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.