Highlights: Catch Mercury’s evening show
When: early evenings in second half of January
What’s special: Mercury, the closest planet to the Sun, opens its observing year in ne style,
becoming visible in the post-sunset sky around the time it pulls out to greatest eastern elongation
(18.6 degrees) on 24 January. ere’s a certain satisfaction to be had when you see Mercury, as many
astronomers have yet to clap eyes on the most elusive planet in the Solar System. It’s never easy to
nd, as it doesn’t stray too far from our star, only eetingly emerging from its overpowering glare a
few times each year.
How to observe: As at every favourable evening apparition of Mercury, you’ll need to track down an
observing site with a reasonably unobstructed view towards the south-west. is is especially true this
month, as this is not Mercury’s best evening showing from mid-northern latitudes; we’ll have to
wait until May for that. (Mercury’s best morning apparition for the UK takes place in October.)
Nevertheless, the planet’s only around two degrees worse off in altitude, so it’s well worth tracking
down Mercury this month.
It’ll be interesting to see how soon you’re able to locate Mercury. Around mid-month from London,
it lies just under 10 degrees above the south-western horizon at sunset (4.20pm). By the time the
Sun is six degrees down (by about 5pm at the end of civil twilight), Mercury lies just 4.5 degrees up
at azimuth 232 degrees. e planet is at its brightest at the beginning of this apparition, shining at
magnitude –0.87. In a transparent sky you should spot it, but if you can view right down to the
horizon, don’t mistake Mercury for brighter Jupiter, which lies ve degrees below. A pair of 10 × 50
binoculars should easily snare Mercury, although it appears merely as an off-white star.
By the time of greatest elongation on 24 January, Mercury has climbed to an altitude of around 13
degrees at sunset from London, though it’s faded to magnitude –0.5. Mercury remains on show for
the rest of the month, fading more rapidly to magnitude +1.0.
Trying to get a telescope on Mercury at such a low elevation after sunset (or in the pre-dawn sky) is
a difficult task. A small telescope can reveal Mercury’s changing phase and occasionally elusive and
subtle markings, so observing the planet in daylight may be the best choice for more experienced
observers (this option is not recommended for beginners).
By mid-month, Mercury’s elongation from the Sun has stretched out to around 15 degrees (don’t try
to nd Mercury when it’s 10 degrees or less from the Sun and NEVER dangerously sweep around
the sky randomly). A polar-aligned GoTo mount with a ‘home’, or ‘park’ feature is the best option,
though you can synch on the Sun (centre it by projecting its image onto a piece of white card
through a correctly aligned nderscope) to then slew to Mercury.
Mercury’s gibbous phase decreases to around a ‘ rst-quarter’ phase at greatest elongation (when
Mercury culminates at 1.30pm at an altitude of 25 degrees) and then to a slim crescent by the end of
the month. A yellow or orange lter should help combat unsteady seeing.
Mercury displayed a 60 per cent gibbous phase when imaged on 5 May 2008. It will show a comparable phase on 22 January.
Mercury imaged in daylight at an altitude of 28 degrees at 18:26 UT on 24 June 2018. It will show a similar phase (72%) on 19
January.
Mercury has a phase of around 30 per cent on 29 January. It culminates at about 1.15pm and can be observed or imaged in daylight.
Mercury pulls out to greatest elongation east of the Sun on 24 January. This is the view at about 5.15pm GMT, when the planet lies
eight to nine degrees above the horizon and shines at magnitude -0.5.
ViewHighlights: Catch Mercury’s even...
January 2021
Astronomy Now