Sun-hugging Mercury is the most elusive of the five classical planets, but in late March and early April 2010 the little planet will be as obvious as it ever becomes for mid-northern observers. Moreover, the bright signpost of Venus will be there to point the way.
The two inner planets remain less than 5° apart from March 28th to April 12th, though Mercury fades greatly during this time: from magnitude –1.1 to +0.7, a loss of four-fifths of its light. They appear closest, 3.0° apart, on the evenings of April 3rd and 4th for North America.
The diagram at right shows the scene. Venus and Mercury are plotted 45 minutes after sunset for a skywatcher near latitude 40° north. For your date, draw a pencil line between the two planets’ curves to set the scene.
On the diagram, the planets are shown with their correct shapes and relative sizes as seen in a telescope at medium-high power. Venus is an almost full disk 10? or 11? wide. Mercury is smaller and strikingly dimmer but more interesting. It wanes rapidly from gibbous to crescent while enlarging from 5? to 10?, measured cusp-to-cusp, from March 22nd to April 16th.