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The Changing Phases of Venus


Last updated 6/21/2023 at 9:19am

Observing the phases of Venus helped Galileo conclude that planets in our solar system orbit the sun, not the Earth.

Ask your friends and neighbors, and most will probably tell you that it was the 17th century scientist Galileo who invented the telescope. This just isn't so. The "optic tube," as it once was called, most likely originated in Holland decades earlier.

The Dutch had cleverly used it to spot approaching ships long before the eye alone could see them.

Galileo was also not the first ever to aim a telescope skyward; what made him unique was that he built one to study the heavens systematically and scientifically. And what he found among the stars was enough to rock the world and overturn long-held concepts of the universe.

One of his great celestial observations was that the moon had craters and mountains; of course, others had found this earlier, but Galileo used the shadows these features cast onto the lunar surface to calculate their sizes.

When he aimed his telescope toward the planet Jupiter, he found something even stranger. He discovered that this distant world played host to four moons that swung around the planet from night to night.

We can see both phenomena today with even a small backyard telescope – when the moon and Jupiter are in our sky, of course.

One of the other celestial sights Galileo can now be seen low in the western sky shortly after sunset. You've almost certainly noticed it there glistening in the waning light of dusk – the brightest of all planets – Venus.

Venus shines so brilliantly because it's a world the size of Earth that now lies less than 70 million miles away. In addition, the planet is shrouded by thick white clouds that reflect into space more than two-thirds of all sunlight that falls on them.

What makes Venus particularly interesting right now is exactly what Galileo found four centuries ago. The planet appears through a telescope not as a round disk but in a quarter phase – not unlike our moon every few weeks. By July 8, Venus will have approached to about 41 million miles and will appear through a telescope as a thick crescent. And by the end of July, it will lie less than 30 million miles from us and appear as a thin crescent that you might even see in binoculars!

While this may not seem like an epic discovery in today's fast-paced, high-tech world, it was this simple observation that led Galileo to conclude that the phases of Venus could not happen if the planet were orbiting the Earth as both the great Aristotle and the Catholic Church had long taught. No, the only way its phases could appear as they did was if Venus circled the sun!

In other words, our world was not the center of the universe as had long been believed. And this ultimately helped to change forever how we view ourselves and our place in the universe.

If you don't have a telescope of your own, go online and search for an amateur astronomy club in your area. Chances are they'll be having a free "star party" sometime soon where you can view through their telescopes all the wonders of the cosmos – including the amazing planet Venus.

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