Uranus

Uranus

Uranus was discovered by William Herschel on March 13, 1781, in the course of a systematic survey of the stars with a comparatively small telescope. Some astronomers had seen the planet before, but had mistaken it for a faint, fixed star. Herschel at first mistook it for a distant comet, but further observations showed that it was a giant planet moving outside the orbit of Saturn and taking just over 84 years for one revolution. Its average distance from the sun is 1,783 million miles, or roughly twice the corresponding distance for Saturn. Its diameter, measured along its equator, is about 29,300 miles. There is a slight polar flattening and the period of rotation, about 10 hours 49 minutes, is comparatively short.

Uranus, like Jupiter and Saturn, is encased in clouds. The cloud layer has an estimated temperature of less than -185 degrees centigrade, and is very rich in methane. At this low temperature free hydrogen would be gaseous but the ammonia must be completely frozen. The overall physical nature of the planet is thought to be similar of Jupiter and Saturn.

Uranus

The five satellites of Uranus

Uranus has five satellites. Two of them, Titania and Oberon, the brightest and most distant from the planet, were discovered by William Herschel. Two more, Ariel and Umbriel, were detected by William Lassell in 1851. Miranda, the fifth and nearest satellite to Uranus, was discovered by the American astronomer Gerard Kuiper in 1948. The estimated diameters of the satellites range from 200 to 700 and their orbits are nearly circular.

A curious feature of the Uranian system is that the planes of the orbits of the satellites, and also the planes of the planet’s equator, are tilted almost at right angles to the plane of the planet’s orbit. As a result the satellites sometimes appear to move in straight lines (when their orbits are presented edgewise) and sometimes in nearly perfect circles (when their orbits are presented in plan). Actually, the planet rotates and its satellites revolve in the retrograde sense, or opposite to the direction of the earth’s rotation and revolution. This situation, unique in the solar system, suggests that Uranus and its satellites once conformed to the general rule but have been turned over at some time in the remote past.

Saturn

Saturn

Saturn, the most distant of the five naked-eye planets, revolves about the sun in a period of nearly 29 ½ years. Its average distance from the sun is 866 million miles, or ever nine times the corresponding distance for the Earth.

Saturn is undoubtedly the most spectacular of all the planets. Its great yellowish globe, 74, 160 miles across at the equator and second in size only to Jupiter, is girdled by a magnificent system of rings. The innermost ring, known as the Crepe ring, is dusky and so transparent that the ball of Saturn can be seen through it. The second or middle ring, is broad and bright, and is separated from the narrower outer ring by a gap known as the Cassini division. The rings were once thought to be solid, but we now know that they consist of myriads of tiny satellites. Not only do they rotate about Saturn’s ball, but when on rare occasions they pass over a bright star, the star remains visible, although greatly reduced in brightness. Yet although the three rings have a width of about 41,500 miles, their thickness may be no more than a few inches. When presented edgewise to an observer on the earth the disappear from view, even in the largest telescopes.

Saturn by Paulucci

Saturn is also unusual in the sense that it is the lightest of all the planets – so light, in fact, that it coyld float in water. Its average density is only 0.71 whereas that of Jupiter is 1.33, and that of the Earth, 5.52. In other aspects it is similar to Jupiter. The greater part of its mass is thought to be in the form of liquid and solid hydrogen, and it is completely shrouded in low-temperature clouds, rich in methane. The clouds are arranged in parallel belts, but these are less prominent and detailed than those on Jupiter. Like Jupiter, Saturn sends out sudden bursts of radio energy. It also rotates rapidly in a period of no more than 10 hours 14 minutes at the equator, and this gives it a marked polar flattening.

Ten satellites

Saturn controls a family of ten satellites, all of which lie outside the rings. By far the largest is Titan, a body about 3,000 miles across and therefore larger than our moon. Eight of the others range in diameter from about 100 to 800 miles, and are larger than their counterparts in the system of Jupiter. One of the satellites, a tiny body considerably less than 100 miles across, was discovered just outside the rings as recently in December, 1966, by the French astronomer A. Dollfus.

