The planet must be nearly spherical. Otherwise the planet might become tide-locked, that is, its rotation could slow until its day was comparable to its year. The Moon is elliptical and one side always faces the Earth. This makes a day on the Moon a month long. Mercury is elliptical like a football and on its closest approach to the Sun one of its ends points toward the Sun. This makes Mercury’s day equal two thirds of its year. The Earth’s more rapid rotation provides more uniform heating.