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This planetary close approach is called a conjunction by astronomers. While the planets will appear to be close together in the sky, this is just a line of sight effect caused by the relative positions of Earth, Mars and Mercury. On Thursday night the distance to Mars is nearly 390million km, whereas Mercury is some 200million km from us.
The conjunction between Mercury and Mars will be quite hard to see because the two planets are quite low in the sky. By the time it is properly dark (an hour after sunset) the planets are a mere three degrees above the horizon. Three degrees is about three times the angle subtended by your little finger when held at arm’s length, so any obstructions or low clouds will hide the planets from view.
The best way to find the pair of planets is to use Venus as a guide. If you imagine Venus is the centre of a clock, then Venus and Mars can be found some 20 degrees below at roughly the seven o’clock position. You will almost certainly need a pair of binoculars, or better still a telescope, to give yourself the best chance of catching this event.
Once you find the planets, it will be easy to tell which is which. Mercury will be the brighter of the two appearing some eight times more luminous than its sibling.
- Ian Griffin