Uranus and Neptune Revisited

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Uranus and Neptune Revisited Sethanne Howard USNO, retired

Abstract Uranus and Neptune are two planets not known in ancient times. Once discovered, however, astronomers were eager to obtain their vital statistics. Of course once Voyager flew by them everyone had the information, but before there was Voyager, there were many attempts to measure things like the rotational period, the mass, the brightness, etc. This is the story of the two people who obtained the rotational period of both planets before Voyager got there. They confirmed that the spin of Uranus is retrograde and that of Neptune direct. Uranus rotates on its side. Their estimates for the periods of rotation are, for Uranus, 24 ±3 hr., and for Neptune, 15 ±3 hr.

Introduction FOR MOST OF HUMAN HISTORY humanity knew of only five planets (plus our own): Mercury, Venus, Mars, Jupiter, and Saturn. These are the planets that are visible to the naked eye at various times during the year. We learned relatively recently that there are two other planets in our Solar System: Uranus and Neptune. Note the proper pronunciation of Uranus (accent on the first syllable). First a little history on Uranus and Neptune. The question being what did we know before the Voyager flyby of Uranus. Uranus had been observed on many occasions before its recognition as a planet, but it was generally mistaken for a star. Then Sir William Herschel observed the planet qua planet on March 13, 1781. This was the first planet added to the Solar System since the dawn of history. See Figure 1 for a drawing of the telescope he used. He decided to name the new planet Georgium Sidus (George’s Star), in honor of his new patron, King George III of England. As one might expect, this was not popular outside England. Bode (a German astronomer) opted for Uranus, the Latinized version of the Greek god of the sky, Ouranos (the only planet with a name of Greek origin). Bode argued that just as Saturn was the father of Jupiter, the new planet should be named after the father of Saturn. Ultimately, Bode’s suggestion became the most widely used, and became universal in 1850 when Her Majesty’s Nautical Almanac Office, the final holdout, switched from using Georgium Sidus to Uranus. Summer 2015

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Figure 1 – Discovery telescope for Uranus Uranus is the seventh planet from the Sun. It has the third-largest radius and fourth-largest mass in the Solar System. It revolves around the Sun once every 84 Earth years. Uranus has a ring system and numerous moons. The Uranian system has a unique configuration among the planets because its axis of rotation is tilted sideways, nearly into the plane of its revolution about the Sun. Its north and south poles therefore lie where most other planets have their equators. Uranus’s orbital elements (the shape of its orbit) were first calculated in 1783 by Pierre-Simon Laplace.i Over time, discrepancies began to appear between the predicted and observed orbits, and in 1841, John C. Adamsii first proposed that the differences might be due to the gravitational tug of an unseen planet.iii In 1845, Urbain Le Verrieriv began his own independent research into Uranus’s orbit. On September 23, 1846, Johann G. Gallev was the first to see the new planet close to the position predicted by Le Verrier.vi There was considerable controversy over the name for the new planet. At first, Neptune was simply called “the planet exterior to Uranus” or “Le Verrier’s planet.” However, eventually the name Neptune, Roman god of the sea, was accepted. Neptune is the eighth and farthest planet from the Sun in the Solar System. It is the fourth-largest planet by diameter and the third-largest by

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mass. It revolves around the Sun once every 164.8 Earth years. Like Uranus, Neptune has a ring system and several moons. Uranus – the Details Uranus was known to be a bit strange. It was already suspected that its axis of rotation was off kilter. Most of the Solar System planets have rotation axes that are close to perpendicular to the plane of the orbit. The Earth, for example, has an axis tilted only 23.5° off perpendicular. Uranus, on the other hand, was thought to have a rotation axis almost in its orbital plane. No one was quite sure of this, though. Its major moons are Ariel, Umbriel, Titania, Oberon, and Miranda – names taken from Shakespeare. There are about 27 moons known. The internal structure of Uranus is shown in Figure 2. The wildly off-center magnetic field is shown in Figure 3.

Figure 2 – Structure of Uranus The planet is thought to have a very small central almost rocky core, surrounded by a plasma ocean, surrounded in turn by an atmosphere with lots of hydrogen, helium, and methane (CH4). It is the methane that Summer 2015

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makes Uranus appear cyan in color. Note in Figure 3 that the Magnetic North Pole points “downward” – below the orbital plane. The magnetic field is not centered on or near the center of the planet. This is quite unusual. This unusual geometry results in a highly asymmetric magnetosphere. By comparison, the magnetic field of Earth is roughly the same at either pole, and its “magnetic equator” is roughly parallel with its geographical equator.

Figure 3 – magnetic field for Uranus (left) and Neptune (right) Neptune – the Details Figure 4 shows the structure of Neptune. Although smaller than Uranus as seen from the Earth, when seen with a large telescope it is visible as a disk. Voyager found the axial tilt of Neptune to be 28.32° − similar to the Earth’s tilt. Triton is its major moon – very large as moons go. Unlike other large planetary moons in the Solar System, Triton has a retrograde orbit, indicating that it was captured rather than formed in place. There are about 14 known moons for Neptune. The magnetic field of Neptune (Figure 3) is also a bit off center although not as much as Uranus. Never visible to the naked eye, Neptune requires a 4 meter class telescope to capture its spectra, and a 50 inch telescope to work in the near infrared part of the spectrum. To me it is a beautiful planet because its color is a deep, rich blue. The atmosphere is mainly hydrogen and helium with trace amounts of CH4 that contribute to that beautiful color.

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Planetary Rotation All planets rotate. The Earth rotates about its axis once a day. This is how we define a ‘day’. Planets revolve around the Sun and rotate about their individual axes. It is fairly straightforward to obtain the rotation periods (i.e., the length of the planetary day) of the five naked eye planets – one simply watches them. We can’t watch the more distant Uranus and Neptune. It takes a large telescope to determine their rotational periods.

Figure 4 – The internal structure of Neptune: 1. Upper atmosphere, top clouds 2. Atmosphere consisting of hydrogen, helium, and CH4 gas 3. Mantle consisting of water, ammonia, and CH4 ices 4. Core consisting of rock (silicates and nickel-iron)

By using various techniques people tried to determine the rotation periods for Uranus and Neptune. A visual technique means watching the planet as it spins. This is rather like watching the Great Red Spot on Jupiter as Jupiter rotates: once around is a ‘day’ on Jupiter. Theory means that the rotation period is derived from planetary theory (using the mass and shape of the planet to derive its period), not by using a telescope. Photometry means that a telescope is used with a filter in selected wavelength bands (e.g., a color like infrared) to measure changes in the light from the planet. A regular and repeatable change in the light can Summer 2015

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represent the length of the planetary day. The spectra technique means that the planet’s spectral lines are used to determine the period. This last is the most difficult to do because it means measuring the minute tilt of the spectral lines, and from that tilt, the rotational period. Some of the early attempts are listed in Table I and Table II. Table I – early values for Uranus Date 1872 1900 1902 1912 1916 1930

Period h

12 7h < P