Venus phase
The alternating light shapes of Venus as they appear in the telescope as they move around the sun are called Venus phases . As with the phases of the moon, they are a result of the different directions of illumination.
The Venus sickle probably saw Galileo Galilei for the first time with his telescope , built in 1610 , and through further observations he also observed the change to other "phases of the moon". Venus (and in the more powerful telescope also Mercury ) appears as a narrow, up to 60 "sickle when the planet is between the earth and the sun. Fuller, but smaller, phase shapes appear when Venus is to the side or beyond the sun.
About this Galileo corresponded with the Roman Jesuit Christoph Clavius , who had discovered the Venus phases independently from him with his religious colleagues. Galileo and the astronomers at the Vatican Observatory were largely aware of the consequences of this discovery: if Venus were to be illuminated in different ways, it would have to orbit the sun , which would contradict the Ptolemaic worldview . However, the Tychonic World Model could also explain the phases and the changing sizes of the Venus shape.
Over time, the phases of Venus change more irregularly than with the moon . When Venus is on the other side of the Sun (so far away) near the upper conjunction , it appears very small (about 10 "), but in almost full phase. This changes little in the next six months. Only after 7-8 months, if As a bright evening star, it has the greatest angular distance to the east from the sun ( greatest elongation , about 45 ° to the left of the sun), at about 100 times magnification it looks like a small increasing crescent moon ( first quarter ). The next 2 months it becomes a sickle which gets narrower and narrower and grows rapidly in size. Before it reaches 60 "in diameter, it becomes invisible in the rays of the sun and changes to the other side after the lower conjunction . Appearing as a morning star after a few weeks , the crescent is now shaped like the waning moon and quickly becomes smaller to the greatest western elongation.