Neptune Fast Facts about a Planet in Orbit

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s soon as astronomers discovered Uranus in 1781 and began tracking it carefully, they saw that its orbit deviated from what was expected. This immediately brought them to the hypothesis that another even more distant planet was perturbing Uranus's orbit with its gravity. John Couch Adams was determined to find this more distant planet while still an undergraduate mathematics student at Cambridge University in England. He made careful calculations for two years based on Uranus's orbital perturbations, eventually predicting Neptune's position almost per-fectly.The perfection of his calculations was not known at the time because he had great difficulty in getting any astronomer's attention. He tried for months to reach George Airy, Britain's Royal Astronomer.The story is frequently told that Airy eluded Adams and felt disdainful of him, but in fact Adams failed to make appointments before trying to see the busy Airy, and when Airy read Adams's calculations in the end and wrote asking for additional calculations,Adams never answered. Airy in turn did not use the telescope at the Greenwich Observatory to look for the new planet, and wrote to Adams that the observatory was too busy.

Meanwhile Urbain Le Verrier, an astronomer at the Paris Observatory, was making the same calculations in France. He also failed to get the attention of the astronomical community, even after presenting his results at the French Academy of Sciences. When Airy wrote to Le Verrier asking the same questions he had of Adams, Le Verrier answered.Airy communicated with the Cambridge Observatory, where a search was begun but failed because of incomplete record making. Neptune was actually found but overlooked by the Cambridge group, much to their later distress.

Many solar system objects have simple symbols; this is the symbol for Neptune.

In 1846 Le Verrier finally got the attention of Johann Galle and Heinrich d'Arrest, astronomers at the Berlin Observatory. They, along with observatory director Johann Encke, looked in the area of sky Le Verrier predicted. Over two nights, they saw a small greenish point of light move, and knew they had found the new planet. This discovery, motivated by the work of Le Verrier, was a serious embarrassment to Airy. This embarrassment was exacerbated by the long rivalry between the countries. Airy allegedly took an extended vacation to avoid the controversy.

Soon Le Verrier and Adams were announced as codiscoverers, and they continue to share that honor today. Naming the planet caused even more trouble:There was a motion in France to name the planet Le Verrier, begun by Le Verrier himself, while others suggested Couch, Janus, and Oceanus, but in the end the planet was named after Neptune, the son of Saturn. Each planet, and some other bodies in the solar system (the Sun and certain asteroids), has been given its own symbol as a shorthand in scientific writing. The planetary symbol for Neptune, the triton, is shown below.

Though Adams and Le Verrier might be assumed to be archrivals, Adams in particular seems to have been a modest and companionable person. He even refused a knighthood that was offered him in 1847. Adams met Le Verrier in Oxford in June 1847. According to James Glaisher, president of the London Mathematical Society in the late 19th century and one of Adams's biographers, "He uttered no complaint, he laid no claim to priority, and Le Verrier had no warmer

Symbol for Neptune tlJ

Fundamental Information about Neptune

A s he did with Uranus, Galileo also saw Neptune in 1612, 234 years before its confirmed discovery. His telescope was not powerful enough to distinguish Neptune from the background stars. It is also reported that Galileo mistook Neptune for one of the Jovian moons. Neptune was discovered in 1846, and the planet takes 164.8 Earth years to orbit the Sun. Neptune has therefore made almost one entire orbit since the date of its discovery.

Like Jupiter and Saturn, Uranus and Neptune have enough similarities to be considered as a pair. The two are almost exactly the same size, as noted in the table below, though Neptune is far more dense than Uranus. The density of the two planets sets them apart from the strikingly low density of Jupiter and Saturn. Uranus and Neptune also share the distinctive patterns of their magnetic fields, along with the field strengths.

