Monday, May 14, 2012

Venus Phases

Venus is one of the two inner planets. An inner planets is a planet that its orbit is inside the Earth orbit. There are only too such planets, Mercury and Venus. Inner planets have some major characteristics:
  • Their angular distance from the sun is small and thus they are seen only some hours after sunset or before sunrise. Unless near the poles, it is impossible to see Venus at midnight. As a result, inner planets will never be in opposition, but unlike any other planets they will be twice in conjunction with the sun, one when they are behind the sun (superior conjunction) or in front of the sun (inferior conjunction). When they are directly in from of the sun the inferior conjunction is called a transit.
  • Inner planets has phases just like the moon, and from the same reason. At superior conjunction the planet is totally illuminated but since it is close to the sun we can't see it. As the planets moves it wans until is is half lit and when it gets closer to Earth it become a crescent. Since it get closer to earth it get bigger and bigger. Venus will show its phase with even the smallest magnification. A telescope of even a binoculars at 7x or 10x will do, but the binoculars must be very stable, preferably mounted on a tripod. Mercury is much smaller and a telescope is required to see its phases.
Crescent Venus
Crescent Venus

Since Venus has an atmosphere it can be viewed even in inferior conjunction. Such viewing is dangerous since Venus is very near the sun. It is possible to view it just after sunset when Venus is high or from the poles when Venus Is moving almost parallel to the horizon for many hours. Viewing it will see a disc around Venus. Of course this can't be views during a transit. The next inferior conjunction for Venus are 11-Jan-2014, 15-Aug-2015 and 25-Mar-2017 (every 19 months).