On or about September 21, fall begins in the northern hemisphere. This is the time of the year when the Sun is directly over the equator. This is called the Autumnal Equinox. At this time, the daytime and nighttime are each 12 hours long throughout the world. From September 21 each day becomes progressively shorter until on or about December 21.
Summer begins in the southern hemisphere.
When looking at the Sun, it appears to be moving in the sky. The Sun does not move, it is the Earth which is orbiting the Sun and changes its location daily in its relationship to the Sun. The Earth's speed is 66,000 mph in its orbit around the Sun.