The Earth: viewed from the Radiant-Point of the November Meteors 1. 12h 15m C.M.T. 2. 2h 15m G.M.T. Fig.1 Fig. 2 combination of their own actual motion, and the earth's motion applied in a reversed direction). These suppositions being made, we can have no difficulty in selecting a suitable point of space from which in imagination to view our earth. The "radiant point" is clearly the proper point to select. If the reader, therefore, will suppose himself somewhere in space, between e and u Leonis, and armed with sufficient optical power, he will be prepared for the examination of the illustrative Figs. 1 and 2. In these the earth is supposed to be viewed from such a direction; in Fig. 1, at about a quarter past twelve, and in Fig. 2, at about a quarter past two, Greenwich solar time, on November 14th, in any year. The shaded half of each hemisphere is the portion turned from the sun, the apparent boundary of this portion being a straight line, because the radiant point (as respects its longitude) is situated very nearly in the direction towards which the earth is moving at the time. But since the radiant (as respects latitude) was raised some 10° to the north of the ecliptic, the north pole of the earth is brought more into view than it would be to an observer placed at a point towards which the earth is actually moving at the time. In fact, the presentation of the earth towards the radiant point happens to be almost exactly the same (as to inclination of the polar axis) as the presentation of the earth towards the sun at the time of summer solstice. Without entering further into these points, it will suffice to say that Figs. 1 and 2 are the results of mathematical calculation and careful constructionnot imaginary figures set down partly at random, as is too often the case with illustrations of this sort. I am particular to mention this, because when it is known that an astronomical picture represents actual facts, as closely as possible, the student will undertake the study of the picture with some hope of information and instruction, whereas the study of illustrations (so-called) not carefully constructed-and nine out of ten figures in our works on popular astronomy fall under this category-is often worse than unprofitable. Around London and Calcutta in Map 1, and around London and Cape Town in Map 2, oval and circular spaces are indicated. It is necessary to explain the meaning of these. Assuming the depth of our atmosphere to be about seventy miles—or, at any rate, that meteors are not commonly visible at greater heights-it is easily shown that the segment of atmosphere cut off by a plane touching the earth at any point, has a circular base about 1,500 miles in diameter. neglecting the effects of refraction which would slightly increase the dimensions of the segment, we have this result, that no meteor can be seen from any point of the earth's surface further than 750 miles from the point over which such VOL. XII.-NO. III. Thus |