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The diagram on the right shows Venus' phases as they look through a telescope. Venus is at full phase when this planet is opposite the Sun as seen from Earth. Venus appears in the evening sky after sunset when it's east of the Sun ("left" of the Sun on this diagram).
When first appearing in Earth's evening sky, Venus exhibits a nearly full waning gibbous phase. As Venus continues to orbit the Sun, it comes closer and closer to Earth. Venus' phase continually thins while its disk continually enlarges in size. Inevitably, Venus passes in between the Earth and the Sun, to exit the Earth's evening sky and to enter into the morning sky. Venus appears in the morning sky when it's west of the Sun ("right" of the Sun on this diagram).
Five times every eight years, Venus passes in between the Earth and Sun at what's called inferior conjunction. Depending on the date of the inferior conjunction, Venus can swing directly between the Earth and Sun, as it did on June 8, 2004, and as it will do again on June 6, 2012. As seen from Earth, Venus can be seen as a small black dot silhouette in front of the solar disk.
At inferior conjunction, Venus can swing up to 8 degrees north or south of the Sun, depending on the inferior conjunction date. If the inferior conjunction happens in late March, this world swings a maximum 8 degrees north of the Sun. When Venus swings this far north of the Sun at inferior conjunction, Venus sets after sunset and rises before sunrise in the northern hemisphere as it transitions from the evening to the morning sky.
Tale of Two Latitudes
At middle and far northern latitudes, it is indeed possible to see Venus in both the evening and morning sky for a few days before and after inferior conjunction. Because Venus will be at inferior conjunction on March 27, 2009, people in the northern hemisphere will have a golden opportunity to spot Venus as both an evening and morning star, starting a few days before inferior conjunction.
For example, at 40 degrees north latitude (the latitude of Philadelphia, PA), Venus sets about 1/2 hour after sunset on March 24 and rises about 1/2 hour before sunrise on March 25. On these dates, Venus sets about 15 degrees north (or right) of due west, and rises about 15 degrees north (or left) of due east, given a level horizon. Another way of putting it, the azimuth reading of Venus' setting point is 285 degrees and its rising point is 75 degrees.
At 60 degrees latitude (Seward, Alaska), Venus sets about one hour after the Sun on March 24, and rises about one hour before the Sun on March 25. Given at level horizon at 60 degrees north latitude, Venus sets about 24 degrees north (right) of due west (azimuth: 294 degrees) and rises 24 degrees north (left) of due east (azimuth: 66 degrees).
The Tale at Your Latitude
What to know the setting and rising time of Venus and the Sun in your sky, and the azimuth readings? Simply visit this US Naval Observatory page to obtain the information. Don't forget binoculars, for you're trying to spot Venus close to the horizon and in the glare of dusk and dawn. Best of luck in catching Venus' double feature!
|Venus Appears as Both Morning and Evening Star|
|Diagram of Venus' Phases|
Copyright 2009 by Bruce McClure
February 2009 Feature * April 2009 Feature