Jupiter, Uranus at opposition in May

Press, 27 April 1983, Page 30

Jupiter, Uranus at opposition in May

Jupiter reaches opposition on May 28, and Uranus is at opposition a day later. They will be in the same area of the sky. This will provide a good opportunity for those who have not seen Uranus to find this distant planet, especially on the night of May 16 - 17, when the two planets will be within a degree of each other. At opposition, Jupiter will be 649 million kilometres from Earth while Uranus will be at a distance of 2.68 billion kilometres. This vast distance and its much smaller size makes Uranus less well known than the giant, Jupiter. Jupiter has been well publicised since the Voyager space craft passed close to it with the result that its general features are well known.

Uranus, at opposition, has a magnitude of 5.8, making it just visible to the naked eye under clear, dark skies. It is easily seen with binoculars and a telescope will show its small, greenish disk. The planet was first discovered in 1781 by William Herschel when he was examining a small group of stars in the constellation Gemini. It did not appear like the other stars and subsequent observations showed that it moved against the background of stars. Herschel at first

thought that he had discovered a comet that had neither tail nor coma. All efforts to fit the observations into a cometary orbit failed, but even then it was difficult to get acceptance of the fact that the new object was a planet far out in the solar system. Uranus, along with Jupiter, Saturn, and Neptune form what are often termed the giant planets simply because they are so much larger than the other planets. They are similar in as much as the interiors are maiply hydrogen and helium whilst, their atmospheres are of the same elements with some simple compounds of hydrogen. However, Uranus differs from the other three giant planets in having its axis tipped almost parallel to the plane of the solar system. This means that we see the planet almost pole-on at certain times.

The most surprising discovery came in 1977 when observations were being made from the Kuiper Airborne Observatory of an occultation of a star by Uranus. The purpose of these observations was to try and determine a more accurate diameter of the planet. It was found that at least five rings surrounded the planet.

Another accultation of a star by Uranus occurred in 1978 and it is now known that Uranus is encircled by at least eight rings, and possibly nine, evenly spaced about the planet. They extend from 16,000 to 24,000 kilometres above the surface. Their widths vary from about 100 km for the outer ring to some 5 to 10km for the remainder. They must be very thin since the occulted star did not entirely disappear ■ behind them. This implies that they are much less dense

than Saturn’s rings and they also appear to be much darker.

Uranus requires 84 years to complete one circuit around the Sun. Its equatorial diameter is generally quoted as being 50,800 km, but there is some uncertainty. Some place the diameter smaller than this while others believe it to be larger. If the larger figure proves to be correct then Uranus, like Saturn, has an average density less than that of water. There is considerable uncertainty about the period in which Uranus rotates on its axis, because the pole-on inclination of the axis makes it very difficult to determine the correct rotation period. Some 1983 books still quote the old value of about 11 hours, whilst others give 18 to 24 hours which comes from recent studies.

Uranus is accompanied by five satellites, the largest of which has a diameter of about half that of our own Moon. The smallest is thought to be about 300 km in diameter. None of these satellites are visible in small or moderate size telescopes and they are all very faint.

At present we are looking toward the northern hemisphere of the planet. In two years time we will look almost directly to its north pole. Throughout 1983, Uranus will be close to the borders between the constellations Scorpius and Ophiuchus. Around opposition date is the best time to see this planet.

Three novae have been discovered in recent months. The first in Sagittarius was found by a Japanese amateur astronomer, Minoru Honda, in October, last year. It has now faded to below fourteenth magnitude. This fading was moderately fast. By

contrast another nova, in Serpens, also found in Japan by an amateur astronomer, in February has declined very fast and is now also below fourteenth magnitude. The third nova located in Musca was discovered by W. Liller, a professional astronomer from Harvard while observing in Chile. It was of seventh magnitude when discovered in January and has been fading very slowly. It was tenth magnitude at the end of March. The early evening skies in May will be dominated by Venus, whose magnitude is -3.7. It will set shortly before 8 p.m. throughout the month so it will be visible fairly low in the north-west after sunset. Venus will be only IVideg. north of the Moon at 5 p.m. on May 16. The two objects will make a very pretty picture at that time.

Both Jupiter and Saturn will be in good viewing positions this month and visible throughout the hours of darkness. By 10 p.m. they will be high in the east. Late on the evening of May 6, Jupiter will be six degrees, north of Antares, the bright red star in Scorpius. Mercury reaches inferior conjunction on May 13. By the end of May, it will be visible very low in the north east just before dawn.

One of the best annual meteor showers will be visible in the morning sky during the first week of May. This is the Eta Aquarid shower which has its maximum on May 5-6. The . best time to see meteors from this shower is in the early morning during the two hours before dawn. The meteors move very swiftly and have long trails. This shower has the same orbit as Halley’s comet and will be intensely observed over the next few years as that comet comes closer to the Sun.