November 17 - December 1, 2009: Tilting

The morning here began with ominously dark fog, but has cleared up to become a clear, warm, sunny day. This feels like May in Portland. I'm sitting on a new redwood deck, and the whir of drills and saws in the background is comforting and fun. A low-flying blue airplane circles behind the barn and finally lands at the airport, disappearing behind a low rising hill. A flock of sparrows rushes me like a school of fish, swarming past my head with the flow of air currents; the last one catches my head turning and veers off to the other side of the house, startled. The airplanes return, droning like large, flying beetles. Its a perfect November day in California, with all the carefree warmth of summer. Vultures soar, horses stretch and smile; crow calls and finch chirps fill the air. I find myself surrounded by life, by health, and yes, comfort. The new grass is short and bright, and the sky is just a lighter shade of blue than the plastic buckets that sit in the yard like proud toadstools. What is it that makes a November in California so different from a November in Oregon? While I sit in shirt-sleeves, I know that Schmuel is wearing sweaters indoors and thinking about hot things to make for dinner. I know that highways are flooding, snow is falling, and leaves are freezing in that northern state, but how could the difference of a mere 600 miles (thinking in the scale that the earth curves from north to south pole in 22,000 miles - meaning that the distance from the Bay Area to Portland is approximately 1/37th of this distance) explain the patterns of sunlight, growth, timing, and temperature?


The beginning of the answer is such: that the obliquity of the ecliptic of the earth is approximately 23.44°. "Ecliptic" refers to the plane of orbit; "obliquity", the deviation from perpendicularity. This is the tilt that causes the seasons. Because of this tilt, each half-orbit that our Earth makes around the sun brings either the northern or southern hemisphere closer to the sun - whichever hemisphere is closest receives more sunlight per day, and at an angle which delivers more sunlight energy per unit of surface area. Recall that sunlight allows life on Earth, and think about a difference of 1/37 more or less sun energy being received by California or Oregon. And, like most things about Earth, this 23.44° is not constant, but cyclical. There is a 41,000 year cycle between obliquities of 22.1° and 24.5°. The Earth actually moves like a gyroscope through space, and if you imagine yourself setting a top to spin, imperfectly, so that the handle doesn't stand up as straight as you might like it to, but describes larger and larger circles upon your table, until the handle touches down and your top goes rocketing off into ... outer space. Like your top, the Earth's axis is slowly describing a circle through space.

( This is not, however, in an ever-widening circle - for a more thorough discussion of these reasons, and about the obliquity of the ecliptic during Earth's formation, see "Extraordinary climates of Earth-like planets: three-dimensional climate simulations at extreme obliquity", by Darren M. Williams and David Pollard, and printed in 2003 in the International Journal of Astrobiology. The link is

http://physics.bd.psu.edu/faculty/williams/3DEarthClimate/ija2003.pdf

And you may have calculated that we are approximately in the middle of this cycle. What does this mean? Changes in the degree of tilt will affect the amount of differences between the seasons, and the length of the seasons. But specific effects of differing obliquities are complicated and disputed, and I honestly cannot explore them here, nor do I feel expert enough to judge good or bad astronomy. I will only write that they include effects of nutation, eccentricity, precession, inclination, and insolation, and that they might (or might not) be responsible for ice ages. I can hardly summarize the grace with which some reports are written, so I will direct you to page 182 of "the Change in the OBLIQUITY of the ECLIPTIC; its Influence on the CLIMATE of the POLAR REGIONS, and LEVEL of the SEA." By Mr. JAMES CROLL, published in 1867. The link is

http://docs.lib.noaa.gov/rescue/IPY/IPY_020_pdf/Qc8845A6C761867.pdf

To an ecologist (which is what we think we may be), a knowledge of the amount of sunlight that the earth receives on any given day is paramount to understanding how energy is cycled throughout ecosystems. The obliquity of the ecliptic matters! Not only for the basic needs of human survival (i.e., plant in spring, harvest in autumn) but for understanding the great patterns that underlie the spinning planet upon which we make our home.