The apparent path of the Sun on the celestial sphere is known as the ecliptic. The sinusoidal shape of the ecliptic is due to Earth's axis of rotation being tilted 23.5° with respect to the plane of Earth's orbit around the Sun. Click Animate and observe the apparent motion of the Sun on the flatmap of the celestial sphere in the animation below. The Sun crosses the celestial equator on both equinoxes (March 21 and September 21) but can be as much as 23.5° north of the celestial equator on the summer solstice (June 21) and 23.5° south of the winter solstice (December 21).