the earth's ORBIT and orientation around the Sun affects how solar energy is received. Obliquity is the degree of the Earth's tilt as it completes its daily rotation and yearly revolution around the Sun. It is the angle an imaginary rotational axis would make with the plane of the Earth's orbit. The axial tilt, which varies over time from 21.5 to 24.5 degrees and back again, is the reason for the seasons. A complete tilt cycle takes 41,000 years to complete. In concert with the eccentricity of the orbital path around the Sun and the precession effect, obliquity is a factor of climate change affecting Earth's average surface temperature over the long term.
Chinese astronomers in the 11th century determined Earth's obliquity of the ecliptic. They observed that during the changing seasons, the extreme northern and southern declination of the Sun defined the obliquity. Astronomers also deduced that the difference of the peak height of the Sun on the longest day and shortest day of the year was double the obliquity factor.
The Earth's obliquity is about 23 degrees and 26 minutes. Its axial tilt is consistent during the year as it revolves around the Sun. During an annual orbit, each hemisphere alternates between being tilted towards the Sun and then tilted away from the sun. This changing exposure to the direct rays of the Sun is the cause for the seasonal variety and why the Northern Hemisphere enjoys summer while the Southern Hemisphere experiences winter, and vice versa. The half of the Earth's sphere tilted toward the Sun receives more sunlight in a given day and accepts it an angle closer to the vertical. The longer days and more direct rays of light deliver more heat, which contributes to global warming. Fortunately, the Earth is currently in an orbital cycle where the summer-time tilt occurs when it is furthest from the Sun, thereby moderating potential temperature extremes.
Obliquity is a major factor in climate change with the ebb and flow of ice ages on a planetary scale. When obliquity is low, the higher latitudes receive less solar energy. The days are shorter and the sunlight is less direct. The loss of summer heat is much greater than the milder winters experienced during periods of lower obliquity. Furthermore, the warmer winters do not reduce the snowfall enough in the higher latitudes to offset melting. Over time, ice ages dominate the Northern Hemisphere and as the obliquity increases again, interglacial periods return. Earth is currently experiencing one of these interglacial cycles.
Earth's tilt, or the obliquity of the ecliptic, changes over time. The effect is gradual and its changes are not felt on a daily or yearly basis, but on the scale of thousands of years. Although the obliquity and the precession of the equinoxes are related, their respective movements are independent of one another. The tilt is a back and forth movement in a plane perpendicular to the orbital plane, while the changing of the equinoxes is a wobble, or circular rotation, about a plane at right angles to the line of axial tilt.
Milutin Milankovitch (1879-1958), a mathematician, looked for patterns in Earth's orbital variations.
He determined the amount of light energy received for each aspect of Earth's orbital variations, including its tilt. In his 1930 book, Mathematical Climatology and the Astronomical Theory of Climate Change, Milanko-vitch claimed that ice ages occurred when variations in the Earth's orbit caused the landmass-laden Northern Hemisphere to receive less sunshine during the summer months. Such shorter and cooler summers left some winter snow for the following winter to build upon, thereby accumulating thick ice sheets over the years. The snow's white reflective surface would redirect solar radiation back into space and average surface temperatures would continue to drop over the millennia. An ice age would eventually dominate the Northern Hemisphere. Milankovitch predicted ice age cycles overlapping every 100,000 and 41,000 years, with additional mini-cycles occurring every 19,000 to 23,000 years.
The directional angle that Earth's rotational axis tilts defines, in part, these extreme cold periods and the relatively warm periods the planet experiences. The rare warm period currently being experienced has led to the rise of human civilization on Earth. The complex interaction of orbital variations, solar intensity, and the unprecedented increase in greenhouse gas concentrations in the atmosphere is yet to be fully understood. While humanity cannot control celestial mechanics, it can modify its own behavior when it comes to greenhouse gases emissions.
sEE ALsO: Glaciology; Ice Ages; Milankovitch, Milutin; Orbital Parameters, Eccentricity; Orbital Parameters, Precession.
BIBLIOGRAphY. Robert Henson, The Rough Guide to Climate Change: The Symptoms, The Science, The Solutions (Rough Guides, 2006); E.A. Mathez, ed., Earth: Inside and Out (New Press, 2001); S.F. Singer and D.T. Avery, Unstoppable Global Warming: Every 1500 Years (Littlefield Publishers, 2007).
Robert Karl Koslowsky Independent Scholar
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