86, actually the orbit varies from near circular to an ellipse within a cycle of about 100,000 years. The elliptical orbit is not centred around the Sun but has the Sun centred at one end of the ellipse. This means that when the elliptical orbit is at it's maximum, the time the earth spends during it's closest position to the sun is at it's minimum. A circular orbit means that the warming and cooling periods during the year it takes to complete the orbit are comparatively constant, the summer and winters being controlled by the tilt of the axis. Therefore when the elliptical orbit reaches it's maximum, the time the earth spends closer to the sun than the average distance is less than the time it spends further away than the average distance, hence the tendency for the earth to cool.This is when the ice ages occur. A the present time the orbit is trending towards the circular phase of the cycle which means the solar input from the sun is relatively constant, and hence the warmer part of the natural cycle.
