Observing Venus
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Venus in conjuction with the Moon and Mars on 14th May 2002 at 20:30 BST. |
Venus is the brightest planet in the sky (the only other brighter objects are The Sun and the Moon). This makes Venus a very favourable object to observe amoungst amateur astronomers.
Venus like Mercury, appears in the sky only just after sunset or just before sunrise in the morning sky. This is because Venus and Mercury are both "inferior planets" i.e. the orbit closer to the Sun than Earth. In the 17th Century, Galileo, an Italian astronomer discovered that Venus showed phases in the same way as the Moon. This proved that Venus was an inferior planet. Venus's phases can been clearly seen in amateur telescopes of all sizes, and I have even seen the phases of Venus using an 18th Century 1.5 inch telescope by Jesse Ramsden!
Venus has a thick, dense atmosphere comprising mainly of Carbon Dioxide and Sulphuric Acid clouds. These clouds are only ever seen in large telescopes as thin darks whisps over the planet.
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The diameter of Venus in the sky changes dramatically, depending on which phase is being displayed. If a nearly full phase is present, then the apparant diameter of the planet will be smallest as at these times we are closest to Venus. When we are far away from Venus the phase shown will be Crescent or nearly new - (see left).
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Venus appears in different parts of the
sky depending on where it is in its orbit compared with
Earth. (see right).
When Venus is at point A, then it will be visable at Maximum Eastern Elongation. i.e in the evening sky,
East of the Sun. When Venus is at
B (between the Sun and Earth), then you cannot see the planet. It is at New phase - inferior conjunction.
When Venus and Mercury are at inferior
conjuction, then very rarely they may pass in front of (transit) the face of the Sun. The last time that Venus
crossed the Sun was in June 2004.
Venus appears at Maximum Western Elongation in the Morning sky at point C. At this time Venus will be
visable for a few hours before Sunrise. When Venus is behind the Sun at point D, then it is not visable -
this is known as Superior Conjunction. Venus shows a full phase at this time.
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The phase that Venus shows is directly related to the size of the planet in the sky. Refer to the above diagram to see how this works. When Venus is at Superior Conjuction on the far side of the Sun, then it will only be small. When Venus is at virually New phase (position B) it will show a crescent phase and be larger in the telescopic field of view. The pictures on the left show the apparant size of Venus at different parts of its orbit displayed on the same scale. |
