If you can’t tell your Cassini from your Huygens then you shouldn’t be writing about the history of astronomy.

There I was, mild mannered historian of early modern science, enjoying my first cup of tea on a lazy Sunday morning, whilst cruising the highway and byways of cyberspace, when I espied a statement that caused an explosion of indignation, transforming me into the much feared, fire spitting HISTSCI_HULKTM. What piece of histSTM crap had unleashed the pedantic monster this time and sent him off on a stamping rage?

The object of HSH’s rage was contained in an essay by Vahe Peroomian (Associate Professor of Physics and Astronomy, University of Southern California – Dornsife College of Letters, Arts and Sciences) A brief astronomical history of Saturn’s amazing rings, published simultaneously on both The Conversation and PHYS.ORG 15 August 2019. Peroomian writes:

I am a space scientist with a passion for teaching physics andastronomy, and Saturn’s rings have always fascinated me as they tell the story of how the eyes of humanity were opened to the wonders of our solar system and the cosmos.

He continues:

When Galileo first observed Saturn through his telescope in 1610, he was still basking in the fame of discovering the four moons of Jupiter. But Saturn perplexed him. Peering at the planet through his telescope, it first looked to him as a planet with two very large moons, then as a lone planet, and then again through his newer telescope, in 1616, as a planet with arms or handles.

Galileo_by_leoni

Galileo Portrait by Ottavio Leoni Source: Wikimedia Commons

Galileo actually observed Saturn three times. The first time in 1610 he thought that the rings were handles or large moons on either side of the planet, “I have observed the highest planet [Saturn] to be triple bodied. This is to say to my very great amazement Saturn was seen to me to be not a single star, but three together, which almost touch each other.”

saturn44

Galileo’s 1610 sketch of Saturn and its rings

The second time was in 1612 and whatever it was that he observed in 1610 had simply disappeared, “I do not know what to say in a case so surprising, so unlooked for and so novel.” The Earth’s position relative to Saturn had changed and the rings were no longer visible but Galileo did not know this. In 1616 the rings were back but with a totally altered appearance, “The two companions are no longer two small perfectly round globes … but are present much larger and no longer round … that is, two half eclipses with two little dark triangles in the middle of the figure and contiguous to the middle globe of Saturn, which is seen, as always, perfectly round.” [1]

saturn2

Galileo’s 1616 sketch of Saturn and its rings

There is no mention of a new telescope and it is fairly certain that all three periods of observation were either carried out with the same or very similar telescopes. The differences that Galileo observed were due to the changing visibility of Saturn’s rings caused by its changing relative position to Earth and not to any change of instrument on Galileo’s part.

Although sloppy and annoying, the minor errors in Peroomian’s account of Galileo’s observations of Saturn are in themselves not capable of triggering the HSH’s wrath but what he wrote next is:

Four decades later, Giovanni Cassini first suggested that Saturn was a ringed planet, and what Galileo had seen were different views of Saturn’s rings. Because of the 27 degrees in the tilt of Saturn’s rotation axis relative to the plane of its orbit, the rings appear to tilt toward and away from Earth with the 29-year cycle of Saturn’s revolution about the Sun, giving humanity an ever-changing view of the rings.

giovanni_cassini

Giovanni Cassini (artist unknown) Source: Wikimedia Commons

Now, Giovanni Cassini did record some important observations of Saturn; he discovered four of Saturn’s largest moons and also the gap in the rings that is named after him. Although, Giuseppe Campani, Cassini’s telescope maker, observed the gap before he did without realising that it was a gap. However, it was not Cassini who first suggested that what people had been observing were rings but Christiaan Huygens.

Christiaan Huygens first proposed that Saturn was surrounded by a solid ring in 1655, “a thin, flat ring, nowhere touching, and inclined to the ecliptic.” In 1659 he published his book, Systema Saturnium : sive, De causis mirandorum Saturni phaenomenôn, et comite ejus Planeta Novo detailing how the appearance of the rings varied as the Earth and Saturn orbited the sun.

huygens_phases1

Plate from Huygens’ Systema Saturnium showing the various recorded observations of Saturn made by astronomers before his own times

huygens_phases2

Plate from Huygens’ Systema Saturnium explaining why the appearance of Saturn and its rings changes over time and that all those different appearances can be explained by assuming the existence of the rings

Confusing Cassini and Huygens, two of the greatest observational astronomers of the seventeenth century, who were scientific rivals, is not a trivial error and shouldn’t be made anywhere by anyone. However, to make this error in an essay that is published  on two major Internet websites borders on the criminal. I have no idea what the reach of PHYS.ORG is but The Conversation claims to have a readership of ten million plus. This means that a lot of people are being fed false history of astronomy facts by a supposed expert.

If the good doctor Peroomian had bothered to check his facts, a thing that I thought all scientists were taught to do when receiving their mother milk, he could have easily discovered his crass error and corrected it, even the much maligned Wikipedia gets it right, but apparently he didn’t consider it necessary to do so, after all it’s just history and not real science.

[1]The Galileo and Huygens quotes are taken from Ron Baalke’s excellent time line, Historical Background of Saturn’s Rings.

 

14 Comments

Filed under History of Astronomy, History of science, Myths of Science

14 responses to “If you can’t tell your Cassini from your Huygens then you shouldn’t be writing about the history of astronomy.

  1. Gavin Moodie

    Why didn’t you post a correction in a comment on the Conversation piece?

  2. Sorry, Thony, you write faster this day than I can follow and read all this excavations you put into rows and columns. Did not want to stay away from sleep in far Asia now but, yes, did it!

  3. How crackpot is The Conversation? Get a load of this: https://theconversation.com/keplers-forgotten-ideas-about-symmetry-help-explain-spiral-galaxies-without-the-need-for-dark-matter-new-research-121017

    Morphogenic fields instead of dark matter? Perhaps next week will be a feature on chemtrails and how they cause autism via vaccinations because Hitler is living in Antarctica (which is on the dark side of the Moon).

  4. I’m thinking about writing some articles about astronomy, you know, for popular consumption. I mean, how hard could it be? You google some stuff, talk to a few of your buddies in the faculty lounge, and there you go. Professional qualifications are pretty much interchangeable, aren’t they?

  5. DCA

    I’d guess that this comes from the fact that the NASA spacecraft studying Saturn was called Cassini, so that’s the name he knows. The mission was Cassini-Huygens, the Huygens part being a probe that landed on Titan– almost all the PR just refers to Cassini. Also, the NASA website mentions only that Huygens discovered Titan. Shows you how deep his research was.

    1657 was quite the year for Huygens: publishing the explanation of Saturn’s rings and developing the first pendulum clock (starting at the very end of 1656).

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