The difference between apparent solar time, i.e. The time shown by a clock running uniformly throughout the year. The difference arises from two factors in the Earth's orbital motion around the sun and its rotation about its own axis. The Earth's orbit is not a circle but an ellipse, so that the sun's apparent motion is more rapid in January, when the Earth is nearer the sun, than in July, when it is farther away, the effect being enhanced by the fact that the Earth is moving more rapidly in January than in July, by Kepler's Law. The second factor in the equation of time is due to the Earth's axis being inclined by 23 1/2?to a line perpendicular to its orbit. If we take a viewpoint from the Earth itself we see that, while hour angles must necessarily be measured around the celestial equator, the sun's apparent motion is around the ecliptic, and this motion is not uniform when projected on to the equator, being greater in summer and winter when the sun is moving parallel to the equator and is nearer to the poles than at the equinoxes when its motion is at 23 1/2?to the equator. The two factors of the equation of time are roughly equal in magnitude, but the first goes through one cycle per year, the second two cycles. When plotted on a graph and aded together they give a curve with two major and two minor peaks.