I recently asked a question (in Spanish, sorry) in the Spanish Language stack about the peculiar definition that the Royal Spanish Academy included in its very first dictionary. It goes like this:

Name that is given to the seven celestial bodies, which in their particular orbs each has its own movement, contrary to that of the first mobile: and for this reason they were called wanderers, unlike the other stars that are fixed in the Sky. Their names are Sun, Moon, Mars, Mercury, Jupiter, Venus and Saturn, from whom they took names the seven days of the week.

This definition, that would make most people smile nowadays, was perfectly valid in Spain in 1737, despite the fact that more than 100 years before it was demonstrated that the Earth was just another planet orbiting the Sun (a star). Somebody told me that it would be nice to check the definition for "planet" in another languages, so I ask you:

What was the definition of "planet" like in English around the first half of the XVIII century? I know that the current definition includes a very similar meaning:

any of the seven celestial bodies sun, moon, Venus, Jupiter, Mars, Mercury, and Saturn that in ancient belief have motions of their own among the fixed stars

Please note the "in ancient belief" part that is omitted in the Spanish definition from 1737. When did the English language include the "in ancient belief" part in that definition for "planet", and included the most accurate definition of "any of the large bodies that revolve around the sun in the solar system"?

  • Not important to the question, but apart from not being familiar with the term ‘first mobile’ (is that the earth? I’m guessing probably not?), I don’t see why the Spanish definition would “make anybody laugh nowadays”. It’s a perfectly reasonable definition to someone who doesn’t have the benefit of being aware of how solar systems and gravitational pull work. Feb 9, 2018 at 12:58
  • @JanusBahsJacquet maybe you're right and I am talking from the point of view of someone who has been taught the Solar System at school. But the truth is that everyboy I have shown the definition to so far has ended up laughing. About the "first mobile" in the definition, I am not sure about what it is talking about.
    – Charlie
    Feb 9, 2018 at 13:03
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    @JanusBahsJacquet from the same 1737 dictionary: "First mobile. It is called that upper sphere, which is considered to be higher than the firmament: which, moving continuously from east to west, makes a whole turn in twenty-four hours, taking with it all the other lower spheres: for which reason this movement is called Daytime, and also Rapture."
    – Charlie
    Feb 9, 2018 at 13:28
  • Well, that’s clear as mud. Some notion of a celestial element that we no longer think of as a thing. Got it! Feb 9, 2018 at 13:40
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    This "first mobile" is more commonly referred to as primum mobile (since Latin is obviously the only language for a respectable scientist). And "for this reason they are called wanderers" refers to the etymology of planet: "πλανήτης, planētēs "wanderer"". Sorry I can't help further (being as yet unconvinced by this new-fangled heliocentric theory). Good question though. Feb 9, 2018 at 13:46

2 Answers 2


Your question is less one of language and usage than of the history of science, specifically the reception of Copernican heliocentricity first by educated elites, then by the common people — and, of course, the Roman Catholic Church.

Perhaps the best way to illustrate the difference between Spain and England at the time is John Wilkins (1614–1672), Anglican Bishop of Chester, one of the founders of a new natural theology that attempted to embrace the scientific progress of the Renaissance. In 1640, he wrote A Discovrse concerning a new Planet. Tending to prove, That 'tis probable our Earth is one of the Planets.

This somewhat tentative, though uncontroversial, suggestion of the Earth as a planet becomes established fact in the Lexicon technicum, 1704, by John Harris:

We now number the Earth among the Primary Planets, because we know it moves round the sun. — cited, first OED.

This is likely the first entry in an English dictionary acknowledging that the Earth moves around the sun.

  • And a well-watered plant no less.
    – tchrist
    Feb 9, 2018 at 14:03
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    Well, if the Earth were a plant it would be a heliotrope. Corrected.
    – KarlG
    Feb 9, 2018 at 14:06
  • Why don’t you mention the 1640 usage? A Discovrse concerning a New Planet. Tending to prove, That 'tis probable our Earth is one of the Planets. Feb 9, 2018 at 16:07
  • @user1284969632635: 1640, Wilkins? Mentioned but not cited like Harris. Take another look.
    – KarlG
    Feb 9, 2018 at 16:09
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    You initially asked why I didn't mention Wilkins' work, which is puzzling, since I quite obviously did. I added that although I didn't provide a citation beyond the title of the Discourse, I did provide a citation of Harris. So, are we done now?
    – KarlG
    Feb 9, 2018 at 23:46

This isn't a direct answer to your question, but you can ask when people started using the word "planet" to refer to the Earth, rather than to the other planets that were orbiting the sun.

Looking in Google books, it appeared that this happened earlier in other languages than in English. The first reference I can find in English to "the whole planet" meaning Earth rather than, say, Jupiter, is a 1753 translation from French. The next is a 1795 translation from German. It isn't until 1800 that this expression appears in English, where it seems quite rare for the next 50 years.

  • This may just be a reflection of English not being in use much in scientific works during those centuries. Treatises on astronomy would have been written mostly in Latin.
    – Barmar
    Feb 13, 2018 at 4:30
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    @Barmar: all the quotes I mention in my answer are in the sense of "whole planet" as meaning "whole world"—so not scientific works. I expect Earth was treated as a planet in English scientific writing quite a bit earlier (and probably also in Spanish scientific writing). I assumed the OP's question was about the word as used by the masses, and not about the word as used by scientists. Feb 13, 2018 at 11:12

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