The Story of Eclipses

by George F. Chambers

Almost coincident with the appearance of Baily’s Beads, that is, either just before or just after, and also just before or just after the absolute totality (there seems no certain rule of time) jets of red flame are seen to dart out from behind the disc of the Moon. It is now quite recognised as a certain fact that these “Red Flames” belong to the Sun and are outbursts of hydrogen gas. Moreover, they are now commonly called “Prominences,” and with the improved methods of modern science may be seen almost at any time when the Sun is suitably approached; and they are not restricted in their appearance to the time when the Sun is totally eclipsed as was long supposed.

I may have more to say about these Red Flames later on; but am at present dealing only with the outward appearances of things. Carrington’s description has been considered very apt. One which he saw in 1851 he likened to “a mighty flame bursting through the roof of a house and blown by a strong wind.”

Certain ambiguous phrases made use of in connection with eclipses of ancient date may perhaps in reality have been allusions to the Red Flames; otherwise the first account of them given with anything like scientific precision seems to be due to a Captain Stannyan, who observed them at Berne during the eclipse of 1706. His words are that the Sun at “his getting out of his eclipse was preceded by a blood-red streak from its left limb which continued not longer than six or seven seconds of time; then part of the Sun’s disc appeared all of a sudden.”

Some subsequent observers spoke of the Red Flames as isolated jets of red light appearing here and there; whilst others seem to have thought they had seen an almost or quite continuous ring of red light around the Sun. The last-named idea is now recognised as the more accurate representation of the actual facts, the Red Flames being emanations proceeding from a sort of shell enveloping the Sun, to which shell the name of “Chromosphere” has now come to be applied.

As regards the Moon itself during the continuance of the total phase, all that need be said is that our satellite usually exhibits a disc which is simply black; but on occasions observers have called it purple or purplish. Although during totality the Moon is illuminated by a full allowance of Earth-shine (light reflected by the Earth into space), yet from all accounts this is always insufficient to reveal any traces of the irregularities of mountains and valleys, etc., which exist on the Moon.

When during totality any of the brighter planets, such as Mercury, Venus, Mars, Jupiter, or Saturn, happen to be in the vicinity of the Sun they are generally recognised; but the stars seen are usually very few, and they are only very bright ones of the 1st or 2nd magnitudes. Perhaps an explanation of the paucity of stars noticed is to be found in the fact that the minds of observers are usually too much concentrated on the Sun and Moon for any thought to be given to other things or other parts of the sky.

Perhaps this is a convenient place in which to recall the fact that there has been much controversy in the astronomical world during the last 50 years as to whether there exist any undiscovered planets revolving round the Sun within the orbit of Mercury. Whilst there is some evidence, though slight, that one or more such planets have been seen, opponents of the idea base their scepticism on the fact that with so many total eclipses as there have been since 1859 (when Lescarbault claimed to have found a planet which has been called “Vulcan”), no certain proof has been obtained of the existence of such a planet; and what better occasion for finding one (if one exists of any size) than the darkness of a total solar eclipse? At present it must be confessed that the sceptics have the best of it.

THE CORONA.

We have now to consider what I have already called the central feature of every total eclipse. It was long ago compared to the nimbus often placed by painters around the heads of the Virgin Mary and other saints of old; and as conveying a rough general idea the comparison may still stand. It has been suggested that not a bad idea of it may be obtained by looking at a Full Moon through a wire-gauze window-screen. The Corona comes into view a short time (usually to be measured by seconds) before the total extinction of the Sun’s rays, lasts during totality and endures for a brief interval of seconds (or it might be a minute) after the Sun has reappeared. It was long a matter of discussion whether the Corona belonged to the Sun or the Moon. In the early days of telescopic astronomy there was something to be said perhaps on both sides, but it is now a matter of absolute certainty that it belongs to the Sun, and that the Moon contributes nothing to the spectacle of a total eclipse of the Sun, except its own solid body, which blocks out the Sun’s light, and its shadow, which passes across the Earth.

Of the general appearance of the Corona some idea may be obtained from Fig. 1 (see Frontispiece) which so far as it goes needs little or no verbal description. Stress must however be laid on the word “general” because every Corona may be said to differ from its immediate predecessor and successor, although, as we shall see presently, there is strong reason to believe that there is a periodicity in connection with Coronas as with so many other things in the world of Astronomy. A curious point may here be mentioned as apparently well established, namely, that when long rays are noticed in the Corona they do not seem to radiate from the Sun’s centre as the short rays more or less seem to do. Though the aggregate brilliancy of the Corona varies somewhat yet it may be taken to be much about equal on the whole to the Moon at its full. The Corona is quite unlike the Moon as regards heat for its radiant heat has been found to be very well marked.