Planetary gathering after dusk

 Mercury will be visible for the next couple of weeks. Photo: Getty Images
Mercury will be visible for the next couple of weeks. Photo: Getty Images

This week, keen celestial sleuths can track down the normally elusive innermost planet Mercury, which will be visible after sunset low in the southwestern sky, says Ian Griffin.

Ian Griffin
Ian Griffin
Orbiting our parent star from an average distance of 58 million kilometres, Mercury has a year that is only 88 Earth days long.

The combination of this short orbital period and its proximity to the sun, means that when seen from Earth, this small dense rocky planet is never more than 27 degrees from the sun.

While Mercury will be visible for the next couple of weeks, there will be a really fortuitous alignment tonight, involving the moon and Saturn, that can be used to find it. At 10pm the three-day-old waxing crescent moon will hang some 15 degrees above the western horizon as the sky darkens.

If you are not familiar with angles, 15 degrees is about the same distance as the angle subtended by your little finger and index fingers if held at arms length.

Once you find the moon, imagine it is the centre of a clock face. Cast your gaze downwards in the direction of 8 o'clock. You should pick out a yellow/white ``star'' which is in fact the planet Saturn. To find Mercury, just look a little down and to the left; Mercury is brighter than Saturn and should be visible as a whitish star roughly seven degrees above the southwest horizon.

If you have access to a telescope and point it towards Mercury, you will see that tonight its disk will be nearly two-thirds illuminated, making it look like a tiny waxing gibbous moon. As you gaze upon the moon, Saturn and Mercury, do take a moment to consider their relative distances from us.

Even though they appear close together in the sky they are, in reality, at vastly different distances.

Travelling at more than 300,000km per second, the sunlight you see reflected from the moon's surface left our closest celestial neighbour 1.35 seconds ago.

Mercury's reflected sunlight has taken nearly nine minutes to reach your eyes, while the light from Saturn has taken almost 91 minutes.


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