June 4 1965 - Spinning Worlds

It seems that every celestial object is rotating, even if the speeds of spin are not the same. The programme of June ig6j was prompted by the announcement that Mercury, the closest planet to the Sun, has not, after all, a 'day equal to its 'year', as had always been thought. This has been confirmed since; and neither is there any reason to question the very long rotation period of Venus, though it is possible that in neither case has the last word been said.

The planet Mercury is one of the less prominent members of the Sun's family. It is never a brilliant naked-eye object, since it can never be seen against a dark background; its distance from the Sun is only 36,000,000 miles, so that it may be seen only for a brief period after sunset or before sunrise low down over the horizon. This year the best opportunity will be during the firs: week in September, when Mercury will be a morning star.

Mercury's sidereal period (that is to say, the time taken to complete one revolution round the Sun) is eighty-eight Earth- days. It has long been supposed that this must also be the time taken for the planet to spin once on its axis, in which case 'the 'day' would be equal to the 'year'. The result would be that: Mercury would keep the same face to the Sun all the time, with part of the surface in permanent daylight and another pan plunged into never-ending night. Between these two extremes, there would be a so-called twilight zone over which the Sun would pass above and below the horizon.

Shown on the left, this is not our Moon, but a newly released mosaic (2011) of the planet Mercury as seen in images from Nasa's Messenger probe.

Behaviour of this sort is known as a 'captured1 rotation', and would mean that the ordinary day-and-night conditions as we know them on Earth would not occur. However, new method: of investigation have now been applied by scientists at Puerto Rico, using the radar equipment at Arecibo. Radar echoes were; obtained from Mercury, and have led to the surprising conclusion; that the rotation period is not, after all, the same as the planet's eighty-eight-day 'year'. The true period seems to be only 58 days.

Consequently, all our ideas about Mercury must be revised. There is no area of constant sunlight, no region of permanent darkness, and no twilight zone; the whole planet experiences a regular alternation of day and night. Yet Mercury is an unwel­coming sort of world, mainly because it has little or no atmosphere and the temperature range is very great. Unfortunately our maps of its surface features are rough, because the markings are diffi­cult to study even with large telescopes. Mercury is a mere 2,900 miles in diameter, and never comes much within 50,000,000 miles of us.

In size Mercury is comparable with our Moon (diameter 2,160 miles), while the second planet outward from the Sun, Venus, is almost the same size as the Earth. There have even been suggestions that Mercury is nothing more than an ex- satellite of Venus which broke free for some reason or other and moved off in an independent path, though such a theory will be extremely hard to prove.

The Moon provides an excellent example of a world with a captured rotation. It travels round the Earth once in just over twenty-seven days, and takes the same time to rotate on its axis, so that it keeps the same face to us all the time, and the surface markings appear fixed. Some of these markings may be seen with the naked eye, and it is possible to draw up rough charts without the use of any optical aid. In response to a request in the April Sky at Night programme, various naked-eye drawings of the Moon were sent in, two of which showed the pattern of dark, waterless seas, notably the well-marked Mare Crisium. In view of these and other similar drawings, it is rather curious that ancient astronomers of pre-telescopic days drew up lunar charts which were not even approximately accurate.

From Earth it is possible to study 59 per cent of the Moon's surface, because of effects known as liberations. Though the Moon rotates on its axis at a constant speed, its velocity in orbit is slightly variable, because its path round the Earth is an ellipse instead of a circle; it moves fastest when closest to us. Consequently, the amount of rotation becomes 'out of step' with the position of the Moon in its orbit, and we can see first some distance round the eastern side, then some distance round the western. The remaining 41 per cent of the surface is permanently averted, and remained unknown until the Russians obtained photographs of much of it with their rocket Lunik III in October 1959.

There is no mystery about this behaviour, and there can be no doubt that tidal friction over the ages is responsible for it. Some thousands of millions of years ago it may well be that the Moor rotated much faster than it does now, and we may assume that: it was then in a viscous or semi-solid condition. The Earth raise huge tides in it, and so slowed down the Moon's rate of spin until relative to Earth, the rotation had stopped altogether. Note, however, that the Moon does not keep the same face towards the Sun, so that day and night conditions on the averted hemisphere are the same as on the hemisphere facing us. A lunar day lasts for almost a month instead of only twenty-four hours.

With Venus, the situation is uncertain. There are no permanent surface markings to guide us, since Venus is concealed beneath its obscuring atmosphere, and before the era of space-probes i: was generally thought that the rotation period must be in the order of a month. Since the planet's 'year' is equal to aimer I 225 Earth-days, there would therefore be only seven or eight: I Venus days in every Venus year.

This idea was challenged in late 1963, when the American vehicle Mariner II passed within range of Venus and sent ban information to the effect that the rotation period was very lour indeed — perhaps even longer than 225 days. Radar work 21 Arecibo by R. B. Dyce and G. H. Pettengill, who have beer responsible also for the new studies of Mercury, gives the period of Venus as 247 days, so that we have a planet upon which tit day is longer than the year! This would mean that to an observer – on the surface, the Sun would rise in the west and set in the east; 10 days later. (In point of fact, seeing the Sun would be very difficult, owing to the cloudiness of the atmosphere. Mariner II gave the surface temperature as around 8oo°F., much too hot for any life to survive unprotected there.)

