Astronomy

Press, 27 February 1981, Page 10

Astronomy

Three planets well placed

By

F. M. BATESON

The casual viewer of the bright planets will have a good opportunityin March to view Mercury, Jupiter and Saturn under favourable conditions. Viewing Mercury’ will mean early rising, but Jupiter and Saturn can be viewed throughout the night. Mercury is the most difficult of al) the planets to observe, either with the naked eye or with a telescope. This is because its orbit as viewed from Earth only makes the planet visible in a small zone on either side of the Sun. When it is west of the Sun it is visible as a morning star; when east of the Sun it is visible in the evenings. However not all these elongations are equally favourable. The tilt of the Earth’s axis and the orbit of Mercury’ enable us to get much better views of the planet at certain times of the year than we do at other times In March, especially during the second half of the month. Mercury is very favourably placed in the morning sky rising, around 4 a.m. It will thus be visible for roughly an hour before dawn. Its greatest elongation west of the Sun occurs on March 16. when it will be twenty-eight degrees from the Sun, .which is the maximum angle it can reach. The orbit of Mercury is so elliptical that it has a 24 million mile range be tween its nearest approach to the Sun and its most distant. That has the effect of changing the apparent size of the Sun, as viewed from Mercury, from ten to four times the Sun’s apparent size’ as viewed from Earth', The orbital period of Mercury is 88 days. This combined with its sidereal period of rotation of almost 59 days makes the length of its solar day, as measured from one sunrise to the next, equal to 176 earth days. Mercury has no atmosphere. These facts mean that the temperature of the planet depends entirely on the solar radiation. Its d a v t i m e temperature reaches 430 degrees Celsius. whilst during the 88day night it drops to -180 deg. C. ■ Mercury has a diameter of 4878 kilometres, which is slightly larger than that of the -Moon, but smaller than the Jovian satellite Ganvmede or Saturn’s Titan. Until the space age almost nothing was known about the surface of Mercury. In 1974 the Mariner 10 spacecraft photographed one hemisphere of the planet, showing it to be very heavily cratered. The largest crater, called the’ Caloris Basin, has a diameter of 1300 kilometres. It is generally; believed that the. density' of the craters on Mercury is due to a bombardment of

rocks and dust grains that occurred about, four billion years ago. At that time there was a vast amount of debris in- the solar system left over from the formation of the planets. At Mercury’s distance from the Sun any of this material would be travelling at speeds that probably range from 30 to 120 per second. With no atmosphere to offer resistance these infalling objects pitted the surface with craters. large and small. Not all the craters are old and some are comparatively recent. That is to be expected since debris remains in the solar system even today. Early risers will find Mercury low in the north eastern sky before dawn. In brightness and colour it will be about equal to Saturn. Binoculars are an aid in locating the planet. It is also revealed by its rapid movement against the stellar background from one night to the next. By contrast Jupiter and Saturn are easy to locate. At the beginning of March they rise around 8 p.m. but by the end of the month they are visible all night. They’ are situated in the constellation, Virgo, where they are only a few degrees apart. Jupiter will be much the brighter of the two planets. Even a small telescope will show something of the multi-coloured cloud tops that veil the giant planet, Jupiter. Unlike Mercury it has no solid surface but is a huge globe of helium and hydrogen. It is believed to have a small rocky core, surrounded by a thick mantle of metallic hydrogen, and enveloped in a massive atmosphere. Telescope views of Jupiter show dark belts tying parallel to its equator and separated from one another by’ brighter zones which are actually smeared bands of ammonia clouds. These zones are usually' festooned with spots, large ovals and loops. It is easy to understand why the continuallychanging appearance of the belts and zones occurs when it is realised that the equatorial regions are moving some 400 kilometres per hour faster than the other regions. This causes violent interaction between the different currents. Jupiter has 13 satellites, of which four are bright enough to be viewed with binoculars or the smallest telescope. These have provided endless delight to the casual viewer as on some nights they can be found paired on either side of the planet. Often one or more lie hidden behind Jupiter or its shadow. On other nights a satellite may be seen passing in front of the planet on which it casts a - dark round shadow. Should that

shadow fa!! on a bright zone it becomes very clearly' visible. The appearance of Saturn in small instruments is less interesting than Jupiter. It has a similiar banded appearance but lacks the continually changing aspect of Jupiter. Its beautiful ring system has received wide publicity from the superb photo- • graphs taken by’ the Voyager I spacecraft. At present the inclination of these rings is slowly decreasing. On March 1 they are tilted with respect to the Earth at an angle of 6.5 degrees, but by the end of the month this will have decreased to 5.4 degrees. Saturn has at least 12 satellites of which the largest. Titan, can be seen in small telescop4s. It is best seen when it is furthest west of the planet on March 7 and 23, or at its eastern elongations on March 15 and 31. A number of the other satellites can be seen in telescopes of 20cm aperture. Of these three, Tethys, Rhea, and Dione will on occasions be either eclipsed by Saturn’s shadow or occulted by’ the planet during March. It is fortunate that there is so much to interest the casual viewer during March as Saturday, March 14, will be National Astronomy 7 Day. Then and for a week all observatories and Astronomical societies will provide demonstrations and displays of interest to the public.