Mercury

Observing Mercury

Mercury is an elusive planet, always close to and in the glare of the Sun. When visible it is always low down for a short time before sunrise or after sunset, shining through a thick layer of atmoshere so even through a telecope it is a disappointing object. It is likely to be best placed in the northern hemisphere in spring evenings or autumn mornings. When Mercury is well placed, choose somewhere with an unobstructed and cloudless horizon for the best chance of spotting it.

Mercury is the planet closest to the Sun, orbiting in 88 days at a mean of distance 56 million km. From our viewpoint at a distance of 150 million km, Mercury is always close to the Sun and at best sets or rises two hours after or before it. It is a challenge to see Mercury except when viewing conditions are ideal. Remember that any celestial object close to the horizon will be difficult to spot. Not only may there be trees, hills or buildings in the way but also the light has to pass through a greater thickness of air (figure 1).

Figure 1. Light from something high in the sky (a) has to pass through less atmosphere than something low down on the horizon (b).

For practical purposes Mercury can only be seen near its 'greatest elongation' from the Sun, when its orbit takes it as far away from the Sun as possible from our point of view (figure 2a and 2b). Mercury can never be seen more than 28° away from the Sun (roughly the length of The Plough) and is usually closer. Greatest elongations occur about six times a year, three times west of the Sun (when Mercury can be sees before sunrise) and three times east of the Sun (after sunset).

Figure 2a. The orbit of Mercury round the Sun.

Figure 2b: The positions of Mercury in figure 2a as seen from the earth: 1; inferior conjunction. 2; greatest elongation west. 3; superior conjunction. 4; greatest elongation east.

So when the calendar notes that (for example) "Mercury (morning object) at greatest elongation, 27.5° W", you'll know that Mercury has swung away from the Sun in it's orbit as viewed form the Earth and is 27.5 degrees away from it. However, this does not mean that the planet will be easily seen as it's height above the horizon is also important.

The height of Mercury above the observer's horizon will depend the angle of the ecliptic to the horizon (figure 3). The ecliptic is the path taken by the Sun through the stars and is marked by the 12 constellations of the zodiac. In northern hemisphere spring evenings and autumn mornings this angle is very steep. Mercury will be well placed at these times as it can be several degrees above the horizon for some time after sunset/sunrise. In spring mornings or autumn evenings however the angle is very shallow. Mercury skims the horizon and is low down, even with a large greatest elongation.

Figure 3: Left, after sunset in northern hemisphere spring when the ecliptic (red line) is at a high angle to the horizon with Mercury correspondingly high. Right, after sunset in autumn when the angle of the ecliptic is small and Mercury only skims the horizon, no matter how far from the Sun it is. The angle is also high in autumn mornings and low in autumn evenings.