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It is said that the precession cycle of the Earth's axis takes about 26,000 years to complete. It is also said that the ancient Greeks could see Crux (aka the Southern Cross) from where they lived. It is a constellation resembling a cross:

enter image description here

It is also believed that Vasco da Gama was the first European in a long time to see it as he traveled south along the African coast, but he somehow expected to see it. There were rumors and legends about it.

Wouldn't this mean that he, and the astronomers of his time, knew about precession (almost a century before the advent of Kepler's and Galileo's theories)?

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    Why don't you ask ONE question at a time? What is the relation between precession and Southern cross? If you look at a map (where the Greeks lived and still live) you will immediately see that they could not see Southern cross. – Alex Dec 4 '15 at 23:20
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    @Alex He has a single question. He merely proposes his own answer and is in doubt about it. It is correct and allowed here. The real problem of QA is in logic, but being illogical is not forbidden on the StackExchange sites. – Gangnus Dec 4 '15 at 23:38
  • The Southern Cross is visible above the latitude of the equator. – Tyler Durden Dec 5 '15 at 0:23
  • I still don't see what all this has to do with precession, and still assert that Greeks could not see S. Cross (from where they live). – Alex Dec 5 '15 at 1:45
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    Precession is describes in Claudius Ptolemy's Almagest , as is Crux (as part of Centaurus). The Almagest was still the main source of astronomical knowledge up to the 16th C. Crux had been visible from Athens ~1000 BCE but did not rise above the horizon there by 400 CE. But then Ptolemy lived and worked in Egypt. – Conrad Turner Dec 6 '15 at 6:59
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In 1460, at the time of the death of Prince Henry, the Navigator, the Portuguese had mapped the western coast of Africa down to the 8 N parallel. The Southern Cross is well seen at this latitude. Really, all stars can be seen between the tropics, and the Northern Tropic was reached even earlier. In 1471, they crossed the Equator and began to be guided by Crux.

And Vasco da Gama's travel started in 1497. Of course, he expected and simply knew he would see the Southern Cross. Europeans had looked at it for more than 40 years already. He counted on it. He was well prepared to use it for navigation, as they already did. Only the Cross was higher in the sky for him, than for his predecessors, thus being in the more convenient position.

And even if he were the first European to see it, he would simply have heard about the Cross from Arab sailors. And buy navigation charts from them, that would say how to use the Southern Cross for the Southern Pole placement. (It is not so simple as with the Polar Star)

The two pieces of knowledge - about the existence of Cross and about the precession - are practically independent. Or worse than independent, because the precession means that in a thousand years the points of North/South will move noticeably. So, if da Gama had information about usability of Southern Cross in the far past, and used precession data, he would conclude that the Cross is NOT usable now.

And yes, Europeans knew about precession for minimally 13 centuries at that moment, for already Ptolemy used it to wrongly fake his catalogue on the base of Hipparchos catalogue. History of Star Catalogs, page 5

  • great answer. people are often not told that navigating to India was the result of a multi-generational national effort, not just of one lucky stroke or an individual hero accomplishment. – Luiz Mar 8 at 16:42

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