It would be idle to pretend that all astronomers are satisfied with this picture, but again we must await the results of further work, carried out probably by means of space-probes. When we come to Mars, the first planet beyond the Earth's orbit, we are on much firmer ground. There are visible surface markings, which are carried across the disk by virtue of the planet's rotation, and it is not hard to measure the rotation period, which proves to be twenty-four hours thirty-seven minutes. Mars, at least, has a day which is not very different from our own, and the seasons are also of the same type, since the axis is tilted at about twenty- five degrees as compared with the twenty-three and a half degrees of Earth. The main difference is that the seasons are much longer, since the Martian year is equal to 687 Earth-days.

Jupiter, first of the giant planets, is entirely different. It has a diameter of 88,700 miles as measured through the equator, but only 83,800 miles as measured through the poles, so that it is obviously flattened. This flattening may be seen with a small telescope when Jupiter is on view. Again there is no mystery; Jupiter is gaseous, at least in its outer layers, and the flattening is due to its rapid rotation. The Jovian day is less than ten hours long, so that particles at the planet's equator are being whirled round at some 28,000 miles an hour, and the effects of centri­fugal force cause the equatorial region to bulge out. There is a similar effect for the Earth, but with our world the difference between the polar and equatorial diameters is only about twenty- seven miles, as against 5,000 miles for Jupiter.

Different zones of Jupiter rotate at somewhat different speeds; the period for the equator is five minutes shorter than for the rest of the planet, while definite features, such as the famous Great Red Spot, have periods of their own, though all the values lie between nine hours fifty minutes and less than ten hours. It is also worth noting that the tilt of the axis is only three degrees from the vertical, so that there are no terrestrial-type seasons. In any case, seasons would be of minor importance on Jupiter, where the year is almost twelve times as long as ours, and where the temperature never rises above - 200°F.

Jupiter has four large satellites, again providing us with ex­amples of captured rotation, since all keep the same hemisphere turned towards their huge primary planet. This is also the case with the satellites of Saturn, and, indeed, with all major satellites in the Solar System. Saturn itself is of the same general type as Jupiter, with a gaseous surface and a relatively quick rotation of only ten and quarter hours for the equatorial zone; here, too the globe is markedly flattened.

Beyond Saturn lies Uranus, discovered by Herschel in 1781. At present it lies in the constellation Leo, and earlier in the year appeared close to Mars, but it is not easy to see without a tele­scope, and large instruments are needed to show any detail upon its pale, somewhat greenish disk. It is a giant world 29,300 miles in diameter, with a year eighty-four times as long as ours.

With regard to rotation, Uranus is a celestial oddity. The period is conventional enough for a giant planet (ten and three- quarter hours), but the axial tilt is not. The axis is inclined a: ninety-eight degrees, or more than a right angle, so that the rotation is technically retrograde, and the Uranian seasons are most peculiar. From Earth, we see sometimes the pole, sometimes the equator in the centre of the disk. The reason for this remark­able attitude is unknown, and Neptune, the last of the giants does not have a similar tilt; the angle is twenty-nine degrees, much the same as that of the Earth, while the Neptunian day amounts to about fourteen hours. Accurate data for Neptune are not easy to obtain, since there are no surface features defiant enough to be used as 'markers', and indirect methods have to be employed.

The boundary of the planetary system is the orbit of Pluto, a strange body discovered in 1930 and too faint to be seen in small telescopes. Even its diameter is uncertain, since it appears as little more than a star like point, though recent studies indicate that it may be as large or larger than the Earth instead of a smaller world such as Mercury or Mars.

At the Lowell Observatory in Arizona, where Pluto was first identified, M. F. Walker and R. Hardie have studied the slight variations in magnitude, and have found a period of six days nine hours. This, presumably, must be the rotation period of Pluto; the 'year' is known accurately, and is 2481 times as long as that of the Earth. The method of light-variations has also been applied to the minor planets, or asteroids, most of which keep to a well- defined zone between the orbits of Mars and Jupiter, and periods of a few hours seem to be the general rule. Vesta, the bright­est of the asteroids, has a rotation period of ten and a half, hours.

It is not only the planets and satellites which spin; in fact, rotation seems to be an invariable characteristic of all celestial bodies. The Sun's period may be measured easily enough by the positions of the sunspots, which are carried across the disk from day to day; here, the period amounts to about three and a half weeks, though different zones have different rates of movement. (Sunspots have been rare of late, since we have been passing through the minimum of the eleven-year solar cycle, but activity may now be expected to increase rapidly towards the next maxi­mum.) And spectroscopic work shows that the other stars also rotate, some of them with surprising speed. Even the Galaxy in which we live is in a state of rotation round its nucleus; the Sun takes about 225,000,000 million years to complete one journey, a period which has been graphically termed the 'cosmic year'.

The lesson to be learned from all this is that we must not attri­bute any real significance to a 365-day 'year' and a twenty-four hour 'day'. These periods apply only to our Earth, which is of no importance whatsoever in the universe as a whole. If we lived elsewhere, our time-standards would be quite different, since every celestial body has a rotation period peculiarly its own.

Note added in proof. In 1967 two more Venus probes were successfully sent to the planet, one of which (Venus IV. from Russia) landed there, while the other (Mariner V) by-passec Venus and repeated the Mariner II experiments with much greater precision. The results of these probes, together with radar measures from Earth, give a retrograde rotation period of 243 days and a surface temperature of at least 500 degrees F.; not perhaps so hot as Mariner II indicated, but still hot by Earth/- standards. The planet's atmosphere appears to be very dense indeed. Russian measures give the ground pressure as 15 to 2; times as great as that of the Earth's air at sea-level.