From last september through to May of this year, the dramatic proximity of Castor and Pollux with Mars forms a literally moving memorial to two of the major advances in astronomy—namely, the heliocentric solar system and the Universal Law of Gravitation.
These in turn were to be the observational and theoretical foundations upon which the Cosmological Principle was erected: our belief that the laws of physics and chemistry as discovered here on Earth apply equally throughout space and time.
Using this assumption, we have been able to make some sort of sense of the observable universe, its structure and its history. Great areas of ignorance remain, many observations need refining, disagreements are sometimes profound and even passionate, but, nevertheless, the general picture is secure and will continue to remain recognisable, even if there are quite major revisions of it in the future. Certainly, it will not be shattered and coached Heracles in the warlike arts. After joining in the famous pig hunt in which Artemis was the heroine, they sailed with Jason in search of the Golden Fleece.
Mortal Castor was killed in a quarrel over booty after cattle rustling in Arcadia with his cousins, whose wives the Dioscuri, as the twins were known, had earlier abducted. The semi-divine Pollux prayed to his father that he might not outlive his twin. Zeus granted the prayer and set them on the Ecliptic as a constellation which is now known as Gemini. being the Latin for twins.
Since the end of October 1992, we have been able to watch one of the mysteries which exercised astronomers throughout the centuries—until
Copernicus solved the puzzle by proposing a heliocentric. or sun-centred, model of the solar system. The puzzle was this: Seen from the Earth, the outer planets generally move eastwards against the background of the constellations along the zodiac. However, periodically, there is a time when this eastward drift slows, and for a few days the planet seems to stop, and then to move westwards for a time, before halting again and renewing its eastward progress.
It was to account for this irregularity that Ptolemy had to devise his complex system of cycles and epicycles, with deferent and efferent. This system, based on the work of his Greek predecessors, Aristotle, Pythagoras and Hipparchus, amongst others, was published about 100 A.D. in his Magale Syntaxis tes Astronomias. which later the Arabs were to call Al Magest, The Greatest, and which became the foundation of astronomy for the next 1400 years.
However, amend it how they would, astronomers could not make Ptolemy’s machinery yield accurate predictions for more than a few years ahead, for the Earth is not the centre of the solar system, and nor do the celestial bodies move with uniform circular motion (the two assumptions fundamental to the classical model).
At the end of November 1992, Mars, looking ever brighter and redder, stopped its eastward motion along the zodiac and appeared to stand still against the background stars forming a straight line with Castor and Pollux, the bright stars marking the heads of the Dioscuri.
Throughout December it drifted westwards, until on January 8, the date of opposition, it made a striking isosceles triangle with the twins. Since then, it has drawn increasingly to the west, stretching the triangle, so that by February 28, when it appeared to stand still again, it formed a distinctive elongated right-angled triangle.
Thereafter, through March and April Mars will sweep eastwards through Gemini, and then the pattern with Castor and Pollux will be lost as it moves off into Cancer and thence into Leo.
Being deeper in the solar gravitational well than Mars is, we orbit faster around the Sun, at just under one degree of arc per day. Mars, 50 per cent further away, orbits at little more than half a degree per day. Thus Earth is forever catching up and overtaking Mars, as it does the other outer planets.
We generally see Mars drifting eastwards amongst the stars, but as we approach opposition, the position when the Sun, Earth and Mars lie in line, this eastward drift first halts and then apparently becomes a westward motion, due to our rapid movement relative to Mars as we overtake it at our closest approach. As we draw ahead and curve away, the effect gradually decreases, and Mars appears to drift east along the zodiac again.
Although Copernicus had placed the Sun at the centre of the solar system, he was still wedded to the idea of circular motions, and so predictions of planetary positions remained unsatisfactory. Not until 1609, after years of unremitting toil, analysing Tycho Brahe’s observations of the orbit of Mars, was Kepler able to publish his Astronomia Nova, containing his first two laws of planetary motion.
He demonstrated that planets move in elliptical orbits, and that the radius vector from the primary focus, the Sun, sweeps out equal areas in equal times. These two laws, in conjunction with his third law, which states that for any two planets the squares of their times of revolution are proportional to the cubes of their mean distances from the Sun, gave us an accurate model of the system.
In 1665, Isaac Newton, who had retired to the country to escape the Great Plague, developed his theory of gravitation, to be published as the Universal Law of Gravitation in his Philosophiae Naturalis Principia Mathematica, 1687, which, together with his ideas of mass and inertia, showed not only the what but the why of solar system motions.
Within the observational accuracy obtainable with the instruments of the time, Newtonian dynamics completely described the system, and only one major question remained: was the Law of Gravitation really universal? Did it hold true in the depths of space? Were the motions of stars describable by the same laws as those which enabled us to analyse the motion of masses here on Earth? Could it be that the architecture of the universe was consistent, that our understanding could reach out into the depths of space and make comprehensible those myriad points of light which we call the stars?
Castor, alpha Geminorum, which is the more northerly of the twins, and not quite as bright as Pollux, beta Geminorum, was first seen to be a double star by Cassini in 1678. But, apart from observing the fact that what to the naked eye or a small telescope appeared as a single star was in fact two stars very close together, he took the matter no further.
However, beginning in 1718, James Bradley, the third Astronomer Royal, commenced a series of measurements of this pair, recording their angular separation and position angle for the next 40 years. By 1803, sufficient data had been accumulated for Sir William Herschel to announce that the two stars were gravitationally bound to each other, and that their motions conformed to Newton’s laws. Here was the first secure evidence that the laws of physics are not limited to our solar system, but apply even in the depths of space, and hence form a key to the riddle of the observable universe.
Today, we have established that Castor A and B orbit about one another every 420 years at a mean distance rather more than the diameter of the solar system—about 13.4 billion kilometres.
While these partners waltz together through space, they are accompanied by a faint red dwarf, Castor C, YY Geminorum, which orbits their common centre of gravity at a distance of about 160 billion kilometres, and is estimated to take perhaps 10,000 years for each revolution.
But these three stars are not the only occupants of this part of the celestial dance floor, for this century it was discovered that both Castor A and B are themselves double stars, each pair being so close that they cannot be separated visually, but only by the spectroscopic analysis of their light. The pair forming Castor A are of equal size, about twice the diameter of the Sun, and rotate about one another in just over 9 days at a mean distance of only 6,400,000 kilometres, a ninth the distance of Mercury from the Sun.
The components of Castor B are likewise equal in size, but slightly
smaller, and hurtle about each other in less than three days at just under 5,000,000 kilometres apart.
Astonishingly, staid little Castor C has also turned out to be a spectroscopic binary. It is a pair of red dwarfs a mere 2.7 million kilometres apart and having an orbital speed of more than
400,000 kph and a period of just 19.5 hours. Because the plane of the orbit is in our line of sight, even quite a modest telescope enables one to detect the changes in apparent brightness as the components eclipse one another.
For many years, Pollux was thought to be a multiple star as well, for it has a number of fainter stars close to it, and the initial measurements of their motions suggested that they were gravitationally associated with alpha Geminorum. However, later and more accurate observations and analysis of the various motions, including that of alpha itself, suggest that this is not so, and that time will show them all to be going their separate ways.
While astrologers may read calamity and character into this present conjunction of Mars with the Dioscuri, for astronomers it is a vivid reminder of those first secure steps towards the boundaries of the observable universe.