Planetary rotation

The rate and direction of a planet’s rotation record major incidents from that planet’s past and determines, to a large extent, the climate that a planet experiences. Study of the rotation also yields clues to the generation of the planet’s magnetic field.

Overview

Physicists recognize two possible changes in an extended rigid object. Such an object can change position, orientation, or both. The first is called displacement and the second is termed rotation. A total description of the motion of an object requires a determination of the movement of that extended body’s center of mass and the specification of three angles relative to a coordinate system fixed with its origin at the center of mass within the body of the rigid object. Thus, full space motion of an object requires the specification of six degrees of freedom: three coordinates of movement and three angles defining orientation.

Much of what is known about planetary rotation has been learned, not surprisingly, by observation and study of the Earth. Earth rotates on its axis, but the time to spin once around its axis depends on the point of reference used. If stars are used as an example, the Earth rotates twenty-three hours, fifty-six minutes, and four seconds back to alignment with the fixed stars. This rotation is known as the sidereal period. If, on the other hand, the sun is used as the reference, the Earth rotates in twenty-four hours, from one noon to the next; this rotation is a solar day. The reason that the solar day and the sidereal period are not the same is that the Earth revolves around the sun. It takes a shorter period of time for the Earth to realign with the stars than it does for it to realign with the sun.

The Earth’s axis wobbles. If the positions of stars were observed overhead at the north pole for several months, one would notice that they change position slightly. Actually, they are not changing position; rather, the spin axis of the Earth is changing. It is not known precisely what causes this “nutation,” but winds and seismic activity may be factors.

The tides are produced by the gravitational attraction of the Earth’s oceans by the sun and the moon. This attraction also produces tides in the atmosphere and the solid the Earth, but these two tides are not normally noticed. As ocean water moves across shallow seas on the Earth, it rubs against the bottom of the basin. This action produces a friction that slows down the Earth, but angular momentum must be conserved. Angular momentum is the product of an object’s mass, angular velocity, and an appropriate measure of its size. For rotation, the appropriate measure would be the Earth’s radius. However, for a revolution that measure would be its orbital distance from the sun. The moon is locked into its present rotation and cannot speed up or slow down. Since the moon and the Earth are gravitationally bound to each other, that is where the momentum is conserved. The Earth loses angular momentum by slowing down, but the Earth-moon system gains it. For this to occur, the Earth-moon distance is increasing by roughly four centimeters per year. At one time, the moon rotated much faster, but tides in the solid moon produced by the Earth’s gravitational pull slowed the moon to its present state with the same face always pointed toward the Earth.

The Earth has been slowing down for hundreds of millions of years. Scientists have looked at growth rings in coral fossils 400 million years old; each ring was deposited in a day and varied in thickness depending on the time of year. Four hundred rings were found for one year’s growth, which corresponds to a twenty-two-hour day. Presumably, the Earth was rotating even faster before that time. Observations of other objects in the solar system indicate that a rotation of six to ten hours could be normal. Any rate less than that is caused by a process, such as the tides, slowing down the object.

The Earth’s axis of rotation is tipped roughly 23.5 degrees from the perpendicular to the Earth’s orbital plane. This tip produces the seasons. When the Earth’s northern hemisphere is pointing toward the sun, it is summer, while the southern hemisphere endures winter. As the Earth orbits the sun, six months later, the southern hemisphere points toward the sun. It will experience summer, and the northern hemisphere will undergo winter. If the Earth were not tilted, there would be no drastic changes in the weather.

The rotation of the Earth also creates the Coriolis effect. If a freely suspended pendulum is observed over several hours, it appears to shift its position relative to the ground. In fact, it is not the pendulum but the Earth that is rotating beneath the pendulum. This effect also is important in directing ocean currents. In the northern hemisphere, as the current moves north, the Coriolis effect pushes water to the right or eastward. Water moving east is pushed south, water moving south is pushed west, and water moving west is moved to the north. This produces a clockwise motion of major ocean currents in the northern hemisphere. In the southern hemisphere, the motion is counterclockwise. The effect also explains the way in which areas of atmospheric high and low pressure spiral clockwise and counterclockwise, respectively, in the northern hemisphere. Water spiraling down a drain does not do so because of the Coriolis effect. The Coriolis force is much too weak in comparison to other forces affecting the water’s motion.

