Question

Why isn't the earth the same distance from sun all year long?

Answer 1

Answer:

Earth's orbit is elliptical

Explanation:

The earth's orbit is not a straight circle and the sun is not in the very center of it. The orbit is more of a wonky oval with the sun closer to one side of it than the other causing the earth's distance from the sun to vary throughout the year.

Answer 2

Because the planets (and everything else in the solar system) don't travel in circular orbits with the sun at the center of the circle.  

They travel in elliptical orbits, with the sun at one focus of the ellipse.

Different points on the ellipse have different distances from the focus where the sun is.