Jupiter

Jupiter

Jupiter is the largest of all the planets. The distance across its equator is 88,760 miles or more than 11 times the corresponding distance on the Earth. Although feebly lit because of its great distance from the sun (about 483 million miles of the average), it can be surpassed in brightness among the planets only ve Venus, and sometimes by Mars at its brightest. Like Mars and the other so-called “superior planets” whose orbits lie outside the orbit of the Earth, Jupiter can be seen at all hours of the night.

Cloud Belts

In a good telescope Jupiter presents a bright yellowish disk crossed by bands or belts of various shades of gray and brown, arranged parallel to the planet’s equator. The disk is slightly flattened, the distance from pole to pole being smaller than the diameter at the equator by about 6,000 miles. After about an hour’s observation, it is apparent that the planet has turned slightly, for Jupiter rotates with remarkable rapidity. The time of one rotation at the equator is only nine hours, 50 minutes, 30 seconds, representing a speed of about 27,000 miles an hour. Away from the equator the rate of rotation is different in different latitudes.

Jupiter and its satellites

Changes in the appearance and positions of spots and other features in the belts show quite clearly that we are looking at the tops of clouds in a highly unsettled atmosphere. The latter is rich in gaseous ammonia and methane, and since these are compounds of hydrogen, it is reasonable to conclude that hydrogen gas is abundant. According to one theory, Jupiter consists largely of hydrogen, which, under great pressure in the planet’s interior, exists in the liquid and solid states. This assumes that Jupiter has little or no internal heat, and requires that its atmosphere should be comparatively shallow. The average temperature of the clouds is as low as – 145 degrees centigrade, but it does not follow that the extreme degree of cold extends to the central regions.

The Great Red Spot

One remarkable feature of the cloud belts is a large marking known as the Great Red Spot. During the century or so over which is has been regularly observed the spot has changed considerably in colour and position, sometimes appearing as a prominent red-brick oval patch, and at other times disappearing completely. It is about 30,000 miles long and 7,000 miles wide, but little is known about its structure.

Jupiter emits energy in the form of radio waves and can therefore be studied with radio telescopes. Part of this energy comes in sudden bursts whose occurrence seems to be influenced by the position of Io, one of Jupiter`s satellites. Another part is steadier in intensity but nevertheless varies as the planet rotates. The precise origin of these emissions is a subject for much speculation but they are thought to come from a strong and complex magnetic field associated with Jupiter itself. The field, like that of the Earth, deflects (and is deflected by) the solar wind, and also acts as a kind of reservoir for charged electrified particles.

Satellites

Jupiter has twelve satellites, the four largest of which were discovered by Galileo in 1610 with the newly-invented telescope. These four revolve about their parent in periods ranging from 1 ¾ to 16 ¾ days, and in doing so frequently travel in front of and behind the disk. When they pass in front they cast round, black shadows on the cloud belts. When they pass through the shadow of Jupiter they disappear and are said to be eclipsed. From variations in the predicted times of these eclipses, the Danish astronomer Olaus Roemer in 1675 discovered that light takes time to travel from Jupiter to the Earth. One of the four major satellites, Ganymede, is about the size of Mercury and slightly more than twice as massive as our moon; the other three are about equal in size to the moon. The other eight satellites range in diameter from 100 miles to about 10 miles and can therefore be seen only in large telescopes.

Planets in August

Planets in August

(1976)

Located in the constellation Taurus, the largest planet raises shortly before midnight during most of August. Its diameter increases from 35.2 to 38.5 seconds of arc, while its distance from Earth changes from 485 million miles on August 1 to 443 million miles by month’s end.