Ancient Greek astronomer Hipparchus is credited with the discovery that the Earth precesses. Precession is a slow change in the direction of a planet’s axis of rotation. In general, precession is a change in the direction of the axis of rotation of an object resulting from the application of a torque. Torque is any force not acting through a body’s center of mass. For example, if a toy top is spun, its spin axis will point in one direction. As the top slows down, the spin axis moves in a circular motion. The top takes several seconds to precess through one cycle. The Earth’s period of precession is much greater, roughly twenty-six thousand years. The reason the top precesses results from gravity trying to pull it over. The top responds, however, by precessing in a direction perpendicular to the spin axis and the direction of the force. The Earth precesses because the sun and the moon are gravitationally pulling on the Earth’s equatorial bulge. This pull tends to change the direction of the Earth’s rotation, which responds by precessing.

If one imagines that the heavens are actually a sphere—the celestial sphere—with the stars and other objects on it, then one can project the Earth’s spin axis onto the celestial sphere. It passes near the pole star, Polaris, in the constellation Ursa Minor. In the past, because of the Earth’s precession, other stars such as Vega in the constellation Lyra have been the pole stars. In the future, as the Earth continues to precess, these stars will again become the Earth’s pole star.

As a result of sending spacecraft to other planets, surprising data has emerged, particularly information about their rotation. Before 1965, the rotation rate of Mercury was not known. Astronomers assumed that it was the same as its rate of revolution—in other words, that it kept the same face toward the sun. Mariner 10 determined that Mercury’s period of rotation is 58.65 Earth days. Mercury’s period of revolution is 87.97 days. This means that for every two revolutions that Mercury makes around the sun, it rotates about its axis three times. Mercury is locked into what is called a 2:3 spin-orbit coupling. The Earth’s moon is locked into a 1:1 coupling. It would take tremendous energy to make either of them rotate at a different rate.

In 1974, Mariner 10 arrived at Mercury to take photographs and to measure other characteristics. It was suggested that, although the spacecraft could not be put into orbit around Mercury, it could be placed in an orbit with a period that returned it to Mercury once every two revolutions of the planet. This calculation was made prior to the discovery of Mercury’s period of rotation. As Mariner 10 approached, it took pictures of one face. Two revolutions later, it returned and photographed the same face, and again two revolutions later, the same face was photographed. Photographs were taken of only one hemisphere of Mercury.

The rotation of Venus was another mystery, since the planet’s surface could not be observed because of its dense cloud cover. That cloud cover is also uniform in color, which prevents determination of the rotation by looking at a feature, such as a spot. Radar pulses sent to Venus and reflected back to antennae on the Earth have been used to determine the rate. When the radio pulse (electromagnetic radiation) is transmitted, it has a specific frequency. As the beam strikes the surface of the planet, it is reflected, but there is a shift in the frequency depending on the velocity of the surface at the reflection point. This shift is a result of the Doppler effect. For example, if a siren is heard approaching someone, its pitch rises; as it passes, the pitch becomes lower. In fact, the sound waves emitted by the siren have not changed, but they have been compressed relative to the person hearing them as the siren approaches. They are perceived as expanded, or stretched apart (hence, lower in pitch), as the siren recedes from the listener. A similar process affects the radar pulse. If the surface is moving toward the Earth, the frequency of the reflected beam is raised in proportion to the surface velocity. As it moves away, the frequency drops in proportion to velocity. The received pulse is analyzed for frequency shift, which is converted into a rotation rate.

Since Venus is similar to the Earth in size and mass, scientists assumed that its rotation rate would be similar to that of the Earth. They soon realized, however, that Venus rotates once every 243 Earth-days (sidereal), which is greater than its orbital period. They found that the Earth and most other planets rotate in the counterclockwise direction, as viewed from the north pole of the Earth, whereas Venus rotates in a clockwise direction. Thus, Venus’s rotation is retrograde. Astronomers have studied other objects in the solar system to develop models for the origin of Venus’s retrograde rotation. One possible explanation for this retrograde rotation is that a large object, perhaps the size of Mars, struck a glancing blow to Venus. This impact acted like a brake to slow Venus down and reverse its direction. A similar event is thought to have occurred to the Earth, but in this case, material was ejected from the Earth and some of that material was later pulled together by gravity to form the moon.

Mars rotates once every twenty-four hours and 37.5 minutes, or somewhat longer than the Earth’s rate of rotation. The tilt of the axis, or equatorial inclination to orbit, is 25.2 degrees, two degrees more than that of the Earth. Like the Earth, Mars has seasons, but they are longer, since Mars takes 1.88 Earth-years to orbit the sun. A season is an average of 5.6 Earth-months long.

