The tilted earth revolves around the Sun on an elliptical path. Over the course of a year the orientation of the axis remains fixed in space, producing changes in the distribution of solar radiation. These changes in the pattern of radiation reaching earth's surface cause the succession of the seasons. The Earth's orbital geometry, however, is not fixed over time. Indeed, long-term variations of the Earth's orbit may help explain the waxing and waning of global climate in the last several million years.
Laura Nielsen for Frontier Scientists –
In the 1930s, Serbian mathematician Milutin Milankovitch theorized that slow changes in the way the Earth moves through space about the Sun could have influenced our planet's climate past. The Earth has experienced a string of ice ages in the past, interrupted by shorter, warmer, interglacial periods. How --and how much-- have the Earths' orbital parameters (including eccentricity, obliquity, and precession) influenced global climate in the past?
Eccentricity describes the shape of Earth's orbit around the Sun. Sometimes the path the Earth travels around the Sun is a nearly perfect circle; other times, it travels an elliptical, or oval, path. A complex gravitational balance dictates that path; the location and mass of other planets and celelestial bodies in the solar system can inflict gravitational forces on our planet.
Eccentricity measures how elliptical an orbit is. A nearly circular orbit has an eccentricity approaching zero (minimum e = 0.0005). An elongated elliptical orbit has a slightly higher eccentricity (maximum e = 0.0607). Earth's orbit fluctuates between the two extreme values roughly every 100.000 years. Currently, our planet travels a fairly circular orbit (e = 0.017).
The path Earth travels around the Sun is important because it dictates how much solar radiation reaches the surface, and where. The Sun does not sit at the center of the elliptical orbit, so the Earth sometimes travels extra-close or extra-far from its energy source. When Earth's orbital path is nearest to the sun, that point is known as perihelion. Currently, we reach perihelion in January when the large landmasses of the northern hemisphere are tilted away from the sun, making for more moderate winter temperatures. In contrast Earth reaches its furthest point from the Sun in July, making for less sweltering summer temperatures in the northern hemisphere. On the flip side of the coin is the southern hemisphere: near to the sun during southern summer, far from the sun during southern winter. Luckily, the expansive oceans of the southern hemisphere help moderate the more extreme temperature trends.
Seasons and the Earth's tilt
We experience seasons because Earth sits at an angle. The tilted planet exposes more of one hemisphere to the Sun's rays. As Earth orbits to the other side of the Sun, more of the other hemisphere is exposed to solar radiance while the first hemisphere experiences less sunlight. That's why the seasons are reversed across the hemispheres... while the south bathes in summer light, the north experiences winter. If the Earth sat straight up and down and had no tilt, then there would be no seasons because light would strike the land at every latitude equally for the entire year.
Graphical examples of variations in orbital eccentricity (the shape of Earth's orbit), axial obliquity (the degree that Earth's axis tilts away from vertical), and polar precession (the slow conical 'wobble' of Earth's axis).
Earth's tilt is not always precisely the same. Every 41,000 years the tilt fluctuates between 22.1 degrees and 24.5 degrees from vertical. The tilt of the earth's axis is described by obliquity. Earth's current angle is about 23.45 degrees, that is, we're tilted 23.45 degrees away from a hypothetical vertical line, and we're very slowly becoming less oblique, shrinking the angle.
Those few degrees don't seem like much, but if the planet is less tilted then the solar intensity that reaches the far reaches of the hemispheres is lessened. Insolation is a word for the amount of solar radiation received in a given area. When the axial tilt is low, the north pole and south pole receive less solar radiation overall, which lessens the relative strength of the seasons.
The precession of the equinoxes
The earth's angle is fixed at 23.45 degrees. However, within a period of 22,000 to 26,000 years the tilted axis rotates very slowly. This precession has been described as a 'wobble' of the Earth on its rotational axis. Precession causes the timing of each solstice to advance slowly to a new point on the planet's orbital path.
Make a fist with your left hand to represent the Sun, then hold a pen point-up with your right hand. The pen is the Earth's axis. Over time, it moves in a circular motion, as though you were drawing a circle on the ceiling. Tilt the point toward the right, and orbit the pen around your fist without changing its orientation. That's the Earth, now. Our axis points at the star Polaris, and when perihelion occurs (our closest approach to the Sun), the northern hemisphere is experiencing winter, which reduces the extremity of winter. Now, turn the pen's tip through precession until it is tilted to the left. That's the Earth, about 13,000 years from now. The axis will point at the star Vega, and when perihelion occurs, the northern hemisphere will be in the middle of summer... which means a hotter, more extreme summer.
What does it mean for climate today?
Next week, we'll take a look at how well Milankovitch's theories explain past ice ages, and how much these orbital processes impact modern-day climate change...
In the meantime, here's a NASA graph of temperatures anomalies:
This map shows the difference in surface temperature in 2006 compared to the average from 1951 to 1980. Most of the globe is anomalously warm, with the greatest temperature increases in the Arctic Ocean, Antarctic Peninsula, and central Asia. NASA’s effort to track temperature changes will help societies evaluate the consequences of global climate change.
Frontier Scientists: presenting scientific discovery in the Arctic and beyond
- ‘Astronomical Theory of Climate Change' National Climate Data Center, NOAA Paleoclimatology (2009)
- 'Climate Science: Investigating Climatic and Environmental Processes: Orbital Dynamics' National Climate Data Center, NOAA Paleoclimatology (2008)
- 'How is Today’s Warming Different from the Past?' NASA Earth Observatory's feature: 'Global Warming' (2010)
- 'Milutin Milankovitch: Orbital Variations' NASA Earth Observatory (accessed 2013)