The Astronomy of Milton's 'Paradise Lost'

by Thomas Nathaniel Orchard

The revolution of two suns in one sphere presents to our observation a scheme of creative design entirely different to the single-star system with which we are familiar—one of a higher and more complex order in the ascending scale of celestial architecture. For, if we assume that around each revolving sun there circles a retinue of planetary worlds, it is obvious that a much more complicated arrangement must exist among the orbs which enter into the formation of such a system than is found among those which gravitate round our Sun.

The common centre of gravity of a binary system is situated on a line between both stars, and distant from each in inverse proportion to their respective masses. When the stars are of equal mass their orbits are of equal dimensions, but when the mass of one star exceeds that of the other, the orbit of the larger star is proportionately diminished as compared with the circumference traversed by the smaller star. When their orbits are circular—a rare occurrence—both stars pursue each other in the same path, and invariably occupy it at diametrically opposite points; nor is it possible for one star to approach the other by the minutest interval of space in any duration of time, so long as the synchronous harmony of their revolution remains undisturbed.

When a pair of suns move in an ellipse, their orbits intersect and are of equal dimensions when the stars are of equal mass, their common centre of gravity being then at a point equidistant from each. Consequently, neither star can approach or recede from this point without the other affecting a similar motion, they must be at periastron and apastron together, and any acceleration or retardation of speed must occur simultaneously with each. Stars of unequal magnitude always maintain a proportionate distance from their common focus, and both simultaneously occupy corresponding parts of their orbits.

The nature of the motions of those distant suns, and the form of the orbits which they traverse, have been investigated by several eminent astronomers, and although the subject is one of much difficulty, on account of their extreme remoteness and the minute angles which have to be dealt with, necessitating the carrying out of very refined observations, yet a considerable amount of information has been obtained with regard to the paths which they pursue in the accomplishment of their revolutions round each other.

The orbits of about sixty stellar pairs have been computed, but only with partial success. Some stars have shown themselves to be totally regardless of theory and computation, and have shot ahead far beyond the limits ascribed to them, whilst others, by the slowness of their motions, have upset the calculations of astronomers as much in the opposite direction. So that out of this number the orbits of not more than half a dozen are satisfactorily known.

The dimensions of stellar orbits are of very varied extent. Some pairs are apparently so close that the best optical means which we possess are incapable of dividing them, whilst others revolve in wide and spacious orbits.

The most marked peculiarity of the orbits of binary stars is their high eccentricity; they are usually much more eccentric than are those of the planets, and in some instances approach in form that of a comet.

The finest binary star in the northern heavens is Castor, the brighter of the two leading stars in the constellation Gemini. The components are of the second and third magnitudes, and over five seconds apart. They are of a brilliant white colour, and form a beautiful object in the telescope.