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This is a pretty straightforward question, when the first observations of the night sky were being made, who was the first person to suggest that a planet, say Mars, was not a star, in the sense that their physical compositions were different? I thought Tycho Brahe, but he lived in the 16th century, whilst a large part of such observations was made in ancient Greece, so could it be someone else?

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    $\begingroup$ It is certainly very much before Tycho Brahe, before Ptolemy, and before Hypparchus. Any person who looked at the sky systematically, surely discovered this. History did not preserve the name of this person. $\endgroup$ – Alexandre Eremenko Jan 13 '15 at 0:05
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    $\begingroup$ Why would you down vote this? This forum is specifically for this kind of questions. $\endgroup$ – L.R. Jan 13 '15 at 5:05
  • $\begingroup$ I fail to see how this is off-topic; it fits here perfectly. @Alexandre - regarding your point - I think the answers go against your point. $\endgroup$ – HDE 226868 Jan 13 '15 at 16:35
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    $\begingroup$ HDE226868: And I think the answers have nothing to do with the question "WHO FIRST distinguished...". And it was clear from the beginning that this question, as stated, has no reasonable answer. "Who was the first to observe the sky"? "Who was the first to observe that the Sun rises in the east and sets in the West?" $\endgroup$ – Alexandre Eremenko Jan 13 '15 at 22:14
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    $\begingroup$ @AlexandreEremenko, I might have been unclear, the question was not who first realized that stats and planets have different motions. As you said yourself, the first person to observe the sky. What I meant was who first suggested that the two types of bodies were different, as in physically different. One emits light, the other doesn't. This has a much less intuitive answer, and likely there are records of ancient astronomers who studied the night sky with precision. $\endgroup$ – L.R. Jan 13 '15 at 22:23
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We can try to find the necessary informations browsing some History of Astronomy books, like :

  • Anton Pannekoek, A History of Astronomy (1961, original ed : 1961)

  • Christopher Linton, From Eudoxus to Einstein A History of Mathematical Astronomy (2004)

and the "classical" studies of Otto Neugebauer on ancient science and astronomy, mainly :

  • Otto Neugebauer, The Exact Sciences in Antiquity (2nd ed, 1957).

I think that it is necessary to separate at least three different "stages" :

(i) the "planets" have irregular motions (the retrograde ones) compared to the movement of the "fixed stars".

It is due to Babylonian astronomy; see Lipton, page :

Ancient astronomers were also aware that five of the star-like objects in the sky changed their position relative to the other stars. These five objects – now named after the Roman gods Mercury, Venus, Mars, Jupiter and Saturn – are the planets, from the Greek for ‘wanderer’. Careful observations of these objects reveal that, like the Sun, as well as participating in the daily rotation of the heavens, they, too, move around the celestial sphere though with differing periods, and also that while they move predominantly in the same direction as the Sun – from west to east – they sometimes switch back and, for a time, move from east to west in so-called retrograde motions. [...] The planets also remain close to the ecliptic, the maximum deviation for any of them being 8◦, and thus all the wandering heavenly bodies can be found within a strip on the celestial sphere 16◦ across centred on the ecliptic. This strip,therefore, is very important, and is known as the zodiac and was divided by the Babylonians into twelve equal parts: the signs of the zodiac.

Despite their astronomical and mathematical ability, it seems to me that their is no trace of "physical" Babylonian theorie regerding the nature of "celestial objects".

(ii) some planet, like the Moon, does not shine by his own light (compared to the Sun and the stars); see Pannekoek, page 100 :

[the pre-socratic philosopher] Anaxagoas of Clazomenae (c.500-428 BC) [...] was the first to state clearly that the moon shines by the light it receives from the sun and that lunar eclipses occur when the earth (or another dark body) intercepts the sun's light.

See also Lipton, page 21 :

He [Anaxagoras] was the first to think of the seven wandering heavenly bodies in the order Sun, Moon, followed by the five other planets, an arrangement adopted by a number of later astronomers.

