<P> On terrestrial planets and other solid celestial bodies with negligible atmospheric effects, the distance to the horizon for a "standard observer" varies as the square root of the planet's radius . Thus, the horizon on Mercury is 62% as far away from the observer as it is on Earth, on Mars the figure is 73%, on the Moon the figure is 52%, on Mimas the figure is 18%, and so on . For smaller, non-spherical objects, the observer's height must be taken into account when calculating the distance to the horizon . </P> <P> Because Mercury has little or no atmosphere, a view of the planet's skies would be no different from viewing space from orbit . Mercury has a southern pole star, α Pictoris, a magnitude 3.2 star . It is fainter than Earth's Polaris (α Ursae Minoris). </P> <P> On average, the visible diameter of the Sun on Mercury is 2.5 times as large as it appears from Earth, and its total brightness is more than 6 times as great . Because of the planet's eccentric orbit, the Sun's apparent size in the sky would vary from 2.2 times that from Earth at aphelion (with total brightness 4.8 times as great), to 3.2 times at perihelion (with total brightness 10.2 times as great). </P> <P> Mercury has a 3: 2 spin--orbit resonance . This means that although a sidereal day (the period of rotation) lasts around 58.7 Earth days, a solar day (the length between two meridian transits of the Sun) lasts around 176 Earth days . (In contrast, the difference between Earth's sidereal and solar days is only 4 minutes .) </P>

How big is the sun in mercury's sky