Noon is the time of day when the Sun crosses the observer's meridian and is at its highest point above the horizon; at this point, the Sun lies due south of an observer in the northern hemisphere and due north of an observer in the southern hemisphere. The observed crossing of the meridian is known as apparent noon. Between one observed meridian crossing and the next is an apparent solar day. However, because Earth's orbit is elliptical and Earth's spin axis is tilted with respect to the ecliptic, the length of the apparent solar day varies throughout the year. To overcome this problem, the mean solar day is defined as the yearly average of the apparent solar day and used to defined a fictitious mean sun, which moves along the celestial equator at a constant rate. The passage of this mean sun across the meridian defines mean noon; at Greenwich, the mean noon defines 12.00 hours Greenwich Mean Time, also known as Universal Time.