Changing of appearance of a planet imply?

Answer 1

If a planet has phases like Mercury and Venus, that would imply it's within the orbit of Earth (closer than Earth to the Sun) such that it might take a crescent shape from time to time as does the moon, and also indicates that the planet itself might rarely transit the solar disk as seen from Earth. Changes in its apparent size or magnitude can indicate orbital position, or orbital eccentricity. Changes in appearance of other planets can imply weather events - for example during Martian seasons the carbon dioxide ice cap warms and sublimates generating gas quickly enough to cause dust storms to sweep across the planet's surface. The outer gas giants (with the exception of Uranus) have easily visible cloud markings which reflect changing temperature or weather patterns on the planet and can be used to infer internal conditions, cloud pattern changes might imply seasonal changes or other mechanisms; the direction of cloud bands also can be used to determine axial tilt, rotational period, shifts in relative chemical composition; the sudden appearance of certain types of markings may imply recent meteorite impact or for inner planets, geological activity such as volcanism. Changes in appearance of planet Earth might also imply human or other biological activity such as algal blooms or large fires, etc.