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Hesperus (Roman Vesper) is the name ancient Greeks gave to the evening star that appears in the sky for an hour after the Sun sets. Phosphorus (Roman Lucifer, sic!), was the name of the morning star that appears shortly before the dawn. Depending on who you read, at some point Greeks either learned from Babylonians or figured out on their own that they are one and the same, and renamed it Aphrodite (Roman Venus). More recently this became a quintessential example in semantics for identity of apparently different objects, analyzed by eminent logicians like Frege and Kripke.

Today we have telescopes to discern fine details of distant objects, and satellites that can observe them continuously. But how did Babylonians figure it out watching with a naked eye from the surface? When did it happen? What was it that convinced the Greeks that Hesperus is Phosphorus?

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  • $\begingroup$ "Today we have telescopes to discern fine details of distant objects... But how did Babylonians figure it out watching with a naked eye from the surface?" After the Sun and Moon, Venus is the brightest 'star' in the night sky. With a little experience, it's actually not that hard for naked eye observers to recognize Venus from its appearance alone. Surely this was the basis of initial guesses by ancient astronomers that Hesperus=Phosphorus. $\endgroup$
    – David H
    Jan 6, 2015 at 0:02
  • $\begingroup$ @David H Greeks believed otherwise for a while apparently. And they were pretty attentive star gazers. If somebody can confirm that there are enough features seen with a naked eye for identification that would be interesting. We also know that Babylonian observations were more systematic than Greek ones, it might have been something about Venus's motion that suggested it. $\endgroup$
    – Conifold
    Jan 6, 2015 at 0:40
  • $\begingroup$ Is there any reason to believe that it wasn't merely a lucky guess? $\endgroup$
    – Mark Dominus
    Jan 6, 2015 at 17:22
  • $\begingroup$ @MJD Even if it was earlier, after Eudoxus and especially in Hellenistic period Greek astronomy got pretty sophisticated. They wouldn't have accepted a guess without a serious supporting evidence. $\endgroup$
    – Conifold
    Jan 7, 2015 at 1:02
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    $\begingroup$ See J.L.E. Dreyer, A History of Astronomy from Thales to Kepler (1906), page 38 : "we are told [by Diog.L.viii.14 and Pliny, H.N.ii.37] that Pythagoras was the first to recognise that Phosphorus and Hesperus were one and the same body." $\endgroup$
    – Mauro ALLEGRANZA
    Jan 16, 2015 at 12:17

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I doubt that many Babylonians or Greeks or any others who cared about such things ever thought Hesperus and Phosphorus were different objects any more than we think the Morning Star and Evening Star are different. Is there any evidence apart from the use of two names?

I think we today underrate what it meant to see the stars clear and bright almost every night and at any part of the night you were awake outdoors -- and with many herdsmen and sailors and revelers and soldiers awake outdoors much of the night, besides people deliberately studying the night sky.

Every slightly interested party knew the circumpolar stars are up all the time, and just get blotted out by sunlight during the day. They had every reason to believe the adjacent stars that rise and set in the North are travelling 'behind' the earth from the time they set until they rise again -- and indeed that the whole sphere of fixed stars exists all the time and moves around.

They knew that Hesperus, when it is seen, moves away from the sun and then back towards it. And they knew that shortly after Hesperus disappears towards the sun, Phosphorus appears on the other side of the sun. Of course stars on one side of the sun are seen just before dawn, moving towards the sun until they disappear and then reappear on the other side just after sunset.

All the major wandering planets obviously move past the sun sometimes, appearing just east of it just after sunset at times, with the sun approaching, disappearing as the sun passes it, and and then appearing just west shortly before sunrise. That is the moon, Mars, Jupiter, and Saturn.

I doubt any great number of people who cared about the question ever thought Hesperus was an exception which ceased to exist each time it moved towards the sun and then Phosphorus appeared, or that Hesperus suddenly went "somewhere else" where we could not see it. These were two names for two roles for one star.

Kuhn's book Copernican Revolution makes these points about the plainly visible motions of the stars -- including the wandering stars. Well, that is plainly visible before artificial lighting, in parts of the world with generally clear skies. He has a nice diagram explaining how people could not possibly have missed the constant existence of circumpolar stars, or the very similar motion of the stars near them that do rise and set.

B. L. van der Waerden studied Babylonian astronomy and simply reports they knew Venus as a planet -- long before the Greek astronomers that we know. (You can see this in his "Babylonian Astronomy. II. The Thirty-Six Stars," Journal of Near Eastern Studies, Vol. 8, No. 1 (Jan., 1949), pp. 6-26) available at www.jstor.org/stable/542436 .

A lot of sources on-line say the Egyptians and Greeks thought the morning star and evening stars were different stars. But what they actually show is that those people associated different gods to the morning and evening stars. The Egyptians also associated Khepera to the rising sun and Atum to the setting sun, without thinking the rising and setting suns were two different things.

I do not see any evidence of a time when people tried to individuate "stars" as objects but thought the morning star and evening star were two different ones.

Added: Plato's Laws 821-822 has the interlocutors discuss the "impious error" that Greeks make when they believe "Hesperus and Phosphorus and the other planets" wander rather than following a single course. One character asserts "The truth is precisely the opposite: each of them always travels in a circle one and the same path." The other does not disagree. The Greek is no more explicit than the English given here. It does not say anyone thinks "Hesperus and Phoshorus" are two things, and anyway it is discussing an error without saying which Greeks make this mistake. To believe Hesperus follows one circular orbit you would have to believe half of that orbit goes under the name of Phosphorus.

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    $\begingroup$ That does sound pretty convincing, but is there any chance that you would have some references to support this view? $\endgroup$
    – Danu ♦
    Jan 6, 2015 at 21:51
  • $\begingroup$ So the reason was its motion rather than appearence. I was unsure because it's a planet so it doesn't move as regularly as the stars to make immediate connection. We also happen to know already not only that they are are the same, but also that Venus revolves around the Sun and is closer to it than Earth, so we expect it to follow the Sun in the sky. It's hard to filter out all of that and imagine how things would appear to a raw observer. But you make a good argument. $\endgroup$
    – Conifold
    Jan 7, 2015 at 1:04
  • $\begingroup$ Yes, the motion, as well as the look. Anyone who wants to follow Hesperus will notice the exact complementarity of its motion with Phosphorus -- same small (optical) distances from the sun, same speed, opposite times, as well as same size, same color. $\endgroup$
    – Colin McLarty
    Jan 7, 2015 at 1:37
  • $\begingroup$ Would phases also be observable? Not directly perhaps but as changes in brightness. $\endgroup$
    – Conifold
    Jan 7, 2015 at 2:20
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    $\begingroup$ "Near its inferior conjunction with the Sun, the apparent disk of Venus can measure up to 68 seconds of arc when its horns will be extremely slender. Under favourable atmospheric conditions, it appears to be possible for observers with an acute eyesight to detect the crescent form of Venus with the naked eye". staff.science.uu.nl/~gent0113/babylon/babybibl_planets.htm If they are looking for it. It seems that actual naked eye detections date to after telescopes were discovered. $\endgroup$
    – Conifold
    Jan 21, 2015 at 1:52

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