SPA Planetary Section

Observing Mercury

It is only comparatively recently that much was discovered concerning the true nature of this mysterious little world. It was long believed that Mercury spun once on it's axis in the same time as it took to make one revolution of the Sun (88 Earth days) but radar investigations have established a correct value of just over 58 days. This mistake was not really surprising as through the telescope, Mercury is very difficult to observe as the 'messenger of the gods' never strays further than 28° away from the Sun and has a maximum apparent diameter of only 13 seconds of arc. Thus when a naked-eye object, Mercury is always low down and a clear horizon is necessary to see the planet in the dawn or dusk sky. For observers in the northern hemisphere, favourable elongations take place during the spring for evening observations and autumn for morning views but it is worth trying to locate Mercury at one of it's less favourable elongations during the year.

Mercury is best seen through the telescope against a bright sky. Little will be seen apart from the phase but it is always a thrill to glimpse this elusive world for one's self. Full details concerning the current visibility of Mercury are given in "SkyDiary" in Popular Astronomy magazine.

Warning: When observing Mercury with a telescope or binoculars, only attempt to do so when the whole of the Suns disk is below the horizon. As Mercury lies close to the Sun in the sky there could be a danger of getting the Sun in the view. This could result in eye damage or loss of eyesight.

The Phases of Mercury

Positions of the Inferior Planets

An Inferior Planet moves along an orbit closer to the Sun than that of the Earth, i.e. Mercury and Venus. They are best placed for observation when near to greatest elongation east or west of the Sun. At greatest elongation west the planet will be to the west of the Sun and hence visible in the morning sky whilst at greatest elongation east it will be to the east of the Sun being visible in the evening sky. When located between the Sun and Earth the planet is said to be at inferior conjunction and when on the far side of the Sun, at superior conjunction. An inferior planet will be highest in the sky for northern hemisphere observers when at greatest elongation east during the spring and greatest elongation west during the autumn.

Maintained by Michael Oates Email: mike@ph.u-net.com
Last modified 5th Sept 1996