For any observer, beginning or experienced, Jupiter’s moon rank among the most fascinating phenomena of any of the sky. One reason for this is that the line-up of satellites changes from night to night, giving telescopic observers an excellent chance to actually watch astronomy in motion. In addition to the motion of four major moons, an added attraction is provided by the satellite’s shadows which frequently cross the Jovian disk. The combination of a satellite’s motion and Jupiter’s own rotation will show the shadow creeping across the cloud bands. This month Astronomy resumes its “Satellites of Jupiter” and “Satellite Shadows” charts, both of which can be found in Sky Almanac.

Jupiter will shine near magnitude-2 throughout August. Watch Jupiter and the last quarter moon on the night of August 17-18; the moon will slowly creep up on the planet, and the two will be separated by only one degree of arc at 5 a.m. EDT. Those watching the event from southern South America will see an occultation.

Space Lady. Illustration by Elena

Mercury

The difficulties inherent in seeing Mercury are well-known to potential observers, many of whom have probably failed to glimpse the little planet even when conditions seemed excellent. The situation is not helped by circumstances such as those that prevail this month, when Mercury, despite its being at greatest eastern elongation on the 26th, will remain beyond the reach of all but the most efficient planet watchers. Because the ecliptic is very low above the western horizon in August. Mercury does not stand high enough to be easily glimpsed: it can easily be blotted out by horizon haze or obscuring trees or buildings. A much better opportunity for observing Mercury will come at October’s greatest western elongation.

Venus

During August, Venus won’t get much higher above the horizon than Mercury will – only about eight degrees. Setting within an hour of the sun, Venus should be easier to see than its tiny inner neighbor, since its magnitude is – 3.3. For the rest of this year, the planet will remain an evening star, slowly brightening and gradually becoming more dominant in the western sky.

Mars

Observation of the red planet in 1976 has effectively ended, for Mars is now so faint (magnitude + 1.9) and low in the West at sunset that it is hard to spot. Its distance from Earth increases from 218 to 228 million miles; the planet moves from Leo to Virgo on August 12. Lack of a prominent guide star will make identification difficult for those who are unsure of their constellations. Mars’ visual appearance is, in fact, in direct inverse proportion to the interest evoked by the Viking mission to that planet.

Saturn

Too close to the sun to be observed for most of the month, Saturn might be glimpsed be determined observers during the last mornings of August when, located in Cancer, it rises shortly before the Sun.

Comet West and Comet d'Arrest

Comet West and Comet d’Arrest

For those unfortunates – and they must be few in number – who missed comet West in March and April 1976, the close passage by comet d’Arrest of Earth in August 1976 will provide another chance at comet observation. The comet will be making its most favorable appearance in our skies since its 1851 discovery.

This does not mean, however, that comet d’Arrest will put on anywhere near as spectacular a show as West provided; at best, d’Arrest will be about sixth magnitude, visible in richest field telescopes, small telescopes, or (if you’re lucky) binoculars. Its nearest approach to Earth will take it to within about 14 million miles (about half the closest Earth-Venus distance). The comet will reach perihelion on August 12, 1976.

Comet West. These photos of comet West, taken one day apart, demonstrate the changes in the comet’s position and in tail structure that took place in a short space of time. At left, one bright streamer extends back from the nucleus in a photo taken on March 27, 1976. The next day (night), the streamer has shifted slightly toward the right as the comet itself has moved against the background of stars. In both pictures, the comet is located in the constellation Delphinus. A home built six inch reflector at f/6 was used for each of these 20 minute exposures on GAF 200 film. Photographs by Brad Wallis.

Comet d’Arrest is a periodic comet: its orbit has been accurately calculated and it returns to the vicinity of the sun on a regular schedule – every 6.2 years. Watch for it as it moves southeast through the constellations Delphinus, Aquarius and Sculptor. By early September, it will be fading and close to the horizon for United States observers. The comet’s predicted positions are given in the accompanying table.

Predicted Positions for Comet D’Arrest. Predicted magnitudes are approximate (table from Astronomy magazine, August, 1976).

Table, Comet D'Arrest, predicted positions