View Full Version : Perihelion Date as Climate Marker

Robert Tulip
2014-Aug-18, 02:43 PM
In looking for a single simple scientific marker for orbital climate change, how well does the perihelion date serve?

My reasons for asking this question are as follows.

Perihelion, earth’s closest annual approach to the sun, is now about 3 January every year. It moves forward on average by one day every 59.14 years, by my calculation. The motion of the perihelion is due to apsidal precession. As discussed at http://en.wikipedia.org/wiki/Apsidal_precession#Long-term_climate , apsidal precession combines with precession of the equinoxes to drive a 21,600 year cycle of glaciation. On this simplified model, when the perihelion is at the summer solstice, glaciation is at minimum, and when perihelion is at the winter solstice, glaciation is at maximum, due to the cycle of increased and decreased summer ice melt in response to northern insolation discovered by Milankovitch.

I have mapped the movement of the perihelion over precessional time in the astronomy software SkyGazer 4.5, by setting the period of movement as 365.259 days. I calculated this as the time the sun takes to return to the perihelion point, or the perihelion year, which by my calculation is 24.4 minutes longer than the tropical year of 365.242 days, based on the standard estimate that the perihelion takes 21,600 years to complete a solsticial cycle.

Tracking the movement of the perihelion shows its slow advance through the months of fall and winter over historical time from before the Bronze Age.

My question seeks feedback on how reasonable it is to use this main 20,000 year period, when glaciation also responds to the slower influences of obliquity and ellipticity. Over the last ten thousand years, the natural cycle has been masked to some extent by human activity (eg Neolithic methane from rice), but in earlier periods shown in the ice core record there appears to be some correlation between a spring-summer perihelion and warming, and between an autumn-winter perihelion and cooling. I am interested to know how strong and valid is this causal relation.