astronomers usually use different words for the orbit of a planet around a star, and for the spin of a planet about its own axis. if an item is in orbit, it is "revolving", while the spin of a planet is called "rotating". the amount of time it takes for an object to revolve around the sun is a year, and the amount of time it takes to revolve around its axis is one day. according to the gregorian calendar, a year on earth is equal to 365 days, with an extra day added every four years.