Neith

Neith

a moon of Venus

Three of Stroobant's charts, 1887. Venus is the crescent; the lettered stars are those that stood where somebody had reported a moon. This is the whole of the refutation, and no telescope was pointed at anything to produce it.

Type	Hypothetical satellite
Of	Venus
First reported	1645, by Francesco Fontana at Naples[1]
Announced	1686, by Cassini, who had seen it in 1672[2]
Sightings	About thirty, the last in 1768
Observers	Six of standing, and others
Its name
Named	1884, by Jean-Charles Houzeau[5]
Named after	The goddess of Sais, whose veil no mortal lifted
Named by a man who thought it	Not a moon at all
What became of it
Seen at the transits of 1761 and 1769	No
Accounted for	1887, by Paul Stroobant[6]
What it was	Chi Orionis, M Tauri, 71 Orionis, Nu Geminorum, Theta Librae, and others
Exists	No

Neith was a moon of Venus. It was reported some thirty times between 1645 and 1768, by observers including Cassini, and it is not there. Every sighting that can still be checked has been matched to a star that stood near Venus at the hour the observation was made.^[6]

The case is not one of carelessness, and this is the whole of its interest. Venus is the brightest thing in the sky after the Sun and the Moon; a telescope of the period showed a small brilliant disc surrounded by a good deal of glare; and a fifth-magnitude star a few minutes of arc away, seen through that, looks exactly like a companion. The observers did the work correctly. What they had no way of doing was finding out afterwards what else had been in the field.

The sightings

Francesco Fontana at Naples reported it three times in 1645. Giovanni Cassini saw it at Paris in 1672, said nothing, saw it again in 1686 and then announced it.^[2] James Short had it at London in 1740, Andreas Mayer at Greifswald in 1759, Jacques Montaigne at Limoges on four nights in 1761, and Christian Horrebow at Copenhagen in 1764 and again in 1768.^[3]

That is a century and a quarter of sightings by people whose other observations are still good. Cassini found four moons of Saturn and the division in its rings; Short made the finest telescopes of his generation and sold them to everybody.

It was a reasonable thing to look for

By 1686 the Earth had a moon, Jupiter had four, and Saturn had five. Venus is the planet most like the Earth in size, sits next to it, and is easier to see than either. A satellite was not a wild expectation; the absence of one was the thing that would have wanted explaining.^[4]

It is worth saying plainly, because the story is usually told as a comedy of credulous men. They were looking for something they had good reason to expect, in the one place it would have been easiest to find, and they kept finding it.

Nor was any of it fraud, which is a different trade with its own literature. For the exhibitor who dispensed with the sky entirely, clipping a painted card over the open end of his telescope and charging admission to the result until a sceptic got past him to the front of the instrument in 1769, see C. Fome.

The transits

Venus crossed the face of the Sun in 1761 and again in 1769, and both transits were watched by more observers, from more places, than any astronomical event before them.

A transit is the one occasion on which a satellite cannot hide. Venus appears as a black disc on a bright ground, and anything going round it must appear as a second black disc, smaller, on the same ground.

Friedrich Artzt's drawing of the 1761 transit. Venus is the notch at the top of the Sun's limb; the small mark lettered near the centre is what he took for its moon. It is the only report of the object from either transit, and it is a report of a spot on the Sun.

Essentially nothing did. Friedrich Artzt reported a small spot following Venus across the disc in 1761, and drew it; the hundreds of others watching that day, and the greater number who watched in 1769, saw no companion at all.^[7]

That should have closed the matter and did not, quite. Sightings continued to 1768, between the two transits.

The name

Jean-Charles Houzeau, lately director of the Royal Observatory at Brussels, gave it a name in 1884, two centuries after Cassini and a hundred and sixteen years after the last sighting.^[5]

He did not think it was a moon. He thought it a small planet going round the Sun in about two hundred and eighty-three days, which would bring it near Venus at intervals and account for its coming and going. And he called it Neith, after the goddess of Sais, whose veil no mortal had lifted.

So the object was named for its invisibility, by a man proposing the wrong explanation of it, three years before it was shown not to be there at all.

The arithmetic

In 1887 Paul Stroobant, at the Belgian Academy, took the reports one at a time and worked out what had been standing near Venus on each of those nights.^[6]

Nearly all of them came out. Chi Orionis, M Tauri, 71 Orionis, Nu Geminorum, Theta Librae: ordinary catalogue stars of about the right brightness, in the right place, at the right hour. Cassini's, Short's, Mayer's, Montaigne's, Horrebow's. The observations were accurate. They were simply of something else.

No telescope was involved in settling it. The instrument was a star catalogue and a list of dates, and it did in one paper what two hundred and forty years of looking had not.

Since

Venus has no moon, and neither has Mercury; they are the only two planets that have none. Nor is there anything inside Mercury's orbit, though that took a further fifty years of eclipses to establish: see Vulcan. Venus has nonetheless been observed to have one more often, and by better people, than several planets that do.

The last word belongs to the observations rather than to the object. Thirty of them were made, and thirty of them were right.

See also

• Vulcan – the other body that was announced and was not there, killed by absence rather than by arithmetic
• Peter van de Kamp – a later companion that was not there, produced by the telescope rather than by the sky
• N-rays – where the observations were the thing at fault, which here they were not
• Moon – the satellite that set the expectation

References

1. ^ Francesco Fontana, at Naples, in 1645; three sightings, the earliest on record. Fontana's telescopes were his own and he reported a good deal that was not there, including markings on Venus, so his are the reports the case rests on least.
2. ^ Giovanni Domenico Cassini observed the object at Paris in 1672 and made nothing of it publicly. He saw it again in 1686 and then announced it. The fourteen-year silence is the conduct of a careful man, and it is why the announcement carried the weight it did.
3. ^ Short in 1740, Mayer in 1759, Montaigne on four nights in 1761, Horrebow in 1764 and 1768. Something under thirty sightings are recorded in all, the last of them in 1768.
4. ^ Cassini himself had found four of Saturn's moons between 1671 and 1684, so the man announcing a moon of Venus was the man most practised in the world at finding moons. This tells against the usual reading of the episode rather than for it.
5. ^ J.-C. Houzeau, 1884. His proposal was of a small planet with a period near 283 days rather than a satellite, which would have made the object's appearances near Venus a coincidence of geometry repeated at intervals. Neith was the goddess of Sais; the epithet Houzeau had in mind is the one about the veil.
6. ^ P. Stroobant, 1887, for the Belgian Academy: a review of every recorded sighting against the positions of stars near Venus at the stated times. The identifications include Chi Orionis, M Tauri, 71 Orionis, Nu Geminorum and Theta Librae. A few reports resisted matching, which is what one would expect of any list of this age, and none of them requires a moon.
7. ^ The lone transit report is Artzt's, of 1761. A spot seen against the Sun's disc is not the same class of observation as a point of light beside Venus at night, and the far larger campaign of 1769 produced nothing comparable. It is recorded here because an article that says "nothing was seen" would be saying something slightly untrue.

External links

• Neith (hypothetical moon) – general reference for the sighting list and the 1887 review
• Stroobant's star maps – the charts reproduced above, from the 1887 review, via Wikimedia Commons
• Artzt's drawing of the 1761 transit – via Wikimedia Commons