The largest "proper motion" now known is that of a star of the 8½ magnitude in the southern hemisphere, known as Cordoba Zone V. No. 243. Its proper motion is 8·07 seconds of arc per annum, thus exceeding that of the famous "runaway star," 1830 Groombridge, which has a proper motion of 7·05 seconds per annum. This greater motion is, however, only apparent. Measures of parallax show that the southern "runaway" is much nearer to us than its northern rival, its parallax being 0"·32, while that of Groombridge 1830 is only 0"·14. With these data the actual velocity across the line of sight can be easily computed. That of the southern star comes out 80 miles a second, while that of Groombridge 1830 is 148 miles a second. The actual velocity of Arcturus is probably still greater. The poet Barton has well said-- "The stars! the stars! go forth at night, Lift up thine eyes on high, And view the countless orbs of light, Which gem the midnight sky. Go forth in silence and alone, This glorious sight to scan, And bid the humbled spirit own The littleness of man." CHAPTER XV Double and Binary Stars Prof. R. G. Aitken, the eminent American observer of double stars, finds that of all the stars down to the 9th magnitude--about the faintest visible in a powerful binocular field-glass--1 in 18, or 1 in 20, on the average, are double, with the component stars less than 5 seconds of arc apart. This proportion of double stars is not, however, the same for all parts of the sky; while in some regions double stars are very scarce, in other places the proportion rises to 1 in 8. For the well-known binary star Castor (α Geminorum), several orbits have been computed with periods ranging from 232 years (Mädler) to 1001 years (Doberck). But Burnham finds that "the orbit is absolutely indeterminate at this time, and likely to remain so for another century or longer."[314] Both components are spectroscopic binaries, and the system is a most interesting one. The well-known companion of Sirius became invisible in all telescopes in the year 1890, owing to its near approach to its brilliant primary. It remained invisible until August 20, 1896, when it was again seen by Dr. See at the Lowell Observatory.[315] Since then its distance has been increasing, and it has been regularly measured. The maximum distance will be attained about the year 1922. The star β Cephei has recently been discovered to be a spectroscopic binary with the wonderfully short period of 4{h} 34{m} 11{s}. The orbital velocity is about 10½ miles a second, and as this velocity is not very great, the distance between the components must be very small, and possibly the two component bodies are revolving in actual contact. The spectrum is of the "Orion type."[316]