Jupiter’s rotation is completed in 9.93 Earth-hours and its axis of rotation is inclined by only 3.1 degrees. It is the fastest rotating planet in the solar system, a fact revealed in its shape. If Jupiter is observed through a telescope, it is not disk-shaped; it has a definite oval shape. As Jupiter spins, it develops an equatorial bulge. The faster the spin, the larger the planet, and the less rigid it is (Jupiter is a “gas giant”), the greater the bulge will be. Jupiter’s equatorial radius is 71,492 kilometers, whereas its polar radius is 66,854 kilometers. The Earth, a rocky planet, by contrast, has a polar radius of 6,357 kilometers and an equatorial radius of 6,378 kilometers, which results in a difference of only twenty-one kilometers.

Another effect of Jupiter’s fast rotation is the banding of its atmosphere, with light-colored zones and dark-colored belts. Zones are areas of high pressure and high elevation. They are the upwelling section of a convection cell in the planet’s atmosphere. Belts are low-pressure and lower-elevation sections of the cell. Rotation also is instrumental in producing the many ovals and eddies visible in Jupiter’s atmosphere. The Great Red Spot, which is several times larger than the Earth, is the largest and oldest of these features. Jupiter’s rapid rotation is also responsible for its intense magnetic field. This rapid circulation of the planet’s liquid metallic hydrogen core generates this huge field by means of a dynamo effect.

The planet Saturn rotates in 10.67 hours, which is 0.74 hours more than Jupiter’s rotation. Since Saturn is also smaller, its equatorial bulge is less prominent than Jupiter’s, but an oval outline can still be observed when the planet is viewed through a telescope. Saturn also has banding, but it is much less noticeable than Jupiter’s. Saturn’s lower gravitational field compresses the atmosphere less than Jupiter’s does. Saturn’s magnetic field is caused by the rapid rotation of its liquid metallic hydrogen layer, which is similar to Jupiter’s.

With an inclination of the spin axis of 26.7 degrees, Saturn should have seasons. One phenomenon that may be caused by the seasonal shift was observed in late 1990. The Hubble Space Telescope photographed Saturn and revealed a large cloud of ammonia spreading across the planet’s atmosphere. Scientists speculated that Saturn “burped” ammonia from the atmosphere in response to the seasonal change of sunlight. Similar clouds have been observed since, including from the Cassini orbiter; they occur at the time of the northern hemisphere’s winter as a result of storms.

Uranus poses a major puzzle to astronomers, since it has a retrograde rotation of once in 17.24 hours but an inclination of 97.8 degrees. Scientists speculate that a large object must have collided with Uranus and knocked it on its side. At this time, material was thrown from the planet, with some of it accreting (gradually building up) into the satellites orbiting Uranus. The satellites orbit the planet in the equatorial plane, which indicates that they formed after the collision. Uranus has seasons that are each twenty-one years long, since it revolves around the sun in eighty-four years. For twenty-one years, one pole is pointed toward the sun. During the next twenty-one years the equator faces the sun, then the other pole, and finally the equator again. Although the planet rotates rapidly, it probably does not have a liquid metallic hydrogen layer. As a result, it does not have a strong magnetic field.

The rotation rate of Neptune is 16.11 hours, with an inclination of 28.3 degrees. Neptune’s satellites have strange characteristics. The largest, Triton, has a clockwise direction for its orbit which is not the norm for the solar system. It is the only known large moon with a retrograde orbit to its planet's rotation. Nereid has a very elongated orbit, which is also unusual for a satellite. Gravitational interactions between the planet and its satellites may have produced this strange situation.

The dwarf planet Pluto is very small, and its orbit is the most elliptical of the planets. It rotates in 6.387 the Earth-days retrograde and has a 122.5-degree inclination. Its orbit takes it closer to the sun than Neptune for twenty years. Neptune was the ninth planet of the solar system from 1979 until 1999, as Pluto was closer to the sun during this time. Its slow rotation and unusual inclination could be caused by a gravitational interaction with Neptune and its satellites. Pluto has another oddity: its satellite Charon. Charon orbits Pluto in the equatorial plane, and the two keep the same face toward each other. Charon may be a section of Pluto that was pulled away during a cataclysmic collision.

