Mercury is (on average) at 0.387 AU from the Sun, while we are at 1. We are 2.58 times further away from the sun.

So, from Mercury, the apparent diameter of the solar disc would be 2.58 times what we see here

(2.58 * 32' = 82.66' = 1 deg. 22' 40")

This means that the sun would appear 6.67 times larger than it does here (the apparent area of the disc varies as the square of the diameter). Therefore it would appear 6.67 times more luminous (two magnitudes brighter) and would feel 6.67 times hotter (solar flux at Mercury = 9140 W/m^2, while it is 1370 W/m^2 just outside our atmosphere).

Our atmosphere brings this down to 1000 W/m^2 by the time it gets to the surface, with the ozone layer, when it works, removing some harmful UV rays and the water vapour removing some infra-red (heat), so that sunlight is "just right" for most of us.

On the surface of Mercury, you'd get 9 times what we get on the surface of Earth. Bring sunblock.

All the numbers above are based on average distances. Mercury's orbit is eccentric with e=0.206

(Earth's e=0.017 where e=0 is a perfect circle).

This means that at perihelion (closest to the sun) Mercury is only 0.387(1-0.206)= 0.307 AU from the Sun while at aphelion, it is at 0.387(1+0.206)=0.466

An observer on Mercury would see the sun at perihelion with 1.5 times the apparent diameter it has as aphelion. In area, that's like 2.3 times (one magnitude of brightness... and heat).

Mercury's spin period is exactly 2/3 of its orbital period. It spins on its own axis three times while going around the sun twice.

This is because of the torque imposed by the Sun's tidal effect at (and around) the time of perihelion where it is greatest (tidal effect varies as the cube of the distance: 1.5^3 = almost 3.4); at perihelion, Mercury is moving fastest on its orbit, imposing a spin that is faster than the average orbital period.

Because of the rotational inertia, the rotation (spin) period does not change, but the orbital speed does change with the planet's distance from the Sun (the closer, the faster). With such an eccentric orbit, there will be a time in its year where the Sun's apparent (annual) movement along Mercury's ecliptic will be faster than the apparent (diurnal) movement accross the local sky.

So, if you pick your observing location with care (longitude being the key), you could observe a double sunrise: the sun rises, then just after clearing the horizon, starts back down again, dips below the horizon, then starts up again to rise for good and proceed for the very long "Mercurian" day.

The sky would not look as dark as it does from the Moon. There is a tenuous atmosphere on Mercury: mostly ionised gasses forced out of the surface rocks by the intense bombardment of charged particles from the solar wind. The ionised atoms capture free electrons (in the process, emitting what, on Earth, we see at "Northern lights"), while other atoms, struck by charged particles, lose their electrons. Near the surface,tiny grains of dust would become electrically charged (by the same type of process).

Therefore, sunlight would be (a little bit) diffracted and scattered by atoms and some very fine dust electrically suspended in the atmosphere, to which one must add the faint light of reionisation.

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Just makes me want to return there for another vacation...