The sun slowly drifts in front of the celestial sphere (the background of stars) in such a way that it comes back in front of the same star once a year. Thus it does a slow 360^o circuit in the space of 365 1/4 days, or approximately 1^o/day. Because these motions are relative, we can just as well say that the celestial sphere drifts behind the sun by 1^o/day. Remember that from rotation, we know that a change in location of one degree amounts also to a time difference of 4 minutes. Thus, when we look at the night sky, one consequence of revolution is that stars rise 4 minutes earlier each night. Another way of saying this, is that a star day (sidereal day) is only 23 hours, 56 minutes long, whereas the day based on rotation relative to the sun (the solar day) is 24 hours long. 

Moreover, the sun does not appear to travel along the celestial equator.  The apparent path of the sun, called the ecliptic, is tilted by 23.5 degrees in relation to the Celestial Equator. Thus, over one revolution in the span of a year, the path of the sun is above the Celestial Equator (N of it) for half the year and below (S of it)  for the other half. That also means it is on it (crosses it twice a year. This tilt of the ecliptic, along with the yearly revolution, is the fundamental cause of the seasons.  

The Egyptians were the first to use revolution as a way of fixing the year. They noticed that, once a year, the star Sothis (Sirius to us) rises in line with the rising sun. They used this yearly occurrence as a way to set their calendar and, for them, this astronomical event marked the beginning of the new year.  

	The second consequence of revolution is a change in the position in the sun in comparison to the celestial equator. The earth/sun line does not lie on the celestial equator. In fact, this imaginary line, called the ecliptic, is tilted 23 1/2 degrees in relation to the celestial equator.
	Thus the apparent path of the sun against the celestial sphere, called the ecliptic, varies over the course of a year, from 23.5o above the celestial equator, to 23.5o degrees below it, crossing it twice.
	This leads to a change in the position of the sunrise and sunset throughout the year, and a related change in the length of the day. Note that the sun will rise due east and west only when it is on the Celestial Equator. When the sun is above the CE, it will rise north of east and set north of west. Conversely, when the sun's path is below the CE it will rise south of east and set south of west.
	The combination of the 23.5o angle between the ecliptic and the celestial equator, and revolution also leads to the seasons. When the earth is in the right hand position in the diagram, the northern hemisphere is getting most of the sunlight, and it is June 21st, the day of the summer solstice. Six months later, on December 21st, the day of the winter solstice, the sun is 23.5o below the celestial equator, and it is the beginning of winter in the northern hemisphere. In the intermediate positions, the sun is on the celestial equator and both hemispheres are receiving the same amount of sunlight. These are the equinoxes, days of equal day and equal nights. March the 21st is the day of the vernal equinox, and September 21st is the day of the autumnal equinox.