Figure 2-22: Why the Sun Is a Poor Timekeeper There are two main reasons that the Sun is a poor timekeeper. (a) Earth’s speed along its orbit varies during the year. It moves fastest when closest to the Sun in January and slowest when farthest from the Sun in July. Hence, the apparent speed of the Sun along the ecliptic is not constant. (b) Because of the tilt of Earth’s rotation axis, the ecliptic is inclined with respect to the celestial equator. Therefore, the projection of the Sun’s daily progress (shown in orange) along the ecliptic onto the celestial equator (shown in blue) varies during the year. This causes further variations in the length of the apparent solar day.