Applications

Orientations and rates of planetary rotation are important to the solar system. Planetary rotation provides clues to events that have occurred in the solar system. From a practical point, the rate and orientation of the Earth’s rotation are very important to life on this planet. The rate of rotation sets the rhythm of life. It provides periods of light and darkness that govern life’s biorhythms. Some creatures use the cover of darkness while foraging, while others (such as lizards) make use of the sunlight to warm themselves. It is difficult to imagine human life on a world where the day is ten hours long, or life on a planet where one face is always pointed toward the sun. If the Earth rotated more slowly, perhaps with a day of thirty-six hours, the days would be hotter for a longer period of time. The nights would be colder and longer. A greater temperature difference would, in turn, have produced greater wind speeds and weather in general would be more extreme. Life would have had to evolve to handle these harsher conditions and expend more energy on survival.

Orientation of the rotational axis is also important in determining the Earth’s seasons. If the Earth’s equatorial inclination were zero degrees, the climate would vary little. The reason there are such fierce storms in spring is the difference in the temperature of the cold ground and the warming atmosphere. As the sun warms a hemisphere experiencing a spring season, atmospheric instability and subsequent violent weather result. With zero inclination, the same amount of warming per surface area would occur each day. The area near the equator would be very warm and rainy, while the areas thirty degrees north and south of the equator would be more arid than now, with even less rainfall. Polar areas would be colder, and birds and other animals would not migrate as far north for reproduction purposes. On the other hand, if the poles were oriented at ninety degrees, the climate would be very different. For three months, the north polar region would receive sunlight for twenty-four hours a day and would be very warm. The south polar region would be in total darkness and would be very cold. As the Earth revolves around the sun, sunlight would strike the entire surface for twelve hours, and then there would be twelve hours of darkness. Winter for the northern hemisphere would follow with continual darkness and cold. Then three months of spring would follow with twelve hours of day and twelve hours of night. Such severe changes over a twelve-month period would be difficult to withstand for life as we know it.

Context

Recognizing that the Earth rotates was a major scientific advancement. By looking at the sky, with its moving stars, planets, and the sun, one gets the impression that the Earth is stationary. The ancient Greeks believed in a geocentric theory of the cosmos, where the Earth indeed stands still and all other objects in the cosmos move around it. This Ptolemaic theory, named for the Greek astronomer Ptolemy, held sway for more than twelve hundred years. It eventually became part of the accepted dogma of Catholicism.

In the mid-sixteenth century, Nicolaus Copernicus stated that the Earth and the other planets revolve around the sun in circular orbits. It took 150 years of work by various scientists for this new “heliocentric” theory of the solar system to be accepted by the scientific community. Also in the sixteenth century, Tycho Brahe compiled the most accurate positional data for the planets. This data was used in the early seventeenth century by Johannes Kepler to derive his three laws of planetary motion. Galileo Galilei made telescopic observations that verified the heliocentric theory and defended his observations in the early seventeenth century. Sir Isaac Newton provided the theoretical basis for the motion of the planets with his law of universal gravitation in the late seventeenth century. He also used it to reveal the cause of the tides. The moon and the sun pull on different parts of the Earth with different amounts of force, which places the Earth in a tug-of-war that pulls the oceans into the bulges known as the tides.

James Bradley, an English astronomer, discovered the nutation of the Earth in the mid-1700s, when he was making observations of stellar positions. He published his results after nineteen years of careful study. The Coriolis effect was explained by French physicist Gaspard-Gustave de Coriolis in 1835, when he investigated motion on a spinning surface such as the Earth.

Although much study of the rotation of the other planets was done before 1957, it was early spacecraft, such as Mariner 10, the Vikings, the Pioneers, and the Voyagers, that returned data that has enabled astronomers to refine our knowledge of the planets’ rotation. With the Magellan spacecraft sent to Venus, Ulysses and Galileo to Jupiter, Cassini to Saturn, New Horizons out to Pluto and beyond, and other planetary spacecraft of the future, scientists will continue to learn more about the physical properties of the planets.

Much of what has been learned of the planets in the Earth's solar system is being applied to studies of planets in other galaxies. These are termed "exoplanets." As of 2023, over 5,000 such exoplanets have been identified. While there are some planetary characteristics that are common, there are also very notable differences. For example, the planets of the Earth's solar system seem to be different from others with their relative distance to the sun. Most exoplanets are thought to be much closer to their parent star which results in yearly orbits of less than 50 days. Rocky planets such as Mercury, Venus, the Earth, and Mars, are not thought to be commonplace. Instead, gas giants such as Jupiter and Saturn are more the norm.

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