and winter in the Northern Hemisphere occur at closest approach, the Earth is moving at its maximum velocity and therefore autumn and winter are slightly shorter than spring and summer. Today in the Northern Hemisphere, summer is 4.66 days longer than winter and spring is 2.9 days longer than autumn. As axial precession changes the place in the Earth's orbit where the solstices and equinoxes occur, Northern Hemisphere winters will get longer and summers will get shorter, eventually creating conditions believed to be favourable for triggering the next glacial period. The arrangements of land masses on the Earth's surface are believed to reinforce the orbital forcing effects. Comparisons of plate tectonic continent reconstructions and paleoclimatic studies show that the Milankovitch cycles have the greatest effect during geologic eras when landmasses have been concentrated in polar regions, as is the case today. Greenland, Antarctica, and the northern portions of Europe, Asia, and North America are situated such that a minor change in solar energy will tip the balance in the climate of the Arctic, between year-round snow/ice preservation and complete summer melting. The presence or absence of snow and ice is a well-understood positive feedback mechanism for climate. References Further reading Cionco, Rodolfo G., and Pablo Abuin. "On planetary torque signals and sub-decadal frequencies in the discharges of large rivers." Advances in Space Research 57.6 (2016): 1411–1425. External links The NOAA page on Climate Forcing Data includes (calculated) data on orbital variations over the last 50 million years and for the coming 20 million years The orbital simulations by Varadi, Ghil and Runnegar (2003) provide another, slightly different series for orbital eccentricity Climate forcing