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 page banner (click on post title). However since the Earth’s orbit is on the same plane, or level, as Mercury’s orbit we see Mercury move from one side of the Sun. Mercury’s orbit is divided into four parts based on the planets position (location) relative to the Earth and the Sun somewhat like we describe phases of our Moon. Starting with today Mercury is between the Earth and the Sun at inferior conjunction. In approximately 1/4th of a revolution Mercury will be to the right of the Sun at western elongation – when Mercury rises before sunrise, and is known as a ‘morning star’. One-fourth of a revolution later Mercury is on the opposite side of the Sun, not visible from the Earth, and is at superior conjunction. Continuing along its orbital path Mercury will reach eastern elongation, on the left side of the Sun when it will rise after the Sun rises and sets after the Sun, as an ‘evening star’.
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 30th Mercury reaches western elongation as this graphic shows.
Click here to go to the Qué tal in the Current Skies web site for more observing information.