Precession - The third orbital parameter that influences climate is the Precession of the Equinox. This parameter provides a measure of the timing of the Earth's seasons in relation to the maximum and minimum Earth-Sun distance. Today, Northern Hemisphere summer occurs when the northern hemisphere tilts toward the Sun; This happens when the Earth is at the point on its orbit that is furthest from the Sun.  Through time however, the Earth's axis wobbles (or precesses) along the orbit so that the timing of the seasonal extremes shift to different locations on the Earth's orbit.  For example, around 11,000 years ago, Northern Hemisphere summer occurred when the earth was closest to the Sun.  We'll look more closely at how this affected climate later in the talk.  Like eccentricity, the precession is actually composed of a number of cycles that take between 19,000 and 23,000 years.  Interestingly, the eccentricity of the Earth's orbit affects how much influence precession has on climate. We call this modulation.  Can you think of how eccentricity might modulate precession?