In 1610, with his small telescope, Galileo first saw the planets Venus and Mercury moving through a cycle of phases much like our own Moon. This observation finally killed the ancient Ptolemaic view of the solar system, which held the Earth as the centre of the universe. And it helped bolster the Copernican view of the solar system with the Sun at the centre and the Earth taking its place as the third rock from the Sun.
Like Galileo, you can easily follow the phases of Venus and Mercury as they cycle through the morning and evening skies.
Venus and Mercury exhibit phases because they are closer to the Sun than Earth. Here’s how it works (see image below)…
When Venus, for example, lies on the far side of the Sun from Earth, the planet is fully illuminated from our point of view. But its disk is small, just 10″ across, because the planet is nearly 300 million kilometres away.
When Venus is almost closest to Earth, on the near side of the Sun, it’s about 60 million kilometres away. Then it appears as a slender but much brighter crescent with a disk nearly 50″ across.
In between the planet can appear half-lit, like the first-quarter Moon, and in various gibbous phases. For Venus, the cycle from full to full takes 584 days.
Of course, the planet cannot be observed when it is directly between the Sun and the Earth. But in rare circumstances, the dark disks of Venus and Mercury can transit the face of the Sun.
Mercury exhibits the same series of phases, though it moves through the cycle more quickly, in approximately 116 days, because of its speedier orbit around the Sun. Unlike Venus, Mercury appears brightest when it’s full.
The phase of Venus and Mercury are easily visible in a small telescope at 50-100x or more.
Some unconfirmed reports, in modern and ancient times, suggest some sharp-eyed observers have observed the extremely slim and bright crescent phases of Venus without optical aid! The Mesopotamian goddess Ishtar, who was represented by Venus, is described in ancient cuneiform texts as having “horns”. Coincidence?