Time based on the rotation of the Earth with respect to the Sun. Solar time units are slightly longer than sidereal units due to the continuous movement of the Earth along its orbital path. For example, by the time the Earth has completed one full rotation on its axis with respect to the fixed stars, it has also moved a short distance in its orbit and is oriented slightly differently to the Sun, so that it must turn slightly more on its axis to complete a full rotation with respect to the Sun. &diamf3; The time it takes the Earth to rotate fully with respect to the Sun is called a solar day . The length of a solar day varies throughout the year due to variations in the Earth's orbital speed and other factors. &diamf3; The average value of all solar days in the solar year is called a mean solar day ; it is 24 hours long and by convention is measured from midnight to midnight. &diamf3; A solar year is the period of time required for the Earth to make a complete orbit with respect to the Sun as measured from one vernal equinox to the next; it is equal to 365 days, 5 hours, 48 minutes, 45.51 seconds. A solar year is also called an astronomical year and a tropical year . &diamf3; A solar month is one twelfth of a solar year, totaling 30 days, 10 hours, 29 minutes, 3.8 seconds.