Examples of Typical Quotes Showing That Moons Are Planets, 1610 – 1920s
Examples from the 1600s
Galileo in 1610:
“Four Planets revolving around the star of Jupiter…I should disclose and publish to the world the occasion of discovering and observing four PLANETS, never seen from the very beginning of the world up to our own times… These are my observations upon the four Medicean planets…” (all caps in the original)
“…for now we have not [only] one planet only revolving about another, while both traverse a vast orb about the Sun, but our sense of sight presents to us four stars circling about Jupiter, like the Moon about the Earth…”
Kepler in 1620:
“How are the planets divided among themselves? Into the primary and the secondary. The primary planets are those whose bodies are borne around the sun…; the secondary planets are those whose own circles are arranged not around the sun but around one of the primary planets and who also share in the movement of the primary planet around the sun….Jupiter has four such planets around itself…”
Galileo in 1632:
“…many detailed parallels were drawn between the earth and the moon. More comparisons were made with the moon than with other planets, perhaps from our having more and better sensible evidence about the former by reason of its lesser distance.”
English astronomer Vincent Wing in 1654:
“Twice shall this primary Planet the Earth which we inhabit, be deprived in part of the Sun’s illuminating Raies [via eclipses], and twice also shall the Earth’s Planet, the Moon, lose part of her Sun-borrowed luster, by the interposition of the Earth between the Sun and her.”
Harvard’s Almanac by Bridgen, 1659:
“[A]bout the Earth as its center, the secondary Planet the Moon is carryed, which goes from, and returnes to, the same point in the Zodiaque in the space of 27 dayes, which measures the periodical month. The next primary Planet is Mars, who finisheth his course in 2 yeares. Jupiter takes his place next after Mars and paceth round the Sun in 12 yeares; about whom as in an epicycle move 4 other Planets [Jupiter’s moons], not lyable to the eye without the help of the Telescope. The last primary Planet, which is highest in place, and consequently slowest in motion, is Saturn, who runs his circuit in no less than 30 yeares: and as the Earth and Jupiter had their moons or concomitans, so is the body of this Planet environed by two secondary Planets, not visible without the help of the forenamed Instrument.”
Huygens, 1659:
“For it appeared that the two neighbouring appendages clinging to Saturn were by no means two planets, but rather something different [i.e., the rings], while, distinct from these there was a single planet [Titan], at a greater distance from Saturn and revolving round him in sixteen days.”
Cassini, 1673:
“This Passage of Saturn gave us occasion to discover in the same place,…Eleven other smaller Stars, one of which, by its particular motion, shew’d it self to be a true Planet [Iapetus]…we have described the way of Saturn, and that of the New Planet….The greatest digression [distance from Saturn] of this Planet of treble to that of the ordinary Satellit [Titan], and this enabled us to judge the Time of his revolution to be quintuple, applying to the Satellits that proportion, which Keplerhath noted in the Principle Planets, between the periodical Times and their Distances….[Y]et it might well be, that the Circle of this Planet [Iapteus] might have some declination from the Circle of the other Satellit [Titan], as it comes to pass in the Principal planets, the Circles of which are inclined to one another…A Discovery of another New Planet[Rhea]…Our application to observe the Planet nearest to Saturn [Rhea], in the small time we had at evenings, by reason of his proximity to the Sun-beams, had diverted us from the other more remote Planet [Iapteus].”
Bernard Le Bovier de Fontenelle of the French Academy of Science in 1686:
“But what a beautiful object is Jupiter, surrounded by his four little moons, or satellites! These moons are four little planets which, whilst Jupiter revolves in twelve years round the sun, constantly go round him as the moon does round the earth…”
Isaac Newton in 1692/3:
“…the several Distances of the primary Planets from the Sun, and of the secondary ones from Saturn,Jupiter, and the Earth; and the Velocities with which these Planets could revolve about those Quantities of Matter in the central Bodies…”
“…The same power, whether natural or supernatural, which placed the sun in the center of the six primary Planets, placed Saturn in the center of the orbs of his five secondary planets, and Jupiter in the center of his family of secondary Planets, and the Earth in the Center of the Moon’s Orb;…”
Examples from the 1700s
Cotes in 1712:
“Now it is confess’d by all Astronomers that the Primary Planets about ye Sun and the Secondary about their respective primary doe describe areas proportional to the times…Therefore the centripetal forces of the Primary Planets revolving about the Sun & of the Secondary Planets revolving about their Primary ones, are in duplicate proportion &c.”
