Generalities: it is the planet nearest  to the Sun. It has rather small dimensions and it is a rocky planet.
It is the thickest planet of the Solar System and for this reason  its metallic nucleus is supposed to occupy great part of the ray of the planet. It is practically deprived of atmosphere: the presence of helium has been noticed, probably captured by the solar wind in extremely small quantity. Because of its proximity to the Sun, its diurnal temperatures are extremely high. The lack of atmosphere  exposing the planet to solar radiations during the day, enables  the heat to be  quickly dispersed across the space during the night, making  the temperature fall  toward very low values. This procedure is helped by the slow rotation of the planet. The proximity to the Sun also exposes Mercury  to the bombardment of the meteorites attracted by the strong solar gravity. For this reason  the small planet has a  surface very rich in craters.

Observation: Mercury  is a very elusive object in the terrestrial sky . The difficulty of observation lies in the fact that its permanence in the sky not illuminated by the Sun is limited to brief times after sunset or before dawn. For this reason Mercury  cannot be ever observed in a completely dark sky. Being next to the sunset, or being risen by a little time it  must then  be observed when it is rather low in the sky. Its magnitude is very varied, since Mercury shows  phases. Unlike Venus, which varies its distance from the Earth a lot, even if it shows phases,compensating, as far as  brightness is concerned, the small phase with the greatest proximity to the Earth, Mercury, which varies less its distance from the Earth, is more influenced by the phase, showing different brightness. The magnitude can overcome -2 at the superior conjunction (completely illuminated, but not observable for the perspective proximity to the Sun) and go down beyond the value of +4 at the inferior conjunction (it shows the dark hemisphere). The visibility interval of interest (next to the maximum elongations) varies from -1 to +1. Because of its quick motion the observation  periods  last few days, but they are shown more times a year.

- Visibility of planets during this month

- Mutual position of Earth and internal planets

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