Naked-Eye Observations of Jupiter’s Moons

Sky and Telescope, December, 1976, pp. 482-84.

Denis Dutton

HISTORY HAS IT that the four bright satellites of Jupiter were discovered independently by Galileo and the German astronomer Simon Mayer in the early seventeenth century. These initial glimpses of what we now call the Galilean moons of Jupiter are among the first great revelations to have accrued from pointing the newly invented telescope toward the heavens. Yet, were these men the first to observe Jupiter’s satellites? There have been persistent reports, particularly in the nineteenth century, that these moons can be seen with the naked eye.

Skepticism regarding the possibility of naked-eye observations of Jupiter’s larger satellites is entirely understandable, since there is an enormous difference in brightness between the planet and its moons. Under all but the most exceptional conditions, we would expect that the satellites would be lost in Jupiter’s glare.

Indeed, John Herschel regarded all reports of naked-eye observations as “in some degree apocryphal.” And the redoubtable Admiral Smyth was also inclined to scoff, remarking of those who claimed to have made sightings that he would “rather question their knowing what they looked at, than suspect any willful breach of veracity.”

Still, against these largely offhand dismissals must be set the naked-eye sightings by such able and well-informed observers as Rev. T.W. Webb (albeit with his spectacles), the British meteor observer W.F. Denning, and more recently Richard Baum (Sky and Telescope, April, 1976, page 235).

A lengthy list of reported naked-eye observations can be found in Vol. 1 of Webb’s Celestial Objects far Common Telescopes, and a critical evaluation of nine cases was made by C.T. Whitmell in the Journal of the British Astronomical Association, 14, 361, 1904. Earlier accounts had been published by Alexander von Humboldt in his Kosmos (1845-62) and by François Arago in Astronomie Populaire (1857).

The latter work repeated the legend told of the Russian explorer Ferdinand Wrangel, who in Siberia encountered a hunter who pointed to Jupiter and explained that he had just seen the planet “swallow a small star next to it and vomit it up shortly afterwards.” Of course, this observation may refer to the close appulse of a star and have nothing to do with the Galilean satellites.

Arago also mentions an interesting case of a fraudulent report. Two sisters in Hamburg, Germany, at the beginning of the last century claimed to see the satellites with unassisted vision. The configurations they described, however, were always found to be the reverse of what was actually visible at the moment. In fact, the ladies were surreptitiously deriving the satellite positions from the Berliner Jahrbuch, an ephemeris which, like the chart in Sky and Telescope, gave predictions for an inverting telescope!

While the mendacious sisters were foiled by their ignorance, the fact that astronomical telescopes invert images has helped render all the more persuasive one of the most striking accounts of naked-eye sightings. E. Talmadge Mentall of Dorchester, Massachusetts, related the following incident in a letter to me dated November 18, 1975. A few years ago, Mr. Mentall was standing on his back porch with his eight-year-old daughter, Valerie. They had just finished putting together a small refracting telescope, and he focused it on Jupiter. He explained to his daughter that when she looked into the telescope she would see the moons extending upwards from the planet. But she replied, “No, Daddy, the moons go down from Jupiter.” Mr. Mentall then noticed that she was gazing into the sky, and he asked whether she could see the moons without the telescope. Valerie replied, “Yes, they’re real tiny, right next to Jupiter.”

If it were not for their proximity to Jupiter, all the Galilean satellites would be visible to the naked eye. Their visual magnitudes when Jupiter is at mean opposition are I, Io, 4.8; II, Europa, 5.2; III, Ganymede, 4.5; and IV, Callisto, S.S. In mid-December of this year, Jupiter will be slightly closer to the earth and sun than at mean opposition, and the satellites will be 0.1 magnitude brighter than given above.

At mean opposition, the four satellites have the following maximum angular seporations from the center of Jupiter, which itself would be 47 seconds of arc in equatorial diameter: I, 138; II, 220; III, 351; and IV, 618 seconds. This December, the maximum separations are about three percent larger.

The Galilean satellites

The traditional naked-eye test of visual acuity, Epsilon Lyrae, consists of two nearly equally bright stars separated by 208 seconds. Hence, there seems to be no possibility of glimpsing Io without optical aid. Though Europa can travel farther than 208 seconds of arc from Jupiter, the moon is probably always masked by the planet’s glare.

Possible sightings thus seem restricted to Ganymede and Callisto. The former is the brightest satellite in the system and can reach an elongation almost as great as the separation of the easy naked-eye double star Alpha Capricorni (380 seconds). Though Callisto is the faintest of the Galilean moons, it can move as far as 10 minutes of arc From Jupiter. Why then are sightings so rare and so difficult? The answer, of course, lies in the aberrations of the eye. which cause the brilliant planet to display spikes and flares that hide the moons. But eyes differ greatly in the amount of spurious flares seen when a bright point source is viewed.

In 1843 Arago tried an experiment that deserves repetition. He constructed a “unit-power” telescope in which the focal length of the ocular nearly matched that of the objective. The result, as Arago reported it, was an unmagnified image of Jupiter in which the spikes and flares were considerably contracted. Pointing this instrument to Jupiter, Arago was able to see Ganymede on the first trial. This result was confirmed by a number of his colleagues at Paris Observatory.

My own suggestion for modern observers would be to hide the planet behind a telephone wire or power line running roughly perpendicular to the plane of the moons’ orbits (approximately east-west). By looking with one eye, it might be possible to occult the disk of Jupiter and thereby see Ganymede or perhaps Callisto. To avoid bias, the planet should not be observed with a telescope beforehand, nor should the satellites’ positions be known.

Were Galileo and Mayer the first to see any of Jupiter’s bright satellites? In light of the evidence reviewed here, probably not. But we need not deprive them of the honor of discovery, for merely to see a big star swallow a small one is not to discover a satellite of the planet Jupiter. The concept of true scientific discovery involves grasping the meaning or importance of what is revealed.


ED. NOTE: Dr. Dutton calls attention to the following articles that contain additional information on the naked-eye visibility of the Galilean satellites: W.F. Denning, Monthly Notices of the Royal Astronomical Society, 34, 309, 1874; E. Holmes, Journal of the British Astronomical Association, II, 339, 1901; and T.J.J. See, Popular Astronomy, 6, 257, 1898.