MILANKOVITCH CYCLES
The main cause of periodic glaciations is from a theory set forth by a Serbian civil engineer, Milutin Milakovitch. He put forth this theory in the early part of the twenteith century. The realition between the Earth and the Sun is affected by a number of varitions in the Earth¸s rotation and orbit. These variations cause slight changes in the amount of solar radiation absorbed by the Earth. (436)The extent of the moat recent North American glacation approximately 20¸000 years ago. Looking down on the North Pole arrows indicate the direction of the glacier´s advances.
The easiest of the orbital effects to understand is the presession of the Earth´s axis rotation. If you ever seen a top spining¸ you know that sometimes as the top spins rapidly around its¸ axis the axis itself describes a lazy circle in space. This circular motion is called precession. In the same way that a top may precess every second or two, the Earth's axis¸ which is tilted at an angle of 23º precess once evety 23¸000 years. While this going on, the axis the Earth's elliptical orbit also moves around the Sun due to the gravitional effects of the other planets. The net effect of these motions is that when the Earth is farthest from the sun, during the summer¸ the axis of the Earth tilts the Northern Hemisphere toward the sun for half
