mercury. It was seen in daylight by the Astronomer Royal, John Bevis, at Greenwich, although he noted that the view was interrupted by clouds. The first recorded occultation of one planet by another in pre-telescopic times happened on 19 September 1170: Mars passed in front of Jupiter. This dramatic naked-eye sight was observed in England and China. No other occultation of one superior planet by another has ever been seen; the next will be a repeat in 2223 of Mars in front of Jupiter.
Both the 1170 and 1737 observations certainly rate a 10, but what about the transit of one planet in front of another? While none have been seen through a telescope, there is one other such event that has been overlooked in computer-generated studies of such things.
It is reported in the 1651 book Almagestum Novum (History of Astronomy) by Giovanni Riccioli. Julianus Ristorius (professor of astrology at Pisa University) viewed Saturn nearly covered by Mars at 2am on 19 May 1536. This transit qualifies as a partial planetary occultation: I rate this event a 9.9. Readers are welcome to provide verified data of other instances, but my search has revealed no other such occultations that have actually been observed. I am not including exoplanet research here, where planetary occultation observations are made by advanced telescopes on Earth and in space.
Looking to the future, one only need to wait another 40 years for a 9.9 event! In 2065, Venus transits Jupiter. And in 2067, the rarest event of all (number 10 on the scale) will happen once again when Mercury occults Neptune.
Since Ristorius (also spelled Ristori) is hardly known in the history of astronomy, a few words about him are in order. He lived from 1492 to 1556, and in 1528 he reportedly issued an astrological prognostication that Duke Alessandro of Florence would die young. When the Duke was assassinated 1537, he was only 27; Ristori’s prediction astonished all of Italy, and he became famous across Europe. Alessandro’s successor was Cosimo de Medici, the Grand Duke who founded the rise of the great Medici family and inaugurated the Golden Age of Florence. Cosimo I became the patron of Ristori, just as Cosimo II became the patron of Galileo in 1610.
Ristori taught at university on Ptolemy’s Tetrabiblos, which established a division between astronomy and astrology. Ristori, however, combined them as a twofold division of astrology. What for Ptolemy was astronomy became speculative astrology; the other half was called by Ristori “judicial astrology,” corresponding to both theoretical and practical astrology. While happily wed by Ristori, the ultimate divorce between astronomy and astrology took a very long time. Well into the next century, even such an eminent astronomer as Johannes Kepler was compelled to practice astrology to earn money. More about Ristori can be found in an excellent 2011 book by Robert Westman: The Copernican Question.
DR. CLIFFORD J CUNNINGHAM is a historian of astronomy and a research fellow at the University of Southern Queensland. He was recently seen chatting with the famed Turkish/Kurdish author Burhan Sönmez, President of PEN International, a worldwide association of writers.
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