Fundamental Facts about Neptune equatorial radius at the height where 15,388 miles (24,764 km), or 3.89 times Earth's atmospheric pressure is one bar radius (Uranus is 15,882 miles or 25,559 km)

polar radius

15,126 miles (24,442 km)


0.017, meaning the planet's equator is almost 2

percent longer than its polar radius


1.50 x 1013 cubic miles (6.253 x 1013 km3), or 57.7

times Earth's volume


2.253 x 1026 pounds (1.024 x 1026 kg)

average density

110 pounds per cubic foot (1,760 kg/m3), or 3.13

times less dense than Earth (Uranus is 79.4

lb/ft3 or 1,270 kg/m3)

acceleration of gravity on the

35.8 feet per second squared (11 m/sec2), or 1.12

surface at the equator

times Earth's gravity

magnetic field strength at the surface

2 x 10-5 tesla




13 presently known

admirer." Le Verrier went on to detect discrepancies in the perihelion of Mercury's orbit that eventually helped prove Einstein's theory of relativity, though Le Verrier sought a (nonexistent) planet closer to the


rotation on its axis ("day")

16 Earth hours, seven minutes

rotation speed at equator

6,000 miles per hour (9,658 km/hour)

rotation direction

prograde (counterclockwise when viewed from above its north pole)

sidereal period ("year")

164.8 Earth years

orbital velocity (average)

3.404 miles per second (5.478 km/sec)

sunlight travel time (average)

four hours, nine minutes, and 59 seconds to reach Neptune

average distance from the Sun

2,795,084,800 miles (4,498,252,900 km), or 30.07 AU


2,771,087,000 miles (4,459,630,000 km), or 29.811 AU from the Sun


2,819,080,000 miles (4,536,870,000 km), or 30.327 AU from the Sun

orbital eccentricity


orbital inclination to the ecliptic

1.77 degrees

obliquity (inclination of equator

to orbit)

29.56 degrees

Sun than Mercury that he thought must cause Mercury's strange orbit.

Even after Neptune's proper discovery in the 19th century, scientists continued to look for another large planet because the mass calculated for Neptune could not explain all of the irregularities in its orbit, and to a lesser extent, in the orbits of Uranus and Saturn. Neptune's mass was finally recalculated using data from its Voyager 2 encounter in 1989, and this new, more precise mass now accounts for all the discrepancies in the orbits of the planets, making it unlikely that another planet of any significant size will be found past Pluto.

Uranus and Neptune differ more in their orbital characteristics than in those of the physical planet. The two rotate at about the same rate (almost 18 hours for Uranus and about 16 hours for Neptune), though

Neptune rotates with an inclination similar to the Earth's while Uranus appears to have been highly disturbed, probably by an impact, and now orbits with its rotation axis almost in the plane of its orbit. Neptune, of course, is much farther from the Sun. Uranus orbits at an average distance of 19 AU, while Neptune orbits at 30 AU. Both planets have near-circular orbits that lie very close to the ecliptic plane, entirely unlike their outer neighbor Pluto.

All orbits are ellipses, not circles. An ellipse can be thought of simply as a squashed circle, resembling an oval. Neptune's orbit is very close to circular, but it is still an ellipse. The proper definition of an ellipse is the set of all points that have the same sum of distances from two given fixed points, called foci.This definition can be demonstrated by taking two pins and pushing them into a piece of stiff cardboard, then looping a string around the pins (see figure below). The two pins are the foci of the ellipse. Pull the string away from the pins with a pencil, and draw the ellipse, keeping the string taut around the pins and the pencil all the way around. Adding the distance along the two string segments from the pencil to each of the pins will give the same answer each time:The ellipse is the set of all points that have the same sum of distances from the two foci.

The mathematical equation for an ellipse is

Making an ellipse with string and two pins: Adding the distance along the two string segments from the pencil to each of the pins will give the same sum at every point around the ellipse. This method creates an ellipse with the pins at its foci.

where x andy are the coordinates of all the points on the ellipse, and a and b are the semimajor and semiminor axes, respectively.The semimajor axis and semiminor axis would both be the radius if the shape was a circle, but two radii are needed for an ellipse. If a and b are equal, then the equation for the ellipse becomes the equation for a circle:

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