THE ANNULAR ECLIPSE

Taranaki Daily News, 18 February 1933, Page 1 (Supplement)

THE ANNULAR ECLIPSE

WHY NOT TOTAL?

(By

Rev. B. Dudley, F.R.A.S.)

The last solar eclipse was total in phase, and took place on August 31, 1932, being visible only in the northern hemisphere (Canada and U.S.A.). On March 7 there was an annular eclipse, visible in the Antarctic and (as a partial eclipse) in Australia. This year there are also two obscurations of the Sun, both annular: that of February 24 being visible in the. south of South America and central Africa, the second taking place on August 21, visible in Persia, India, and northern Australia. Annular eclipses are caused when the moon, passing ventrally across the suns disc, is at or near her remotest distance from the earth. It so happens that the apparent size of the sun and the apparent Size of the moon, as seen from the earth, are approximately the same. Hence, under ordinary conditions, when the moon passes over the solar face she just covers it. giving us the phenomenon of a total eclipse, the sun being completely hidden. When the moon is between us and the sun, being at the same time at her minimum distance from the earth, her apparent size is as great as possible. Such eclipses of the sun have the longest duration because it takes longer for the moon to effect the passage.

At the opposite extreme —when the moon is remotest from the earth—her apparent size is reduced, while that of the sun remains unchanged. The result is that the moon does not then quite cover the sun’s disc. At the moment, of centrality (that is to say, when the line of vision of both sun. and moon would, if projected, pass through the centre of both .bodies) a ring of light is seen round the moon. This is really the margin of the sun which extends beyond the moon’s disc. On such occasions the shadow of the moon cast by the sun in the direction of the earth may fail to reach the earth by more than 23,000 miles. An annular eclipse of the moon is not possible because the shadow of the earth (cast by the sun in the direction of the moon) is always larger than the moon.

An eclipse of the sun is strictly speaking an occultation by the moon, since it is caused, as we have seen, by the passing’of the moon between the earth and the sun, just as a distant star is said to be occulted by a planet when the latter passes in front of the former, occulting (hiding) it; or just as when the moon, which is comparatively near the earth, passes in front of a planet, which is enormously more distant. It will be seen, therefore, that an eclipse of the sun can take place only at “new moon, while that of the moon occurs only at “full moon.” This is true whether the solar eclipse is total or annular.

The reason why such eclipses cannot be seen often is not. because they seldom happen, but because they can be witnessed only from limited areas of the earth’s surface; and . but few people have the opportunity for travelling to observe them..-When .seen, a.Jotal eclipse is admittedly among the grandest and most awe-inspiring sights that can greet the eye. It is not a matter for. surprise that in earlier times events like these should have excited public dread; It would not be necessary for a total eclipse to. coincide many times with the outbreak of a pestinence, or the death of a monarch, to create in the minds of superstitious races the conviction that such sudden darkenings were ■ portents of disaster. An annular eclipse has not the same popular interest, nor does it mean so much for the astronomer. It is only when the sun is completely veiled by the dark body of the moon that the wonderfully beautiful corona is revealed. During an annular eclipse there is too much solar light for the delicate nimbus that surrounds the sun to be seen. The astronomer has found a way by which he can actually see the ‘ prominences” (or great hydrogen flames that leap up many thousands of miles above the sun’s surface) without waiting for eclipses. But this is not the case with regard to the corona. The spectroscope makes visible these prominences, and has revealed their true nature. So enormous are .they that if the earth were enveloped in one of them it would be as a grain of sand in the flame of a candle. The corona appears as a soft effulgence surrounding the black body of the moon, though we know beyond refutation that it belongs to the sun and not to the. moon. While, therefore, the total eclipse of last August called into being numerous expedition parties,, no such elaborate attention will be given to the event of the 24th. inst. There are those who will travel to see the eclipse, being interested in it from a purely scientific standpoint; while others will do so' because of its spectacular value.

The scientific worth of total solar eclipses may be stated as follows: First: the relative positions of the earth, the sun, and the moon can be ascertained more correctly from the position of the borders of the track of a total, eclipse on the earth and from the times of contact of the edge of the moon with that of the sun than by any other known method. In the next place, supposing there was a planet nearer the sun than Mercury, it could scarcely be seen except during a total eclipse. The search for an infra-Mercurial planet has. practically been abandoned: but it is important to keep watch, even though for a negative purpose. There is, further, the fact that just before totality only a very thin portion of the’sun’s edge remains visible; if only for a second or two. This gives the astronomer his opportunity to examine the outer layers of the sun; those layers, that is to say, above the visible surface, including the solar atmosphere. And, finally, there is the corona already mentioned, a perfect knowledge of the nature of which is of extreme importance.

The value of this opportunity may be appreciated when it is borne in mind that the sum total of the fractions of time at all eclipses during which the corona and coronal streamers were visible have amounted to less than an hour since photography was first used in studying’ the sun. The corona is fainter than the illumination of the sky through which it must be seen: hence the difficulty of visual observation. What we know about the corona is that it is not arranged in continuous layers about the sun, as the atmosphere envelopes the earth; but is made up of long streamers reaching out radially to distances of a million miles and more, especially in the plane 1 of the sun’s equator.