The Phases of Venus

2001 Feb 9th:


Few can have failed to see the very bright beacon  shining in the west after dark. It now sets some 4 hours  after  the sun and at magnitude -4.5 can be seen at midday if you know exactly where to find it. Its diameter is currently 32 arcseconds. Here is an image I took this afternoon before sunset, showing the crescent phase of Venus.



I am often asked why does Venus show phases like the moon. The answer to this can best be demonstrated by looking at our solar system from a bird's eye view . As we can see from the graphic above, Venus is now catching up the earth as it is on the inside race track and orbits the Sun at a greater speed than our Earth. Over the coming weeks as it overtakes our planet, the Sun-Venus-Earth angle will diminish and as it does so Venus will show us a narrowing crescent. Because it is also moving nearer to us as well in its orbital path, then it will also appear to grow in apparent size reaching its greatest diameter of 59 arcseconds on March 31st, the date of our next Star Party.


We shall post images of Venus here over the coming weeks to show its changing phases.
Here is an image taken 13 days apart - a long enough interval to show the changing phase and increase in diameter.


 

The image below was taken just 8 days later and now shows the crescent appearing more slender.