Precession
Precession refers to the gradual circular motion of the rotational axis of a spinning body, such as a planet or a gyroscope, around a line that intersects the axis. This phenomenon is notably observed in the Earth's rotational axis, causing it to exhibit a slow "wobble" over thousands of years, which can alter the alignment of stars in the sky and change the position of the Sun relative to background constellations. The concept of precession was first recorded by the Greek astronomer Hipparchus around 134 BCE, who noticed a shift in star positions, although earlier Babylonian astronomers may have observed it as well.
The precession of Earth occurs due to gravitational forces acting on its slightly tilted axis, which is influenced by the planet's equatorial bulge. This wobbling motion takes approximately 25,765 years to complete a full cycle. Different celestial bodies exhibit varying periods of precession; for example, Mars takes about 178,000 years, while Neptune takes around 23 million years. As a result of precession, the north celestial pole shifts over time, changing which stars are visible as the North Star. Additionally, this slow movement affects the positions of the Sun during equinoxes, creating a discrepancy between astrological signs and their astronomical positions.
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Precession
Precession is the circular motion of the rotational axis of a spinning body around a line intersecting the axis. It is most commonly observed in the movements of a spinning top in which an imaginary line from the top's axis traces out a circular, cone-shaped pattern. Precession is caused by the effects of an outside force such as torque or gravity on an object. In large rotating bodies such as Earth and many other objects in the solar system, precession results in a slow "wobble" of the body's axis. On Earth, this wobble occurs over many thousands of years and will eventually change the alignment of the stars in the sky. It also slowly changes the position of the Sun in relation to the background constellations.
Background
Many elements of modern astronomy had their origins in the observations of Babylonian astronomers from the first millennium BCE. The Babylonians were the first to divide the sky into a 360-degree sphere, identify many of the constellations, and keep highly detailed records of the movement of the planets. Many of the accomplishments of Babylonian astronomers were passed down through successive civilizations such as the ancient Greeks. Some researchers have adhered to a theory that a fourth-century BCE Babylonian astronomer named Kidinnu may have been the first to observe the precession of Earth; however, there is no proof to the claim.
The Greek astronomer Hipparchus is most often credited with discovering precession about 134 BCE. Hipparchus had observed the appearance of a seemingly new star in the sky where none had been before. To determine if the star was a new occurrence, Hipparchus made a detailed chart, mapping the position of more than one thousand stars. He compared his chart to one made more than a century and a half earlier and discovered the stars had shifted in the sky by about 2 degrees. Hipparchus calculated that Earth's axis shifts at a rate of thirty-six seconds of angular distance per year. While his numbers were wrong—the planet's shift is closer to fifty seconds per year—he was the first to observe and record the phenomenon.
Overview
Rotating objects demonstrate precession when an outside force acts upon them. In smaller objects, such as a spinning top or a gyroscope, this force can be caused by torque, an amount of force that causes an object to rotate on an axis. An axis is an imaginary centerline around which an object rotates. Without an acting force, a top's axis would remain straight in a vertical position. When a force, such as slight push, is applied, the top's axis will begin a slight circular rotation around the original axis position. On the surface of Earth, the planet's gravity will eventually cause that precession to become greater until the top wobbles and falls over.
Similar to a top, Earth, planets, and many smaller bodies in the solar system also spin around a central axis. Earth rotates at an angle of 23.4 degrees from its axis. This slight tilt in its axis is what causes the seasonal variations, as the Northern Hemisphere leans toward the sun in summer and away in winter. Just as a top would spin upright in the absence of an outside force, Earth's axis would remain vertical if the planet was a perfect sphere. Because of its rotation, the planet is actually wider at the equator and slightly flatter at the poles. This equatorial bulge combines with gravitational influences from the Sun and Moon to act as the "push" to start Earth's precession motion.
The gravitational force that causes a top eventually to topple over on the surface of Earth does not have the same effect on large astronomical bodies. Precession of Earth, the planets, and moons is a slow, gradual wobble that can range anywhere from a few decades to millions of years. Earth's precessional rotation takes about 25,765 years to complete. Larger bodies further from the Sun tend to have longer periods of precession. On Mars, the process takes about 178,000 years, Jupiter takes about 500,000 years, and Neptune about 23 million years. Planetary moons have shorter periods, with Earth's Moon taking 78.5 years and Jupiter's moon, Callisto, about 200 years.
Over Earth's period of precession, the planet's north celestial pole will slowly move along an imaginary circle in the sky, changing the position of the background stars. At the time when ancient human civilizations were building the pyramids in Egypt or Stonehenge in Britain, a star called Thuban in the constellation Draco held the position of the North Star—the star that appears closest to the north celestial pole. In the modern era, Polaris in the constellation Ursa Minor holds that position. About 14,000 years from now, precession will align the north celestial pole with the bright star Vega in the constellation Lyra.
Precessional movement also slowly changes the background stars that the Sun, Moon, and planets appear to move through over the course of a year. The rate of shift amounts to about one day every 72 years. This process called precession of the equinoxes, changes the position of the Sun at the time of the spring and autumn equinoxes. More than 2,000 years ago, the Sun was in the constellation of Aries on the first day of spring. In the modern era, the Sun has shifted into the constellation Pisces by March 20; 600 years in the future, it will have moved into Aquarius. This also means the dates used in the pseudoscience of astrology to determine traditional horoscopes do not match astronomical reality. In astrology, a person born on April 1 is said to have been born under the sign of Aries; however, the Sun on that date is actually in Pisces.
Bibliography
Brain, Marshall. "How Gyroscopes Work." HowStuffWorks.com, 1 Apr. 2000, science.howstuffworks.com/gyroscope.htm. Accessed 25 June 2017.
Dehant, V., and P. M. Mathews. Precession, Nutation and Wobble of the Earth. Cambridge UP, 2015.
Jones, B. W. The Solar System. Pergamon P, 1984.
"Kidinnu, the Chaldaeans, and Babylonian Astronomy." Livius, 19 June 2017, www.livius.org/articles/person/kidinnu-the-chaldaeans-and-babylonian-astronomy/. Accessed 25 June 2017.
"Precession: Free and Forced." Cornell University, astro.cornell.edu/academics/courses/astro6570/Precession‗Free‗and‗Forced.pdf. Accessed 25 June 2017.
"Precession of the Equinox." Western Washington University, www.wwu.edu/depts/skywise/a101‗precession.html. Accessed 25 June 2017.
Stern, David P. "Precession." National Aeronautics and Space Administration, 17 Sept. 2004, www-istp.gsfc.nasa.gov/stargaze/Sprecess.htm. Accessed 25 June 2017.
Violatti, Cristian. "Hipparchus of Nicea." Air & Space Magazine, 2 Apr. 2013, www.ancient.eu/Hipparchus‗of‗Nicea/. Accessed 25 June 2017.