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Saturday, October 3, 2015

Notes on Obliquity

It is usual, when discussing Milankovitch cycles, to dismiss any effect of obliquity on global, annual mean insolation based on the fact that the Earth is a sphere.  I did it myself in a comment on Skeptical Science.  As the Earth is an oblate spheroid, however, it is not strictly accurate.  In particular, on the equinoxes, the Earth presents its minimum aspect to the Sun, showing an eclipsed area of 1.2737 x 1014 m2 to the Sun.  As the Earth moves to the solstice (either winter or summer) it presents its maximum aspect, showing an eclipsed area of 1.2744 x 1014 m2.  That represents a difference of 0.05% in recieved sunlight, or approximately 0.12 W/m2.  That seasonal variation is much less than the 6.8% (16.2 W/m2) seasonal variation due to the eccentricity of the Earth's orbit.