Mercury Spotting Guide for 2007
Submitted: Tuesday, 26th September 2006 by Edosaurus Rex
Altitude at Civil Twilight versus Day of the Year of Greatest Elongation
In hunting for the elusive planet Mercury you have the best chance to catch it near its Greatest Elongations. You will notice however that there are some times of the year when it is easy and others when it is difficult to find. Whether you are looking for a “morning” or an “evening” Mercury will matter as well.
With an orbital eccentricity hovering around 0.2 the resulting maximum elongation is about 27 degrees when Mercury is near aphelion and a minimum of 18 degrees when near perihelion. One would think the near aphelic elongation is the easiest to see but position along the Ecliptic and latitude are also very key players.
Below are two graphs that show the elongation and altitude at Civil Twilight of Mercury when greatest elongation happens on the given day of the year. The Civil Twilight altitude is based on a latitude of S30 degrees and takes atmospheric refraction into account. For example, we just had a Greatest Western Elongation (morning appearance) on Aug 8th. From the graph below you can see that the elongation was about 19 degrees and the altitude at Civil Twilight around 7 degrees – normally a challenge this one was made easier by Mercury’s proximity to bright Venus.
EASTERN 07 FEB 18.2 DEG