Earth motions
Earth motions refer to the dynamic movements of our planet in space, primarily its orbit around the Sun and its rotation on its axis. Earth completes a nearly circular orbit, known as a sidereal year, in about 365.26 days, while rotating on its axis approximately every 23 hours and 56 minutes. This axial rotation, combined with Earth’s tilt of about 23.5°—referred to as obliquity—plays a crucial role in the changing seasons. The orbit is not a perfect circle, exhibiting a slight eccentricity of about 0.017, which leads to variations in Earth's distance from the Sun.
Moreover, both the tilt and the orientation of Earth’s axis can change over time due to phenomena such as precession and nutation, leading to shifts in the distribution of sunlight across the planet. These changes can influence seasonal temperatures and climatic patterns, though they do not directly affect the total amount of solar energy received. The interplay between these motions and climate can also result in long-term changes, with significant implications for the planet's climate systems. Understanding these motions is vital for grasping how Earth’s climate evolves and how it may respond to various influences over time.
Subject Terms
Earth motions
Definition
Earth is not fixed in space. Rather, Earth orbits the Sun in a nearly circular orbit once every 365.26 days, a period of time called the sidereal year. As Earth orbits the Sun, it rotates once every 23 hours 56 minutes about a rotational axis that is tilted at approximately 23.5° with respect to its orbital motion. This axial tilt is called obliquity, and Earth’s daily rotation is called diurnal motion.
Earth’s orbit is nearly circular, but it is not a perfect circle. The measure of how far an orbit deviates from being circular is called its eccentricity. Earth’s is about 0.017, meaning that its orbital distance from the Sun deviates by 1.7 percent from its average distance of 149,597,871 kilometers. Interactions between the Sun, Earth, and other planets can cause the Earth’s eccentricity to change somewhat. The entire orbit shifts a little each time around, causing the point in the orbit where the Earth is closest to the Sun to shift a little each year. This motion is called precession.
Though the Earth’s orbit is slightly elliptical, it is the planet’s obliquity that is responsible for the seasons. That obliquity, however, is not constant. The tilt, or inclination, varies somewhat over time, increasing and decreasing. This change in inclination is called nutation. There are many causes for nutation, with the largest being tidal interactions between the Earth, Sun, and Moon. These interactions create an 18.6-year nutation cycle, shifting Earth’s obliquity by up to 9 arcseconds. Other causes of nutation also exist, with periods ranging from days to thousands of years. Furthermore, Earth’s rotational axis, too, gradually shifts the direction of its obliquity. This shift, also called precession, takes about twenty-six thousand years to go through a complete circle. The combination of the precession of the orbit and the precession of the poles causes the seasons to repeat every 365.24 days, a period of time known as the tropical year, instead of repeating in a sidereal year.
In 2024, two NASA-funded studies found that global climate change had altered the Earth's movement patterns. They found that the climate change-related redistribution of ice and water had shifted the earth's axis by roughly thirty feet over the last 120 years. The same mechanisms have caused Earth's days to grow slightly longer.
Significance for Climate Change
Earth’s climate is governed by the interaction between the Earth and the Sun. Sunlight warms the Earth, and Earth radiates heat into space. Anything that affects this process can have an effect on Earth’s climate. Thus, some long-term climate change can be attributed to the motion of the Earth as a planet.
Changes in Earth’s obliquity can affect seasons by changing the angle of the sunlight reaching Earth’s surface. However, these changes in obliquity change only the distribution of solar energy reaching Earth, not its total amount. As the obliquity decreases, the difference between summer and winter decreases. Conversely, as obliquity increases, the difference between summer and winter becomes more pronounced. This represents a change in climate, but it does not directly result in global warming or cooling.
At present, about two weeks before Earth reaches its aphelion, its greatest distance from the Sun, Earth’s Northern Hemisphere is tilted most toward the Sun, creating summer in the Northern Hemisphere and winter in the Southern Hemisphere. Likewise, Earth’s Southern Hemisphere is pointed most toward the Sun, creating summer in the Southern Hemisphere and winter in the Northern Hemisphere, about two weeks before Earth reaches its perihelion, its closest distance to the Sun. Because of this, the Southern Hemisphere receives more intense solar radiation during its summer and less intense solar radiation during its winter than does the Northern Hemisphere.
Normally, this difference in radiation intensity would be expected to produce more extreme seasons in the Southern Hemisphere than in the Northern Hemisphere. However, the Northern Hemisphere has a larger landmass than does the Southern Hemisphere. The large bodies of water in the Southern Hemisphere act, in part, as a heat sink to moderate the effects of the differences in solar radiation. Over time, though, the difference between the sidereal year and the tropical year causes seasons to shift the point along Earth’s orbit at which they occur. This process is called the precession of the equinoxes. Eventually, the seasons will reverse, with the Northern Hemisphere experiencing the more intense differences in summer and winter solar radiation. Without the buffering effect of the large bodies of water of the Southern Hemisphere, this shift will result in more extreme seasons. These changes, though, do not result in a change in the total solar radiation incident upon Earth, only in how that radiation is distributed.
Eccentricity changes, however, can change the total solar energy incident upon Earth, and such changes can have a dramatic impact on Earth’s climate. Furthermore, global warming or cooling can result in a decrease or increase in ice on the planet. Ice and snow reflect sunlight rather than absorbing it, amplifying the effects of orbital changes. Warming results in less ice, causing more solar energy to be absorbed by the planet’s surface and warming Earth more. Cooling results in more ice and less solar energy absorbed, cooling Earth more. All of these effects, though, are expected to occur on fairly long timescales compared with human experience. The global climate change that is currently affecting the Earth is happening on a much shorter timescale than natural climate changes.
Bibliography
Drake, Frances. Global Warming: The Science of Climate Change. London: Arnold, 2000.
Leroux, Marcel. Global Warming: Myth or Reality? The Erring Ways of Climatology. Chichester, West Sussex, England: Praxis, 2005.
Mathez, Edmond A., and James D. Webster. The Earth Machine: The Science of a Dynamic Planet. New York: Columbia University Press, 2004.
"NASA-Funded Studies Explain How Climate Is Changing Earth's Rotation." NASA, 19 July 2024, www.jpl.nasa.gov/news/nasa-funded-studies-explain-how-climate-is-changing-earths-rotation/. Accessed 21 Dec. 2024.