There is, of course, the common sense of day versus night, in which day means the part of the solar cycle when your location on the Earth faces the sun and it is light outside. There are several definitions of “day”-probably more than you realize. The Earth's rotational axis isn't exactly perpendicular to the orbital plane. Second, it rotates around an axis passing from its north pole to the south pole. First, the planet orbits the sun, with one orbit taking a year. The Earth has two motions that change the length of a day.