Phases of Venus

The phases of Venus are the ways Venus looks brighter or dimmer, similar to how the Moon changes shape, called lunar phases. People have observed these changes for a long time. In 1610, a man named Galileo Galilei was the first to write about what he saw through a telescope.

Sometimes Venus can look very thin, like a thin slice, but we are not sure if people noticed this before telescopes were invented. There are no clear records to tell us for sure.

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, like 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 and saw that Venus had phases, even though it stays close to the Sun in our sky. This helped show that Venus orbits the Sun, not Earth. It supported the ideas of Copernicus about our solar system.

Galileo was the first to see all of Venus’s phases clearly at the end of 1610. He shared his findings in 1613. Before this, older ideas said Venus could never be seen in some ways because they thought Venus moved differently. Galileo’s work showed these older ideas were wrong and matched 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 hard to see. The shape of Venus' crescent 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. Some think this means they saw Venus as a crescent. But other gods were also shown with horns, so this might have been a symbol for their importance.