Whiston in 1717:
“Jupiter’s four Satellites, or secondary Planets… are kept in their Curvilinear Orbits by their Gravity towards Jupiter’s Center, in a duplicate reciprocal Proportion from it ; as is the Case of all the Planets, both Primary and Secondaryabout the Sun. Saturn has five Satellites or secondary Planets… And Saturn, besides his Five Planets, affords us such a Ring encompassing his Body, as seems to be the most singular and curious Spectacle in the whole System.”
Treiber in 1719, transl. from German:
“[T]he Planets have been divided into Primarios or Main-Planets [Ger., Haupt-Planeten]…and in Planetas secondarios, or Minor-Planets [Ger., Neben-Planeten, lit. “Next Planets”]…. namely, the Moon around our Earth, the four small Planets or Moons around Jovem…[etc.]” (Latin as in the original)
Leadbetter in 1727:
“The Moon is a Secondary Planet….This Planet (different from all the rest) respects our Earth for her Center…”
Pemberton in 1728:
“This system is disposed in the following manner. In the middle is placed the sun. About him six globes continually roll. These are the primary planets…Besides these there are discovered in this system ten other bodies, which move about some of these primary planets in the same manner, as they move round the sun. These are called secondary planets…Those which move about Jupiter and Saturn, are usually called satellites… The secondary planets move round their respective primary, much in the same manner as the primary do round the sun.”
d’Alembert in the Encyclopédie, the famous encyclopedia of Enlightenment science, ca. 1765 (transl. from French):
“SATELLITE in Astronomical terms means secondary planets which move around a primary planet [des planetes secondaires qui se meuvent au-tour d’une planete premiere], like the Moon does in relation to the Earth. They are called so because these planets [ces planetes] always accompany their primary planet and make their revolution around the Sun with it… The Satellites of Jupiter are four small secondary planets [quatre petites planetes secondaires] which revolve around this planet as it itself revolves around the Sun… Galileo, to honor his patron, called these planets [ces planetes] the astra Medicea… The Satellites of Saturn are five small planets [cinq petites planetes] which turn around Saturn … One of these planets [Une de ces planetes], namely the fourth counting from Saturn, was discovered by M. Huygens on March 25, 1655…”
Strong in 1784:
“[Gravity] is so also between the grand and more distant Bodies which compose the Planetary System of Worlds; as between the Earth and Moon, the Sun and Earth, and all the other Planets; and between all the Planets, both Primary and Secondary…”
Cousin in 1787, transl. from French:
“The celestial bodies that make up our planetary system are divided into primary planets [planetes principales] with the Sun as the center of their motion, and secondary planets [planetes secondaires], called satellites [italic in original], which revolve around the main planet… the moon’s speed of rotation would be equal to the average speed of this planet around the earth…Before this explanation of the liberation of the moon, which M. de la Grange first gave in his memoir of 1764, we were obliged to suppose that the primitive speed of rotation, imparted to this planet, was exactly equal to its average speed of translation around the earth…. The Royal Academy of Sciences of Paris proposed, for the subject of the prize of the year 1774, to examine whether, having regard not only to the action of the sun, the earth and the other planets on the moon, but even to the non-spherical figure of the moon and the earth, one could explain by the sole theory of gravitation why the moon appears to have a secular equation. M. de la Grange, author of the winning piece, contents himself with examining the effect which may result from the non-sphericity of the two planets…”
Herschel in 1787:
“I confess that this scene appeared to me with additional beauty, as the little secondary planets [Titania and Oberon] seemed to give a dignity to the primary one [Uranus], which raises it into a more conspicuous situation among the great bodies of our solar system.”
