When the orbit of either of these passes exactly between the Earth and the Sun, it can be seen as a black dot moving across the Sun's face. This is called a transit.
The transit of Venus only occurs twice every 113 years because of the tilt of Venus' orbit.
Astronomers at the Royal Observatory have been observing the transit of Venus for centuries. In 1874 several parties set out from Greenwich to remote corners of the globe to observe the transit from different latitudes. They aimed to use their observations and measurements to find out the size of our solar system and the universe.
Click on the diagrams below for a more detailed explanation of the transit of Venus.
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