The shape given to the representations of stars in old pictures and engravings is a puzzle. Why do they represent a star by the shape of a star-fish? No star ever looks like that, or produces a picture of that shape on the retina. The thing is purely conventional. The shape which we call “star-shaped”—a term we apply to flowers and other things—is not in the least like a real star as seen by an unprejudiced person. What one really sees is an ill-defined point of light. The pretended conventional star of ancient drawings perhaps arose from the simple artifice of picturing tongue-like flames around or upon any representation of a fire or a source of light—“to show what it was meant to be.” Then the notions of perfection and symmetry in regard to the celestial bodies led to the “tongues” being arranged for the purposes of draughtsmanship as perfectly symmetrical-pointed rays of a six- or eight-limbed geometrical design—and latterly it is possible that the mystical figure known as the “pentacle” was utilised by astrologers and others as the emblem of a star. However they arose, neither the weird and astonishing representations of mediæval times nor the geometrical decorative “stars” of later date seem to have any relation to an attempt to represent a star as it really appears to the human eye and the interpreting brain behind it.
[Illustration: FIG. 45.—Copied from a portion of the Bayeux Tapestry, showing Harold’s men looking with superstitious wonder at the comet which appeared in the heavens immediately before the departure of William the Conqueror’s expedition to England in A.D. 1066. That comet was none other than Halley’s comet, and has now returned, and is visible from the earth for the twelfth time since the Conquest. (From a lecture on Halley’s comet, delivered in 1908, by Professor H. H. Turner, F.R.S., Halley’s successor in Oxford, published by the Clarendon Press.)]
The orbits of comets, says Professor Turner, of Oxford, in a delightful lecture delivered in Dublin in the summer of 1908, from which I have culled many interesting facts and presented them to my readers, “differ from those of the planets in being far more highly elliptical. Our own path round the sun is nearly a circle, so that our distance from him remains nearly the same all the year round; but the distance of a comet from the sun varies greatly from ‘perihelion,’ when it is near, and consequently bright, to ‘aphelion,’ when he is so distant and faint that we lose sight of him.” The sun is not at the centre of the ellipse described by a comet’s path, but is quite near to one end of it, so that comets approach the sun far more closely than do the planets, some taking so close a turn round the sun that the heat from it to which they are exposed is 2000 times as great as that which the earth receives. If the orbit of a comet is really elliptic, then there at last comes a time, though it may be only after thousands of years, when the comet, having rounded the sun at close quarters, and journeyed off into space, has his journey brought to a turning-point at the other end of the ellipse, and begins to draw near again, advancing towards the sun. The length of the orbit of Halley’s comet is about 3255 million miles, and the breadth at its broadest is about 800 million miles, and he takes about thirty-eight years to travel the full length (along the curve) and thirty-eight years to come back again! Other comets have other lengths and breadths of orbit, and take longer or shorter periods to go round. But the conditions of attraction affecting a comet may be such that the return journey never occurs. They may be such that the comet goes on indefinitely travelling away from our sun, until he is caught by some other star, and his orbit changes its shape, with the new sun as attracting centre. These are the “wandering comets” as distinct from the “periodic comets,” which have been shown to conform to Halley’s scheme of their movement and recurrence.