Venus – Then & Now


The View from Galileo’s Telescope

Galileo made his first telescopic observations of Venus in October 1610.  He was excited to see whether Venus showed different phases, like the Moon. Before the invention of the telescope, Venus and the other planets just looked like bright stars.  Now Galileo could see the shape of Venus as sunlight reflected off its surface. Over the next few months, Galileo could see how the size and shape of Venus appeared to change as the planet moved around the Sun. When Venus was closest to the earth, it looked bigger, but from our perspective we are seeing more of the night side of the planet.  It looks crescent-shaped.  As Venus moves around the Sun, it looks smaller, but we see more of the daylight side.  The planet looks fuller and rounder.




Above: A simulation of how Venus would have looked through Galileo’s telescope from late 1610 and early 1611. (Image Credit: Owen Gingerich)



Why was this so exciting?  Because the range of phases meant that Venus went around the Sun, and not around the Earth.  Although not proof that the Sun, rather than the Earth, was the center of the cosmos, it was a key piece of evidence that Galileo would use to champion the ideas of Nicolas Copernicus.

Above: How the phases of Venus arise from the relative motion of the Earth and Venus.



What did YOU See with MicroObservatory?

How does Venus appear in your image taken by MicroObservatory? Can you tell the phase: crescent, half or “gibbous” (between half and full).  From your image, can you tell where Venus is in its orbit?  Use Galileo’s image above and the diagram below to figure it out. To see your image of Venus more clearly, you may want to open it in our MicroObservatory image processing software.



Above: Archive MicroObservatory image of Venus.  Look like anything familiar?


Venus – 400 Years Later

Venus is a world that at first sight seems to be Earth’s twin. Yet despite being almost exactly the same size and similar distances from the Sun, Venus and Earth could not be more different. Whereas the Earth is covered with water and has an oxygen/nitrogen atmosphere, Venus is completely dry, and its carbon dioxide atmosphere is so thick and dense, that the pressure on the parched surface of Venus is equivalent to the water pressure at a depth of one kilometer in Earth’s oceans!  In the 1970’s the Soviet Union landed several spacecraft on the surface of Venus.  The landers passed through thick clouds that rained sulfuric acid, and were buffeted by 200 mph winds, to touch down on a surface that was a baking 470ľC! The landers survived the heat and pressure long enough take photographs of barren landscape. 

Above: Photograph of the surface of Venus, taken by the Venera 13 lander. (Image credit: Don P. Mitchell)


The surface of Venus is perpetually gloomy because of the thick cloud layer that envelopes the entire planet all the time.  The surface of the planet is forever hidden in visible light, but because radio waves can pass through the cloud cover, it can be mapped by radar. 


NASA’s Magellan spacecraft arrived at Venus in 1990 and over the next four years, mapped 99% of the planets surface by radar. The exploration of Venus is continuing, with the arrival, in 2006, of the European Space Agency’s Venus Express. 




Above: How cloud-covered Venus looks from space (left), and (right), the planet’s surface revealed by Magellan.  The surface color is modified to bring out detail, and would actually be shades of grey. (Image credit: NASA)



Find Out More


2009 is International Year of Astronomy, chosen to commemorate the 400th anniversary of Galileo’s discoveries with the telescope.  Find out what else is happening by visiting these web sites:


The World site for International Year of Astronomy.


The United States national site for International Year of Astronomy.


NASA’s International Year of Astronomy website.


For more information on the Magellan Mission, go to:


For more information on Venus Express, go to:


Several Soviet space probes named Venera explored Venus in the 1960s and 1970s. NASA sent two Pioneer Venus probes in 1978.  For more information on the Soviet missions, go to:


For an interesting history of the Soviet exploration of Venus, go to:


Take a look at the full list of objects in MicroObservatory’s Galileo activity, and see how our understanding has evolved over the last four centuries.