From our perspective on Earth, the inner planet Mercury appears to move back and forth from the left side of the Sun to the right side and then back to the left side – over and over. This is our view of the planet as Mercury revolves around the Sun. From an overhead view we would see Mercury’s orbit circular as shown in the graphic to the right. However since the Earth’s orbit is essentially on the same plane, or level, as Mercury’s orbit we see Mercury move from one side of the Sun to the other side. Sort of back and forth motion from the east to the west side of the Sun with Mercury, today, centered between the two extremes (elongations) and at inferior conjunction.
Viewing Mercury through a telescope will reveal phase changes similar to our Moon with the biggest difference being that there is no full Mercury phase—at least not one that we can see from Earth. Today while at inferior conjunction the Sun side of Mercury is reflecting sunlight and so like our new Moon phase we do not see either. Mercury moves from inferior conjunction toward western elongation and as it does so it starts as a relatively large crescent phase and by western elongation Mercury’s apparent size has decreased but it has waxed toward a quarter phase and 50% illuminated. As Mercury moves toward superior conjunction its apparent size decreases while its phase waxes through the gibbous shape. Once the planet moves away from the Sun and superior conjunction Mercury will be again seen in the gibbous phase but appears relatively small. As it continues along its orbit toward eastern elongation Mercury wanes to a larger appearing quarter phase and 50% illuminated. From eastern elongation to inferior conjunction Mercury’s apparent size increases while it wanes toward a thin crescent shape.
On March 14th Mercury reaches western elongation.
Click here to go to the Qué tal in the Current Skies web site for more observing information for this month.