Observing Mercury

      At its brightest, Mercury can be brighter than any star in the sky. 
Mercury shines by reflecting sunlight, like all the planets and the Moon. 
The albedo, fraction of sunlight a planet reflects, is 0.12 which is like 
weathered asphalt. Mercury doesn't wander far from the glare of the sun, on 
average only 0.387 AU, so it is hard for astronomers to see it clearly.

     Mercury is best seen at either the greatest eastern elongation or the 
greatest western elongation. At these points, which last for a few days at 
a time, Mercury can be seen near the sun at sunrise or sunset. At greatest 
eastern elongation, Mercury can be seen for a short time just above the 
western horizon just after sunset. At greatest western elongation, Mercury 
can be seen low in the eastern sky before the sun rises. The directions 
eastern or western only refers to where in the sky Mercury can be found 
relative to the sun. The points in Mercury’s orbit around the sun called 
the greatest elongation are also very close to the points where Mercury is 
closest, the perihelion, and the farthest, the aphelion.

     The elliptical nature of Mercury’s orbit usually makes the angle 
between the Mercury and the horizon much less than 28°. (see pictures 
below) Observations of Mercury with the naked eye are best made at dawn or 
dusk. However very good telescopic views of Mercury can be made at midday 
when the planet is high in the sky. A yellow filter can easily filter out 
most the blue light from our sky. 

     Mercury travels around the sun every 88-days, so its synodic period, 
as seen from the earth is about 116 days or one-third of a year. You can 
see Mercury near the sun every 116 days either at dawn or dusk (see table). 
This also means that Mercury travels through the inferior conjunction at 
least 3 times a year. This is also called solar transit, although a solar 
transit requires that it directly between the sun and the earth, like in a 
solar eclipse. But because Mercury’s orbital is about 7° to the plane of 
the elliptic, its not that common to find Mercury in solar transit. Usually 
when Mercury passes through the inferior conjunction it is either too far 
below or above the Earth (and our view). 

Year | Western elongations | Eastern elongations
1998 | January 6, May 4, August 31, December 20 | March 20, July 17, November 11
1999 | April 16, August 14, December 2 | March 3, June 28, October 24
2000 | March 28, July 27, November 15 | February 15, June 9, October 6
2001 | March 11, July 9, October 29 | January 28, May 22, September 18
2002 | February 22, June 21, October 13 | January 11, May 4, September 1, December 25
2003 | February 4, June 3, September 26 | April 16, August 14, December 9
2004 | January 17, May 14, September 9, December 29 | March 29, July 27, November 21

     Mercury has phases like the Moon that can be seen with a telescope. 
It will go from crescents to gibbous to a full Mercury. They come from 
having only parts or all of Mercury's lit up side be visible to the Earth 
depending on the Earth point to view.