THE EARTH AS A SPACE STATION

THE EARTH IS A POSSESSIVE MOTHER and we are held down tight to her bosom by the force of gravity. Men may have dreamed of space travel since the days of Lucian of Samosata in ancient Greece, if not earlier still, but few have seriously resented their terrestrial prisonment,or indeed recognized it as such; and yet astronomically and philosophically it represents a very serious limitation.

In our ' Age of science ' it is not easy to visualize the mental climate in which Galileo could be sent to prison and Giordano Bruno burned alive for believing that the Earth revolved about the sun. How cruel,how bigoted,how parochial mankind used to be only four long human lives back! It is easy and gratifying to scoff and marvel at the errors of the past by hindsight; but neither so easy nor perhaps so gratifying to probe the faults of the present by foresight.

A few days ago I was reading a serious scientific work, whose author stated flatly, without a twinge of conscience, that " the human brain is the most developed organic structure in the Universe". one should have thought that by now 'every schoolboy' knew the Earth was only an insignificant little planet circling an insignificant yellow dwarf star, indifferently placed in a galaxy of average size and age, which is but one of the billions of similar star systems within the observable universe ( the galaxies beyond the point at which the cosmic Red Shift equals the velocity of light must remain for ever unobservable).

Space flight, however, has suddenly become a reality and interplanetary travel cannot be far behind. At this juncture it will come to us not unnaturally to think of our Mother Earth as a 'spatial observatory', or an 'orbital space station', moving once round the sun every 365*1/4 days at a men distance of some 93 million miles .

From this position until recently we had no escape, and its implications are deeply impressed on our ideas and the development of planetary astronomy in particular.

Two solar parents,Mercury and venus, circle the sun within the earth's orbit and are referred to as interior or inferior planets.In the telescope they show phases, like the Moon, and in the sky they stay close to the sun, from which mercury can never get any farther away than by an angle (elongation) of 28 degree. Venus at 67 million miles from the sun is nearer to the earth, and its maximum elongation is 47 degree . Even venus, however, remains uncomfortably close to the Sun for telescopic study.In fact, the interior planets are best observed in daytime, when they study stand higher above the horizon and the air is much less agitated than in the twilight zone before sunrise or after sunset where they can be seen against a dark sky.

Mercury and Venus are brilliantly lit by the close sun, and their daytime observation presents no particular difficulty, but it does not wholly solve the problem. An interior planet can be at full phase only at superior conjunction, i.e. when it is directly opposite the earth on the other side of the sun.At such a time the planet will obviously be close to its greatest possible distance from the Earth, and with the sun in between it will be very difficult to observe, even if it can be seen at all. On the other hand, when closest to the Earth at inferior conjunction, the planet is between the sun and the observer, to whom it presents its dark side, so that it is more or less thoroughly invisible, bar the rare treat of a transit when it actually crosses the solar disk.The next transit of Venus is on 8th June,2004,followed by another 8 years later, Mercury is more obliging in this respect and transits the Sun once every few years.

Transits are of some interest for measuring the planet's diameter and studying its atmosphere, but they are a poor compensation for not being able to see the planet when it comes closest to the earth. as can be done in the case of the exterior or superior planets having orbits wider than that of the Earth. Mars and all the remaining major planets belong to this group.

Although the term 'superior' is meant to refer only to their geometrical position relatively to the earth,their position is superior also from the point of view of observation. Indeed, a superior planet may be found in the sky at any angular distance (elongation) from the sun up to 180 degree, when it is exactly opposite it, or in opposition, and attains its highest point above the horizon(culminates) at midnight local time .Not much spatial imagination is needed to see why at opposition the Earth must be exactly between the Sun and the planet, which will be at full phase and also near its closest approach to the earth. Owing to the elliptical shapes of the planetary orbits the moments of opposition and of closest approach will not generally coincide, but they are never far apart.

A superior (exterior) planet will attain its greatest distance from the Earth somewhere near superior conjunction, and, of course, it can never be in an inferior conjunction.

In the case of Jupiter and the planets (extra-Jovian) beyond it the variation in the distance from the Earth does not matter very much, because these bodies are, apart from pluto and the satellites, very large and so remote that their apparent diameter is not altered significantly by the addition or subtraction of a mere 186 million miles of distance. Not so with Mars, which is only 4,219 miles (according to the latest measurements ) 99 across and many come as close to us as 35 million miles at a favorable opposition or withdraw to some 250 million miles at an unfavorable superior conjunction.Here useful observation is limited to a few weeks centered on oppositions, which occur at intervals of about 2 years.

As stated, our 'spatial observatory' is located much more favorably for observing the exterior than the interior planets, and for this reason our knowledge of the surfaces of the former is a good deal more detailed.

But it is not all again. The exterior planets do not pass through all the phases from new to full, like the Moon and the interior planets; they keep showing an almost dull disk all the time. The arc of shadow does not encroach upon the face of Mars by more than 47 degree, and on that of Jupiter by more than 11.7 degree, which does not show at all, as the shadow is entirely swallowed by the twilight in the giant planet's extensive atmosphere. This phase angle grows progressively less with increasing remoteness .

Yet the variation in phase is important for the study of a planet's surface. To take the Moon as a familiar example, when full its face is shadow-less and its mountains do not show to advantage, but at smaller phases even low gentle hills cast long shadows near the terminator, as the line dividing the night from the day hemisphere is called, and so can be readily observed and measured. The surface of Mars may be comparatively smooth and undifferentiated, but there are clear indications of differences of level amounting to several thousand feet, which can never be seen to cast sufficient shadows to be accurately determined. The same applies to the cloud banks of Jupiter, whose relative altitudes remain a matter of conjecture. 

This is only one of the many problems of planetary astronomy that must remain insoluble so long as we are chained down by gravity to our terrestrial 'space station' .

It's self-evident limitation is that it stays put in its orbit, which may be all to the good so far as climate is concerned, or else we might be cooked alive or frozen to death, but it presents a serious obstacle to many lines of astronomical inquiry. Even the diameters of the remoter planet's,which are feebly lit by the distant Sun (this is no more than an extra-bright star as seen from Neptune or 

Pluto ), are difficult to measure, and when it comes to their satellites and the outermost known planet Pluto the figures obtained by various methods and by different investigators are apt to disagree much more than astronomers like . If a planet is too bright, like Venus, We are again in trouble owing to irradiation, which makes the optical image of a bright object encroach upon its background. A white disk looks larger than a black one of the same size.