There are three long term variables in the earth's orbital dynamics with respect to the sun. These were first recognized as climatically significant by James Croll, who published Climate and Time in the Geological Relations in 1875. This work was extended quantitatively and popularized by the work of Milutin Milankovitch, who calculated the variance of insolation due to each of the variables, and their combined effects. The longest period of these variations is a change in the eccentricity of the earth's orbit. The eccentricity of the ellipse is a measure, usually expressed as a percent difference between the major and minor axes of the ellipse, of how far from circular the ellipse is.
Perihelion is the point in the earth's orbit when it is closest in proximity to the sun.
Aphelion is when we are farthest away. The angle between the earth's equator and the plane of the eccliptic, the inclination, also changes with time on a cycle about 41,000 years in length. The variance causes the angle to vary between 22.1 degrees and 24.5 degrees. Right now it is about average, at 23.5 degrees. Ten thousand years ago it was 24.3 degrees.