Why does earth's distance from the sun change?
The Earth's orbit around Sun is under a central force that is inversely proportional to square of distance from Sun.The orbit is not a circle but an ellipse with Sun at one focus.
Thanks for the depiction. The Sun's distance CS = ae from center C is 1 in 60 of the semi-major a. So, for orbit on an A4 paper, S will be quite close to C.
e = 0.01671. CS/a = 0.01671. Also, minor axis b is 0.99986a. If the orbit is drawn to scale on A4 paper, it will appear as a circle. Of course, some elongation is necessary to show what it is, an ellipse.