Du Séjour in 1789, transl. from French:
“The example of our Moon had learned that a Primary Planet could be accompanied by Secondary Planets which serve it as Satellites.”
Herschel in 1795:
“To this it may be objected, that we perceive no large seas in the moon; that its atmosphere…is extremely rare…; that its climates, its seasons, and the length of its days, totally differ from ours;…In short, that, notwithstanding the similarity which has been pointed out, there seems to be a decided difference in the two planets we have compared.”
From a geography textbook by Patillo in 1796:
“Q. 70. What is the distinction of primary and secondary, among the planets?
“A. The Earth is a primary planet; the Moon is a secondary, called a satellite, guard or attendant; and so of the rest.”
Examples from the 1800s:
Woodward in 1801:
“In addition to this general and common vortex, he [Des Cartes] assigned to each particular primary and satellitary planet, an appropriate subordinate vortex, which occasioned its axual [sic] revolution; and still another, to account for the circumvolution of the satellitary round the primary planets… Herschell [sic] of Germany, by his discovery of five additional planetary bodies in our system, two new satellites to a primary before known, and a new primary attended by new satellites, has in rendering that essential service to science, given immortality to his own name… We have unequivocal evidence that the body of each primary planet, and the body of the Sun, have a tendency to come to the same place. We have the same evidence that the body of each satellitary planet, and the body of its primary, have a similar tendency to come to the same place. We have also reason to believe that all the planets, both primary and satellitary, have a tendency to coalesce and occupy one place; because they are found, in their approximations, to disturb the motions of one another…”
von Freygang in 1804, transl. from French:
“Can a volcano placed on the surface of the moon give these stones an impulse capable of transporting them beyond the sphere of activity of this planet (planète) into that of the earth?”
Webster, 1807:
“The moon is a small planet revolving around the earth.”
Phillips in 1817:
“Of all the planets, the path of no one is more delusive than that of the Moon.”
Vose, 1827:
“The discoveries of the last half century warrant the expectation, that the number of planets, both primary and secondary, may yet be greatly increased.”
Fick, 1837 (transl. from German):
“One divides the Planets:
1) in Main-Planets [Haupt-Planeten], and
2) in Lesser-Planets [Neben-Planeten], satellites or moons.”
Dick, 1838:
“A secondary planet or satellite is a body which revolves around a primary planet as the centre of its motion, and which is at the same time carried along with its primary round the sun… It was a law discovered by Kepler, by which all the planets, both primary and secondary, are regulated…”
de Pontécoulant, 1840 (transl. from French):
“Some of the planets are accompanied by smaller stars which circulate around them like the moon around the earth. These secondary planets [planètes secondaires] are called satellites … Secondary planets which circulate around some of the principal planets [planètes principals].”
Wilkins, 1841:
“The true Solar system, or, as it is sometimes called, the Copernican system, consists of the sun and an unknown number of bodies opaque, like our earth; all of which bodies revolve round the sun, and some of which at the same time revolve round others. Those which revolve round the sun only, are called primary planets and comets. Those which revolve round a primary planet, at the same time that they are revolving round the sun, are called secondary planets moons or satellites…. The sun and all the planets, primary and secondary, are globular, though not perfect globes.”
Cornwell, 1847:
“The Sun, with the bodies which go round it, compose the Solar system. These bodies are either Planets or Comets….The Planets are either Primary or Secondary… The Secondary Planets are the moons of the primary planets, round which they revolve. The Earth has one moon, Jupiter four, Saturn seven, and Herschel [Uranus], perhaps, six.”
Mattison, in an 1851 textbook:
“133. How are the Planets divided?
These side effects can generico cialis on line find for source be minimized by increasing the intake of water. They are known to buy cheap levitra reduce allergy symptoms as well. Pain precipitated by improper food mixture of protein, starch, fat, sugar foods or consumption of alcohol. discount generic viagra You can use this herbal pill without any fear of viagra 50 mg side effects.Into Primary and Secondary.
134. What are the Primary Planets?
Such as revolve around the Sun only, as their center of motion.
