Phases of Venus

The phases of Venus are the changes in how much of the planet we can see lit up, just like the way the Moon changes its shape in the sky each night, called lunar phases. People have been watching these changes for a very long time. The first time someone wrote down what they saw was in the year 1610, when a man named Galileo Galilei used a telescope to look closely at Venus. Even though we now know we can sometimes see Venus very thinly lit without any special tools — something called the extreme crescent phase — no one is certain if people noticed this before they had telescopes, as there are no clear records to prove it.

Observation

The orbit of Venus takes 224.7 Earth days, or about 7.4 average Earth months. As Venus moves around the Sun inside the Earth's orbit, we see different amounts of its lit side, similar to the way the Moon changes shape. When Venus is on the far side of the Sun from us, it looks fully lit. As it moves closer to us, it shows a smaller lit part, called a gibbous phase. When it is farthest from the Sun in the sky, it shows a quarter phase. When it passes between the Earth and the Sun, we see a thin crescent, and when it is directly between, it looks dark, called the new phase. The whole cycle from new to full and back to new again takes 584 days.

Venus also appears to change in size, looking smallest when it is far away and largest when it is closest to Earth. Its brightness changes in a special way because of the sulfuric acid in its atmosphere, which reflects extra light at certain times.

History

In 1610, Galileo Galilei used a telescope to observe that Venus showed phases, even though it stays close to the Sun in our sky. This discovery helped prove that Venus orbits the Sun, not Earth, supporting the ideas of Copernicus about how our solar system works.

Galileo was the first to see all of Venus’s phases clearly at the end of 1610, though he shared his findings in 1613. Before this, older ideas suggested Venus could never be seen in certain ways because they thought Venus moved in a way that would never let us see its full face. Galileo’s work showed these older ideas were wrong and matched better with newer theories about the solar system.

Naked eye observations

The extreme crescent phase of Venus can sometimes be seen without a telescope by people with very sharp eyesight, though it is very difficult. The apparent size of Venus' crescent shape is very small, measuring between 60.2 and 66 seconds of arc.

Mesopotamian priest-astronomers described Ishtar (the planet Venus) in cuneiform text as having horns, which some think might mean they saw Venus as a crescent. However, other gods were also shown with horns, so this might just have been a symbol for their importance.