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How was Galileo able to observe Venus's phases?

  1. Sep 24, 2015 #1
    Another piece of compelling evidence of the existence of a heliocentric solar system was the discovery of Venus's phases by Galileo Galilei. How was Galileo able to observe these phases with the Sun in the way? Did he simply view only early waxing and late waning phases? I find it questioning how Sunlight did not prevent him from observing the planet. Venus's phases are as shown in the image below:
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  3. Sep 24, 2015 #2


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    That would be the obvious way

    why ?, it's not too difficult to do, even for an amateur astronomer using just a little bit of careful observing
    Venus can be observed till its very close to the sun

  4. Sep 24, 2015 #3


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    Also, Galileo's sky was probably a lot clearer than it is for most of us nowadays. :oldfrown:
  5. Sep 24, 2015 #4


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    Venus often appears as a star either in the early morning before sunrise or the early evening just after sunset. Venus is far enough away from the sun to be visible from Earth.
  6. Oct 3, 2015 #5


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    He did it the same way you can do it tomorrow morning with a pair of binoculars. Get up an hour before sunrise and aim your binoculars at the brightest object in the eastern sky. That's Venus. It will show a crescent phase.
  7. Oct 25, 2015 #6
    Thanks for all the replies!

    Wouldn't it be possible for us to see phases of Venus in a geocentric Solar System? Suppose that the Sun is on one side of a stationary Earth, and Venus is on the other. You can see a "full Venus". Now imagine both the Sun and Venus on the same side of the Earth, with Venus in between. Now we can see a "new Venus". Can't such an occurrence be described by a geocentric model?
  8. Oct 25, 2015 #7


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    It would, but the phases would look different if both Sun and Venus would have an Earth-centered motion. You need some way to get Venus between Sun and Earth sometimes and behind the Sun at other times.

    Also, Venus is never far away from the sun in the sky (certainly not on the opposite side), you would need something additional weird going on preventing your scenario from happening.
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