Mercury is the smallest and innermost planet in the Solar System. It orbits the Sun once every 88 days, the shortest year of all the planets in the Solar System. The planet is aptly named after the Roman deity, Mercury, the messenger of the Gods who flew on winged sandals.
Observing the Planet Mercury
Mercury is an elusive planet. While it can appear quite bright at -1.9 to 0 magnitude, it is difficult to observe as it is always near the sun. In fact, Mercury never exceeds 28 degrees in angular separation from the sun.
Please be very careful when observing Mercury. It is very close to the sun and a moment of carelessness resulting in a glance at the sun through an instrument like binoculars or a telescope will cause permanent blindness!
We can observe Mercury only during twilight near the Western horizon after sunset or the Eastern horizon before sunrise. The best times are during the weeks before and after greatest elongation, when it is farthest from the Sun. These opportunities come by every 3-4 months. Even at these optimal times, the planet is still only visible near the horizon, forcing us to observe through a much greater thickness of atmosphere.
The extra atmosphere causes poor viewing conditions. Hence, we cannot see any detail on Mercury’s surface. We learned a lot of what we know of Mercury from two spacecraft: Mariner 10 (mid-1970s), and Mercury MESSENGER (early 2010s).
I captured the above image of Mercury just after sunrise, about 2 weeks before greatest elongation. Finding the planet was difficult in itself even with a go-to telescope mount – I could barely see the planet against the brightening sky. It was even more challenging due to the difficulty of focusing through poor seeing. There were also no other reference points (like stars) to assist in adjusting focus.
Planet Facts
Mercury is tidally locked with the Sun in a 3:2 spin–orbit resonance. This means that it rotates on its axis exactly three times for every two revolutions it makes around the Sun. An observer on Mercury would see only one day every two Mercurian years.
Mercury has virtually no atmosphere and is heavily cratered like the Moon. It probably has been geologically inactive for billions of years. With no atmosphere to retain heat, the planet’s diurnal surface temperature varies tremendously, more then any other planet in the Solar System.
The last fact I present here is that Mercury is close enough to the Sun for the effects of general relativity to be measured!
Mercury deviates from the precession (change in the orientation of the rotational axis of a rotating body like what you see when a top spins) predicted by Newtonian effects. This anomalous rate of precession of the perihelion of Mercury’s orbit was first noticed in 1859 by Urbain Le Verrier.
Several scientists proposed solutions, but none were satisfactory. Eventually, Einstein managed to show that general relativity agreed closely with the observations. This became a strong argument for adopting general relativity. In fact, Einstein proposed it as one of the three “classical tests” of general relativity in 1916.