135. What are the Secondaries?
They are small planets that revolve around the Primaries, and accompany them in their revolution around the sun.”
Smith, in an 1856 textbook:
“Q. How many planets are in the solar system?
A. Fifty-two is the number known at present.
Q. How are they divided with respect to their motions?
A. They are divided into two classes, primary and secondary.
Q. What is a primary planet?
A. It is a planet which revolves around the sun only.
Q. What is a secondary planet?
A. It is a planet which revolves around its primary, and with it around the sun.
Q. What are the secondary planets usually called?
A. They are called satellites or moons.
Q. How many primary planets are there?
A. 8 large planets and 23 asteroids or small planets.
Q. What are their names, beginning at the sun?
A. Mercury, Venus, the Earth, Mars, (Twenty-three Asteroids or small planets,) Jupiter, Saturn, Herschel, or Uranus, Leverrier, or Neptune.
Q. How many secondary planets are there?
A. Twenty-one.”
Crampton in 1863:
“…scattered over the [lunar] plain, an infinite number of circular pits…sink in the body or crust of the planet…to a vast distance over the surface of the planet…The moon, although the humble servant of the earth…acknowledges a superior authority in the sun…This kind of solar influence is common to both planets… Satellites are small planets attached to the larger ones, called their primaries…”
From a Guatemalan texxtbook by Gavarrete in 1868 (transl. from Spanis):
“Q. What is the other way the planets are divided?
A. They are divided into primary and secondary.
Q. What are the primary planets [planetas primarios]?
A Those already named, so called because they only revolve around the sun.
Q. What are secondary planets [planetas secundarios]?
A. Those that revolve around a primary, also called satellites.”
Vaughn in 1871:
“In supposing that the same side of a satellite is always turned towards its primary, there might appear some dealing in uncertainty ; for the arrangement which our moon and other secondary planets exhibit could not be assumed as the result of a general law…”
Nasmyth and Carpenter in 1874 wrote a book called The Moon Considered as a Planet, a World, and a Satellte. Considering it as a planet covered geophysics of the Moon. Considering it as a world covered questions of habitability and what it would be like for a human to stand on its surface. Considering it as a satellite covered interactions with the Earth. In this example we see that a planet was fundamentally a geophysical concept whereas being a satellite as a distinct dynamical concept.
From a textbook on logic by Johnstone in 1887:
[First syllogism] “Planets can only be satisfactorily studied through a telescope;
But the moon is a planet;
Therefore the moon can only be satisfactorily studied through a telescope, – and I must study it through a telescope.
[Second syllogism] “The moon is a satellite;
But every satellite is a planet;
Therefore the moon is a planet;
Planets shine by reflected light;
Therefore the moon shines by reflected light;
That which shines by reflected light is not self-luminous;
Therefore the moon is not self-luminous.”
Lynn in 1891:
“In its annual journey round the Sun, the Earth is accompanied by a smaller planet called the Moon, the movement of which relatively to the Earth being in the nature of a motion in an elliptic orbit round the latter, she is considered as a satellite or secondary planet thereto.”
From an astronomical glossary by Gore in 1893:
“Planets, Primary. The planets which revolve round the sun as a centre. These are in order of distance from the sun : (1) Mercury, (2) Venus, (3) the Earth, (4) Mars, (5) the Group of Minor Planets, (6) Jupiter, (7) Saturn, (8) Uranus, (9) Neptune.
“Planets, Secondary. The satellites which revolve round the primary planets as a centre. Our moon is a secondary planet, or satellite of the earth, but from its relatively large size and other reasons, it may be almost considered as a primary planet. Mars has 2 satellites, Jupiter 5, Saturn 8, Uranus 4, and Neptune 2: a total of 22 secondary planets.”
Examples from Early 1900s:
Doolittle, 1901:
“Of satellites, or secondary planets, seven have been added to the list during the century.”
Todd, 1901:
“In its annual journey round the Sun, the Earth is accompanied by a smaller body called the Moon; her movement relatively to the Earth being in the nature of a motion in an elliptic orbit round the latter, she is considered as a satellite or secondary planet thereto.”