(iii) The last stage is the understanding of the "physical" difference between the fixed stars and others "celestial objects", like the planets and the comets.

This process occurred during the Renaissance, starting with the physical-cosmological debate of the 1580s, stimulated by the apparition of a nova in Cassiopeia in 1572, the comets of 1577, 1580, 1582 and 1585, and followed by the nova of 1604.

See :

This debate culminated with Galileo's telescopic observation of the moon and of the phases of Venus (1610), and of the sunspots (1612) :

  • Galileo Galilei & Christoph Scheiner, On Sunspot (translated and edited by Eileen Reeves & Albert Van Helden - 2010).
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I'm not an expert in the field, but I'll give it a go. Unfortunately, my only source is wikipedia, since I learned most of these things from encyclopedias years ago.

The distinction was certainly made centuries before Tycho Brahe. My humble research hasn't been able to exactly pin down who were the first to observe the difference, but I'd bet Babylonians were already aware of it. In this wikipedia article, we can read:

The oldest significant astronomical text that we possess is Tablet 63 of the Enūma Anu Enlil, the Venus tablet of Ammi-saduqa, which lists the first and last visible risings of Venus over a period of about 21 years and is the earliest evidence that the phenomena of a planet were recognized as periodic. The MUL.APIN, contains catalogues of stars and constellations as well as schemes for predicting heliacal risings and the settings of the planets, lengths of daylight measured by a water clock, gnomon, shadows, and intercalations.

Which strongly suggests that Babylonians knew that there two different luminious objects in the night sky: planets and fixed stars.

The word planet comes from greek πλανήτης, which means wandering (or errant, don't know which word is better). This is evidence of the greeks knowing about the difference, which is confirmed in the (very complete) fixed star Wikipedia article, where we particularly read:

The phrase [fixed star or stellae fixae] originated in classical antiquity, when astronomers and natural philosophers divided the lights in the sky into two groups. One group contained the fixed stars, which appear to rise and set but keep the same relative arrangement over time. The other group contained the naked eye planets, which they called wandering stars.

Conclusion: Even though evidence strongly suggests Babylonians were already aware of the difference around 1000 BC, when the Enuma anu enlil was written, we can be sure the differentiation was made sometime between this date and classical antiquity, which ended around 100 BC.

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    $\begingroup$ The information you provided looks correct to me, I suspect you might not have interpreted the question as the OP intended. People have indeed distinguished the so-called wandering-stars from the fixed-stars in this way since time out of memory. But all this really amounts to is a distinction between two types of stars, and the word "planetary" is just an adjective describing a certain kind of star. This is not quite the same thing as distinguishing a planet from a star. The issue is that the question was vague, and we need to distinguish between different possible distinctions. $\endgroup$ – David H Jan 11 '15 at 23:17
  • $\begingroup$ @DavidH I see, you mean he may as well mean the difference between things that are light-years away and planets? $\endgroup$ – hjhjhj57 Jan 11 '15 at 23:23
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    $\begingroup$ I assumed he meant the differences in their physical composition and intrinsic properties, as opposed to geometrical/kinematic ones. For example, the Bible describes the Sun and Moon as, "two great lights—the greater light to govern the day and the lesser light to govern the night"; that is, instances the same basic type of object, differing only in magnitude. The first noteworthy challenge to this idea came when Anaximenes suggested that the Moon isn't itself a source of light; it's merely reflecting the light given off by the Sun. $\endgroup$ – David H Jan 12 '15 at 0:17
  • $\begingroup$ @DavidH I see, let's wait to see what the author says about it. $\endgroup$ – hjhjhj57 Jan 12 '15 at 0:25
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    $\begingroup$ (I'm a girl by the way) :P $\endgroup$ – L.R. Jan 12 '15 at 16:43
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Certainly much before Tyco Brahe. By Hindu Rishis obviously.

