Mercury is the nearest planet to the Sun, being named after the messenger of the Roman gods, whose Greek equivalent is Hermes. He is quite aptly named, as the planet does show quite rapid apparent movement swaying around the Sun, which in turn, moves across the sky as it passes through the zodiacal constellations. Mean solar distance is about 57 million kilometres or 0.39 Astronomical Units (AU), orbiting around the Sun once every eighty-eight (88) days. Any daytime observer observing from surface of Mercury would see a sky full of stars because of the lack of any real atmosphere. Poised somewhere in the sky during daylight hours would be the huge 1.4° solar disk – being some three times the diameter as seen from Earth – which varies by about 0.15° throughout the orbit. Its heavily crater packed surface is bathed in intense solar radiation whose surface is consequently exceedingly hot – popularly and anecdotally often stated enough to melt lead – even through actually evidence in finding liquid lead on the surface of Mercury is now highly unlikely.

Visual observers on Mercury would see the Sun shining about −29.3 magnitude, more than three magnitudes brighter than if observed from Earth at −26. Alternatively, if you could stand on the night time surface, then looking back towards Earth, would find a very bright bluish ‘star’ that during opposition would shine around −4.8 magnitude, while brilliant Venus would exceed −7.7 magnitude!

FINDING MERCURY

For any terrestrial observer, one of Mercury’s biggest drawbacks is that for long periods it remains well hidden within the solar glare.

Like nearby Venus, Mercury is an inferior planet, which lies well inside the Earth’s orbit. For this reason, it never exceeds more than 28° away from the Sun, and cannot stray more than 1 hour 45 minutes in right ascension either east or west of the Sun.

Due to Mercury’s highly eccentric orbit, such greatest elongations are mostly less favourable, and can be as small as 18°. Only for a few weeks during the time of the greatest elongations, can we see Mercury with the naked-eye – either in the morning or evening twilight but always close to the horizon. This short observational period can be often less than thirty minutes in duration, and this is only under ideal conditions.

A further disadvantage is the inclination of the ecliptic to the horizon. If the ecliptic is inclined sharply, then the elevation of the planet is often poor, so therefore Mercury becomes easily lost in the coloured glare of the atmosphere seen after the sun has set. Those living in temperate climates will find Mercury at its best during the times of the equinoxes — namely late March and late September. In the southern hemisphere, the best times are more likely when Mercury is at southerly declinations in April and October. Northern observers will find the times better when the planet has its greatest elongations and also lies at northern declinations. In places like northern Europe, seeing Mercury at all can only be rarely achieved, and this is only when the atmospheric conditions are very good or even perfect, and only in spring or autumn.

ORBIT of MERCURY

Once each 116 day synodic period, the planet experiences an inferior conjunction and enlarges to some 12±1 arc seconds across. At the times during the more distant superior conjunction, this size can shrink to 4.5±1.0 arc seconds. Over this whole cycle, the telescopic appearance of Mercury shows regular phases that slowly wax and wane like the Moon or Venus.

Due to the high orbital eccentricity, both the true distance and elongation of Mercury changes considerably between successive apparitions. Due to the high 7° orbital ecliptic inclination, the planet does not undergo successive transits across the solar disk each orbit. Transits may occur once either every seven, eleven or forty-six years but preferentially around either the 7th May or 9th November. (See Transit of Mercury) Previous recent transits occurred on 15th November 1999 and 07th May 2003, and the last being on the 09th November 2006. Another transit will again happen until 10th May 2016 then again on 12th November 2019.

Fig. 2. Orbits of the Inner Planets : 11th February 2017. This figure shows the orbital proportions and alignments of the Inner Planets ; Mercury (brown area), Venus (light brown area) & Earth (teal area).

NATURE of MERCURY

Being so close to the Sun, the midday temperatures may soar as high as 450°C, or plummets in the darkness of shadows or at night to −170°C. An average daytime temperature is more like 320°C, being no an air temperature by the temperature of the material. (Mercury is airless) Mercury’s axial rotation is about 59.7 days or about two-thirds the orbital period. An average length of the ‘day’ between successive sunrises is roughly 175.4 days.

Proximity to the Sun immediately suggests that more high velocity meteors, comets and asteroids, once heavily bombarded and pulverised Mercury. This is more so than any other planet than our Solar System.

AMATEUR OBSERVATIONS

Mercury appears like a tiny rosy-red or distinct yellow coloured disk in the telescope. This tint is likely a colour contrast effect, as the planet is usually immersed in the yellowish afterglow of sunrise or sunset. Notably, this rosy colour is not as prominent if observed during the daylight hours, and seems much more greyish. The appearance of the disk is featureless, though some observers using larger apertures sometimes report nebulous grey or white regions. As Mercury subtends sizes mostly below ten arc seconds such features will always remain elusive. Since the images taken by spacecraft, we can now attribute most of these surface effects as optical illusions, as they do not correspond to its surface.

Like Venus, observations are much better made during daylight hours where Mercury appears near or on the local meridian. By carefully offsetting the telescope away from the Sun’s position or using setting circles can make finding Mercury easy with a little practice. Should you use either of these methods, use general care and safety precautions as there is the real risk of accidentally exposing the optics of the main telescope to the dangerous rays of the Sun. Amateurs sometimes can estimate the observed time of the half phase or dichotomy but this requires sizeable apertures and reasonably good seeing conditions. Some have claimed to see odd-looking greyish features from time to time, but these are certainly contrast or optical effects.

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Last Update : 22nd September 2019

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