how big is the sun in mercury's sky
Extraterrestrial skies 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).
Astronomical unit The astronomical unit (symbol: au,[1][2][3] ua,[4] or AU) is a unit of length, roughly the distance from Earth to the Sun. However, that distance varies as Earth orbits the Sun, from a maximum (aphelion) to a minimum (perihelion) and back again once a year. Originally conceived as the average of Earth's aphelion and perihelion, since 2012 it has been defined as exactly 7011149597870700000♠149597870700 metres or about 150 million kilometres (93 million miles).[5] The astronomical unit is used primarily for measuring distances within the Solar System or around other stars. However, it is also a fundamental component in the definition of another unit of astronomical length, the parsec.