Leahy, 1910:
“The satellites or secondary planets, over twenty in number, are all too feeble gravitationally to retain at their surface a proper atmosphere…True planets all of these are whatever be their distinguishing names. For they are solidified bodies shining by reflected light. Yet no one of them is enveloped in an atmosphere suited to the maintenance of life…the moon is typical of the entire group above enumerated.” (Bold added)
Chamberlain in 1911:
“The planets are divided into ‘primary’ and ‘secondary.’ By a ‘primary’ planet we mean one which directly circulates round the Sun ; by a ‘secondary’ planet we mean one which in the first instance circulates round a primary planet, and therefore only in a secondary sense circulates round the Sun….The secondary planets are usually termed ‘satellites,’ or, very often, in popular language, ‘moons,’ because they own allegiance to their respective primaries just as our Moon—the Moon—does to the Earth. But the use of the term ‘moon’ is inconvenient, and it is better to stick to ‘satellite.'”
Examples showing that scientists were aware how planets can change dynamical states from primary to secondary, from primary to rogue (free floating), etc.
Bernard Le Bovier de Fontenelle of the French Academy of Science in 1686:
“The comets are merely planets, belonging to another system. Their orbit was towards the extremity of their vortex…These planets, beginning at the upper part to form their circle, did not foresee that it would extend beyond the limit of their vortex…therefore, to continue their circular journey, they were obliged to enter the extremities of the next vortex, which we will supposes is ours.” [At this time, it was not known how large or small the comets are. de Fontenelle thought they are much larger than they are and that they are planets that escaped their home star. He also assumes Descartes’ vortex theory of planetary orbits. He discusses how a planet escapes the vortex of its home star then enters the vortex of another star.]
A conversation with Isaac Newton from 1724/5 recorded by Newton’s nephew, Mr. Conduit:
“He then repeated to me, by way of discourse…as water and other matter, had gathered themselves, by degree, into a body, and attracted more matter from the planets, and at last made a secondary planet (viz: one of those that go round another planet), and then, by gathering to them, and attracting more matter, became a primary planet; and then, by increasing still, became a comet,….He said he took all the planets to be composed of the same matter with this earth, viz: earth, water and stones &c., but variously concocted.”
Gregoray and (Edmund) Halley in 1715:
“There may also be another Effect or Use of a Comet. Namely, if a Comet passes near a Planet … it will so attract it that its Orb will be chang’d. . . whence the Planet’s Period will also be chang’d. But the Comet may also by its Attraction so disturb the Satellite, as to make it leave its Primary Planet and itself become a Primary Planet about the Sun…”
An example showing planet formation theories gained strength by taxonomical identification of moons as planets:
From Trowbridget, 1865, regarding Laplace’s Nebular Hypothesis of planet formation:
“After the rings from which the planets were formed were broken up, and all the parts of any one ring reduced to a planet, such planet as we have shown, would have a rotation on an axis….[A] secondary ring would be abandoned by such planetary bodies….These secondary rings would, in general, break up and form secondary planets, or satellites….In consequence of the forces acting on the planets, both primaries and secondaries, they will become spheroidal in shape as soon as the rings from which they are made are broken up and the parts united.”
An example showing how multi-level taxonomy began developing in which moons are planets:
Kiddle in an 1870 textbook:
“There are two kinds of planets; Primary and Secondary Planets…Primary Planets are those which revolve around the sun only…Secondary Planets, generally called Satellites, are those which revolve around their primaries, and with them, around the sun… There are eight large primary planets in the solar system, besides a great number of smaller ones, called Minor Planets, or Asteroids…The Minor Planets are very small planets which revolve around the sun, between the orbits of Mars and Jupiter. Ninety-six have been discovered (1868)…Major and Terrestrial Planets.—The first four of these planets, it will be seen, are very much larger than the remaining four, and are, for this reason, sometimes called the Major Planets; while the others, being in the vicinity of the earth, are sometimes called the Terrestrial Planets.”