“Indian and Greek Models: We first note that the idea that the sun is roughly 500 or so earth diameters away from us is much more ancient that Ptolemy. So Neugebauer was wrong on two counts: first, he did not know of any Indian connections although he admitted that the “study of Hindu astronomy is still at its beginning;” second, he did not recognize that the tradition regarding the distance of the sun might be much older in Greece itself. This greater antiquity is in accordance with the ideas of van der Waerden,4 who ascribes a primitive epicycle theory to the Pythagoreans. But it is more likely that the epicycle theory is itself much older than the Pythagoreans and it is from this earlier source that the later Greek and Indian modifications to this theory emerged which explains why Greek and Indian models differ in crucial details.

It should also be noted that Ptolemy’s Almagest is a 12th cen- tury Arabic book that is likely to include much information of the first millennium astronomy and its original 2nd century form can- not be established with certainty.

Did the idea that Rs ≈ 500de originate at about the time of Pancavimsa Brahmana (PB), that is from the second millennium BCE, or is it older? Since this notion is in conflict with the data on the periods of the outer planets, it should predate that knowledge. If it is accepted that the planet periods were known by the end of the third millennium BCE, then this knowledge must be assigned an even earlier epoch. Its appearance in Pancavimsa Brahmana (PB) , a book dealing primarily with ritual, must be explained as a remembrance of an old idea. We do know that PB repeats, almost verbatim, the Rgvedic account of a total solar eclipse. PB is in second millennium BCE.

Once the conflict between the planet period information and the supposition that the heavens were 1000 earth diameters away became clear, this supposition was dropped. Presumably, the theory that Rs ≈ 500 de was too entrenched by this time and it became the basis from which different Greek and later Indian models emerged. As mentioned before, Ptolemy considers an Rs equal to 600 de, whereas Ayabhata assumes it to be about 438 de. Thus the Greek and the later Indian modifications to the basic idea proceeded somewhat differently.

The ideas regarding the distance of the sun hardly changed until modern times. The contradictions in the assumption that the luminaries move with uniform mean speed and the requirements imposed by the assumed size of the solar system led to a gradual enlargement of the models of the universe from about twice that of the distance of the sun in PB to one 4.32 × 10^6 times the distance of the sun by the time of Ayabhata. This inflationary model of the universe in AAr makes a distinction between the distance of the sky (edge of the universe) and that of the stars which is taken to be a much smaller sixty times the distance of the sun.

“Beyond the visible universe illuminated by the sun and limited by the sky is the infinite invisible universe” this is stated in a commentary onAryabhabaıya (AAr) by BhaskaraI writing in 629CE. The Puranic literature from India, part of which is contemporaneous with Ayabhata, reconciles the finite estimates of the visible universe with the old R. gvedic notion of an infinite universe by postu- lating the existence of an infinite number of universes. It is possible that the original notion that the heavens are 1000de away from the earth arose as a metaphor for the large extent of the universe, given that a thousand represents a very great size in Indo-European languages. But it is more likely that some measurements and a theory were at the basis of this supposition. “

“The Surya Siddhanta 12.84 says: “Any orbit, multiplied by the earth’s diameter and divided by the earth’s circumference, gives the diameter of that orbit; and this, being diminished by the earth’s diameter and halved, gives the distance of the planet.” The next verses give the distances as follows:

Table 8.1: Distances to planets and stars in yojanas (= 7.5 miles approx )

Moon 324,000

Mercury (conjunction) 1,043,209

Venus (conjunction) 2,664,637

Sun 4,331,500

Mars 8,146,909

Jupiter 51,375,764

Saturn 127,668,255

Asterisms 259,890,012

Brahmanda (Edge of the Universe) 18,712,080,864,000,000 “

from http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.695.536&rep=rep1&type=pdf by Subash Kak The Astronomical Code of the Ṛgveda (Third Edition)

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