Phases of Venus
The phases of Venus are the variations of lighting seen on the planet's surface, similar to lunar phases. The first recorded observations of them are thought to have been telescopic observations by Galileo Galilei in 1610. Although the extreme crescent phase of Venus has since been observed with the naked eye, there are no indisputable historical pre-telescopic records of it being described or known.[1]
Observation[edit]
The orbit of Venus is 224.7 Earth days (7.4 avg. Earth months [30.4 days]). The phases of Venus result from the planet's orbit around the Sun inside the Earth's orbit giving the telescopic observer a sequence of progressive lighting similar in appearance to the Moon's phases. It presents a full image when it is on the opposite side of the Sun. It is a gibbous phase when it approaches or leaves the opposite side of the Sun. It shows a quarter phase when it is at its maximum elongation from the Sun. Venus presents a thin crescent in telescopic views as it comes around to the near side between the Earth and the Sun and presents its new phase when it is between the Earth and the Sun. Since the planet has an atmosphere it can be seen at new in a telescope by the halo of light refracted around the planet. The full cycle from new to full to new again takes 584 days (the time it takes Venus to overtake the Earth in its orbit). Venus (like the Moon) has 4 primary phases of 146 days each.
The planet also changes in apparent size from 9.9 arc seconds at full (superior conjunction) up to a maximum of 68 arc seconds at new (inferior conjunction).[1] Venus reaches its greatest magnitude of about −4.5 when it is an intermediate crescent shape at the point in its orbit, when it is 68 million km away from the Earth, at which point the illuminated part of its disk reaches its greatest angular area as seen from the Earth (a combination of its closeness and the fact that it is 28% illuminated).[2]
Contrary to other planets its apparent magnitude around inferior conjunction does not decrease consistently but rather spikes before dimming further. This is caused by sulfuric acid droplets in Venus' atmosphere reflecting more light at a certain angle and thus phase, an effect similar to a glory on Earth.[3]
Naked eye observations[edit]
The extreme crescent phase of Venus can be seen without a telescope by those with exceptionally acute eyesight, at the limit of human perception. The angular resolution of the naked eye is about 1 minute of arc (60 seconds). The apparent disk of Venus' extreme crescent measures between 60.2 and 66 seconds of arc,[8] depending on the distance from Earth.
Mesopotamian priest-astronomers described Ishtar (Venus) in cuneiform text as having horns which has been interpreted as indicating observation of a crescent. However, other Mesopotamian deities were depicted with horns, so the phrase could have been simply a symbol of divinity.[1]