Mercury at its best, at arm’s length in the west

The moon reached last quarter yesterday, which means the early part of the night is devoid of moonlight and prime time for stargazing. The centre of the Milky Way is high overhead as darkness falls. To my mind, there is nothing better than sitting on a deckchair and gawping at the gorgeous starry tapestry which graces our sky at this time of year.

A week today, Mercury reaches its easternmost elongation from the sun. The next couple of weeks is therefore the best time this year to enjoy views of the innermost planet in the evening sky. Locating the planet is easy. As the sky darkens, find an observing spot with an unobscured view of the western horizon. Hold your fist at arm’s length, aligning the lower part with the horizon. At 7pm this week Mercury will be the brightest object in the western sky that’s roughly a fist above the horizon. Look for a pinkishcoloured ‘‘star’’. With Mercury setting about 8.30pm, the best time to view will be between 7pm and 7.30pm when the sky will be dark and the planet is at a reasonably high altitude.

If you have a telescope, do take a look at Mercury. You will notice that the planet’s tiny disc is just over 50% illuminated, resembling the moon at first quarter. Mercury’s small size and relatively low altitude make it difficult to see any surface features from Earth. Thanks to visiting spacecraft we know that the surface of the planet is pockmarked by craters.

Mercury is presently moving through the constellation Virgo, forming a nightly changing triangle with the stars Beta and Eta Virginis. A line drawn from Beta through Eta points to the star Gamma. Gamma is fascinating. Roman stargazers named the star Porrima after one of the goddesses of prophecy. While the star looks single to the naked eye, small telescopes reveal it is a wonderful double star. Both components are of similar brightness and pale yellow colour. The orbital period of the two stars is just under 169 years.