The solar noon is defined as the time of the highest position of the Sun in the sky and occurs when the Sun crosses the meridian at a given position. The mean length of the day, namely 24 hours, is a little less than the period of rotation of the Earth around its axis, since the Earth makes 366 rotations around its axis during a year of 365 days. The length of the solar day is the time between two consecutive solar noons. If the axis of the Earth were vertical and the orbit of the Earth around the Sun circular, the mean length of the day would correspond to the time between two consecutive solar noons. For Greenwich’s meridian, the official noon is defined as the solar noon at the spring equinox and then applying a period of 24 hours for the other days of the year. The solar noon oscillates during a year: it is only equal to the official noon four days a year. The difference between the solar time and the official time, called mean solar time is called the equation of time. The fact that the equation of time oscillations of amplitude approximately 30 minutes is explained by two phenomena. The first one is the obliquity of the Earth’s axis: if the orbit of the Earth around the Sun would be circular, the official noon would correspond to the solar noon at the equinoxes and at the solstices. It would be after the solar noon in fall and spring and before the solar noon in summer and winter. The second ingredient is that the orbit of the Earth around the Sun is an ellipse and not a circle. When the Earth is closer to the Sun (this is during the winter of the Northern hemisphere), it has a higher angular velocity around the Sun, yielding longer solar days.