We have already discussed elsewhere  the reasons why the famous myth about the three steps of Visnu may represent the knowledge of the synodic and/or sidereal period of Mercury toward the end of the third millennium B.C.E. We have also shown elsewhere that the organization of the Vedic books represents an astronomical code. It has been argued there that this code, which implies a knowledge of the planets, must go back to at least the third-millennium B.C.E.
The early attitude of the historians of astronomy was that such knowledge is perhaps too early. But it is known that the Babylonians observed Venus in the middle of the second millennium B.C.E. References to Venus come also from the even earlier 3000 B.C.E. evidence at Uruk in Sumer. Known by the Sumerians as Inanna, Venus is represented there as an eight-pointed star. Later Mesopotamia also represents Ishtar (Venus) by an eight-pointed star. It has been suggested that this indicates a knowledge of the eight-year cycle of Venus.
It is believed by Greek scholars that "In Greek astronomy as known to Plato very few planetary observations had been made, and this is consistent with the comparatively late recognition of the planets as such and the fact, for example, that we are told nothing of the periods assigned to them in the Philolaic system [ fifth century B.C.E.]; Eudoxus [390-337 B.C.E.] must have been one of the `few men' to study them, but Plato's words support the idea that the data Eudoxus relied on to construct his system came from outside Greece (almost certainly from Babylonian astronomy.)"
Simplicius (6th century C.E.) attributes to Eudoxus knowledge of the sidereal and the synodic periods of the planets. The sidereal periods for Mercury and Venus are each taken to be one year, the other periods are approximately correct in whole years; the synodic periods are generally more accurate excepting that for Mars it is taken to be 260 days rather than the correct 780 days.
The astronomical system of Eudoxus was based on assigning four spheres to each planet, the moon was assigned three spheres; these spheres moved in ingenious ways to approximate the observed motions. In this model, the moon moves at a constant speed around the ecliptic although it had been known for a long time that the speed actually varies. In other words, the system of Eudoxus did not have the capability to explain what was known.
I think the inference that correct sidereal periods for Mercury and Venus must have been obtained only after Eudoxus is not warranted by this evidence. Even if one were persuaded to ignore the evidence from India, it is quite clear from the tablets that have been found in the third-millennium Sumer and the second-millennium B.C.E. Babylon that there was a tradition of observing the planets there. It seems reasonable to assume, therefore, that in the ancient world trade often carried ideas across lands but astronomical details were, in most likelihood, worked out afresh in each nation. The Greek evidence then only gives us the stages in the development of astronomy there, but its many details were known earlier in Mesopotamia, India, and other civilizations.
Mercury and Venus myths
Mercury and Venus, being inner planets, are found always close to the sun. Hermes as Mercury is the messenger of the gods and the inventor of writing whereas Venus is the goddess of love. In India, the dichotomy is more symmetric: Budha (Mercury) is Visnu, the younger brother of Indra, the great god, the sun who is also later represented by Siva; whereas Sukra (Venus) is the teacher of the asuras (demons).
Sukra knows the secret of immortality; this presumably has reference to the fact that Venus emerges again after being swallowed by the sun. In the Saivite glosses of this story Sukra is swallowed up by Siva and later on expelled as semen; this is a play on the etymology of Sukra as "bright." It is noteworthy that the Siva/Visnu split can be best understood in the interiorization of the astronomical frame. Siva now represents the "sun" of consciousness and Visnu represents the cognitive category of intelligence which ultimately draws its "light" from the sun; this explains the etymology of buddha as intelligence.
The names for the planets in the ancient civilizations are generally different. Their difference suggests that the planets were known in various civilizations before trade and migrations brought unifying impulses and common terminology. The fact that RV 10.123 explicitly calls Vena as the son of the sun makes it clear that it could only be Mercury or Venus (only Venus if we recall that later mythology takes Mercury to be the son of the moon). Its association with Sukra in the Katyayana Srauta Sutra makes it certain that it is Venus and not Mercury; later Puranic mythology also remembers Venus as the son of the sun/Siva.
So is the commonality between Vena in India and Venus in Rome an ancient memory? We have seen that the Rigveda describes two aspects of Venus: one, as Gandharva who is the patron of singing and the arts; and the other, who is the son of the sun and an asura. These conceptions, together with the meaning of Vena as "longing" and "love", lead to both the later mythologies to be found in India as well as in West Asia. If we assume that the notion of Aphrodite was borrowed by the Greeks from western Asia, as is generally accepted, then this notion of Venus as a goddess may have been a late innovation, but it was an innovation based on old ideas. This, in turn, implies that Freya, the Norse goddess of love and beauty, must be derived from Aphrodite and Venus, and not the original Vena. Venus for the name of the planet is related to Vena, but its mythology is to be traced to western Asia.
The other possibility is that Venus as a goddess is derived most directly from Surya, the sun-maiden, celebrated in the Rigveda 10.85 or the Atharvaveda 24.1-2, who is really Venus and not a feminine representation of the sun.
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2. Sarup, L., The Nighantu and the Nirukta. Motilal Banarsidass, Delhi, 1984 (1920-7).
3. See e.g. Frawley, D., \Planets in the Vedic literature", Indian Journal of History of Science, 29, 495-506, 1994.
4. Santillana, G. de and Dechend, H. von., Hamlet's Mill: An essay on myth and the frame of time. Gambit, Boston, 1969.
5. Macdonell, A.A. and Keith, A.B., Vedic Index of Names and Subjects. Reprinted by Motilal Banarsidass, Delhi, 1982 (1912).
6. Shukla, K.S., "Main characteristics and achievements of ancient Indian astronomy in historical perspective."
In History of Oriental Astronomy, G. Swaroop, A.K. Bag, K.S. Shukla (eds.). Cambridge University Press, Cam-bridge, 1987.
7. Dimmitt, C.C. and van Buitenen, J.A.B., Classical Hindu Mythology. Temple University Press, Philadelphia, 1978.
8. See 4 above.
9. Macdonell, A.A., Vedic Mythology. Reprinted by Motilal Banarsidass, Delhi, 1981 (1898), pages 136-7.
10. Moore, P., The Planet Venus. Macmillan, New York, 1956.
11. Kak, S.C., \Knowledge of the planets in the third millen-nium BC", Quarterly Journal of the Royal Astronomical Society 37, 709-715, 1996.
12. Kak, S.C., The Astronomical Code of the Rgveda. Aditya, New Delhi, 1994;
Kak, S.C., \The astronomical of the age of geometric al-tars", Quarterly Journal of the Royal Astronomical So-ciety 36, 385-396, 1995.
13. Krupp, E.C., Echoes of the Ancient Skies. Oxford Uni-versity Press, New York, 1983.
14. Dicks, D.R., Early Greek Astronomy to Aristotle. Cornell University Press, Ithaca, 1970, page 130.
15. See above, page 186.