The Sky Guide challenge
Can you spot Mercury with a telescope and log an observation of its phase? If you manage to locate Mercury, the next trick is to turn your telescope on it
Mercury is an elusive planet and one that a surprising number of people have never seen. May 2021 presents a great opportunity to correct this, Mercury being well presented in the evening sky. However, our challenge this month isn’t just to see Mercury, but rather to see it through a telescope and log an observation of its phase.
Mercury reaches greatest eastern elongation on 17 May, the date when the planet appears furthest separated from the Sun in the evening sky. On this date the Mercury–Sun separation will be a very respectable 22˚. However, the planet can be seen right from the start of May, as long as you have clear weather and a good, flat west–northwest horizon.
On 1 May, it shines at mag. –1.0 and sets 90 minutes after the Sun. The setting difference increases as the planet separates from the Sun in the sky, but this is offset by a drop in brightness.
To see Mercury’s phase, you’ll need at least a 60mm telescope, although a 100mm minimum is recommended. We’d recommend at least 100x magnification, more if the conditions allow. For evening viewing, aim to pick up the planet as soon after sunset as possible, but of course this is also made harder because of a brighter sky. On 1 May, Mercury shows an 80%-lit 5 arcsecond disc and as the days click by, the planet dims, grows in apparent size and reduces in phase. By 10 May, setting an impressive 135 minutes after the Sun, Mercury appears at mag. –0.2, has an apparent size of 6 arcseconds and exhibits a 53%-lit phase. On 17 May, the day of greatest elongation, Mercury sets 145 minutes after the Sun, shines at mag. +0.6 and presents an 8 arcsecond disc with a 35%-lit phase.
Spotting a planet and seeing its phase may sound a rather trivial challenge, but in this case, it’s not. Finding Mercury can be a task in itself. Poor weather, low altitude and bright skies, will all conspire to hinder your attempts. If you manage to locate it, the next trick is to turn your telescope on it. This can be a race against time if there are features on the horizon towards which Mercury appears to be running for cover. Then there’s the issue of seeing; if the seeing is poor, Mercury’s tiny size may render it little more than a distorted blob through the eyepiece.
If you do manage to locate Mercury this month (turn to page 48 for positional information) and manage to catch a view of it through the eyepiece of a telescope so that you can see its phase, you can be forgiven for having a definite sense of achievement!