We all know about the lunar phases, the waxing and waning crescent and the first and third quarter Moon. We can see them daily in the sky.
Since Venus is much smaller than the Moon, less people actually know that she also has changing phases of visibility, just as the Moon, and that she has a crescent and a waning phase. All planets have, actually, but they are hard to see without technical equipment.
Now what is fascinating about Venus’ phases is that the visibility of her waxing body is inverted in relation to the Lunar phases.
What does this mean?
When the Moon is waxing, her illuminated body points west.
We can also say the Moon grows full from west to east.
Please note that the New Moon is on top of the image and that the waxing Moon is seen on the left.
For Venus it is just the contrary. After her conjunction, her illuminated body points to the east and we can say she grows “full” from east to west.
Please note that the conjunction is shown on the left and the waxing Venus is shown from right to left.
Both planets are inferior planets, meaning that they move quicker than the Sun. Now, Venus makes two conjunctions with the Sun. After her superior conjunction, her first visibility, being an inferior planet, is in the western night sky . Night after night, Venus will rise higher in the sky. She also will become smaller but brighter and around her max. elongation she reaches the opposition to the earth, where she is at her brightest. This is her equivalent to the Full Moon.
She will then start to be visible in the morning sky, each day a little further down, until she sets into her inferior conjunction while she is retrograde.
As a curious side note, Galileo discovered the phases of Venus and wanted to tell Kepler about them. But he was also worried about copyright (some things apparently never change).
In order to make sure his authorship, he send Kepler an anagram, which was basically a code, thats used the same syllables of a sentence in a different order. So he sent him a letter that said Haec immatura a me iam frustra leguntur o.y. (These are now too young to be read by me). Kepler, of course, did not know what to make of this incomprehensible sentence.
Only when Galileo was sure about his discoveries he would send his college (and concurrent) the “password”, viz. the reassembled sentence , which was Cynthiae figuras aemulatur mater amorum: The mother of love [Venus] imitates the shape of Cynthia [Moon]. In this way, clever Galileo had assured his copyright.
Find out more on the Venus cycle here and here .
Images References:
Moonphases: Andreas Cellarius.
Venus phases : Passage de Vénus sur le disque solaire La Nature – Revue des sciences, N° 27 à 52, 1874 (p. 52-55).