Transient Phenomena

By transient phenomena, we mean impermanent effects or variable events in the sky. They range from atmospheric effects like the "green flash" to "guest stars," the ancient Chinese expression for novae or supernovae. Many ancient astronomers did not distinguish between astronomical and meteorological phenomena. For the Greeks, the distinction between the "lunar sphere," and the atmosphere ("sublunar") did not prevent Aristotle from assigning comets to the latter realm. They were, after all, transient objects, and in his view, clearly out of place in what he considered to be a perfect, changeless realm. This error, as well as the nearly general acceptance of the geocentric universe, can be laid primarily if not exclusively to a lack of parallax measurements of adequate precision. Although the measurement of parallax of even the nearest stars is beyond the capability of naked eye astronomy,1 the parallax motions of comets were not, since Tycho Brahe (1546-1601) carried out a successful determination of the parallax of the comet of 1577 without the benefit of telescopes or photographic images. Much criticism has been leveled at ancient Greek astronomers for placing too little reliance on observation, and too much on preconceived notions, such as the perfection of the sphere and of spherical motion and the heavens as illustrating that perfection. Much of this is justified; yet, there were important exceptions, such as the careful observational work of Timocharis (~3rd century b.c.) and his school in Alexandria, of Eratosthenes who determined observationally the size of the Earth, and of Hipparchos, whose star catalog and theories of the Sun and Moon were founded on observation (see ยง7.2). In fact, in Hellenistic times, early eclipse and occultation observations were highly valued and aided the discov

1 The parallax of even the closest stars beyond the Sun, the triple star system a Centauri, is less than 1 arc second. The first successful stellar parallax determination was made in the 1830s. F.W. Bessel (1784-1846) measured the parallax of 61 Cygni;T. Henderson (1798-1844) measured that of a Centauri;and F.G.W. Struve (1793-1864) measured that of Vega.

ery of precession. These data are still of interest today, as are ancient observations of the Sun's activity and even its size. Eclipses were studied and predicted to some degree in several areas of the world. In general, variations of heavenly objects were carefully observed and recorded, especially when they were spectacular. Such events held then, as they hold now, intimations of the nature of the universe and of our place in it.

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