When actual observations of sun-distance came to be made, innumerable difficulties arose. Foremost stood the huge amount of that distance. This made precise observations more difficult; and at the same time it made every mistake in observation so much the worse. A little mistake in observing the moon might mean only a hundred miles or so wrong in the answer; but a mistake equally small in observing the sun would lead to an error of many millions of miles. Again, to observe the sun’s exact position among the stars, as with the moon, was not possible; because when the sun is visible, the stars are not visible. Then, too, the dazzling brightness of the sun balked the needed exactitude. Halley had a brilliant thought before he died. He could not carry it out himself, but he left it to others as a legacy; that was, by observing the transit of Venus from two different points on the earth’s surface. At his suggestion, on the first opportunity--which was not till after his death--the above mode was tried of measuring the sun’s distance. A certain observer, stationed on one part of earth, saw the tiny dark body of Venus take one particular line across the sun’s bright face. Another observer, standing on quite another and a far-off part of earth, saw the little dark body take quite another line across the sun’s bright face. Not that there were two little dark bodies, but that the one body was seen by different men in different places. From these separate views of the path of Venus across the face of the sun, in connection with what was already known of the earth’s diameter, and therefore of the length of base-line between the two places, the distance of the sun was reckoned to be about 95,000,000 miles. Since that date many fresh attempts have been made, and errors have been set right. Mercury as well as Venus has been used in this matter, and other newer modes of measurement have also been successfully tried. We know now, with tolerable accuracy, that the sun’s greater distance from earth is between 92,000,000 and 93,000,000 miles. A curious illustration of sun-distance has been offered by one writer. Sound and light, heat and sensation, all require _time_ for journeying. When a child puts his finger into a candle-flame, he immediately shrieks with pain. Yet, quickly as the cry follows the action, his brain is not really aware of the burn until a certain interval has elapsed. True, the interval is extremely minute; still it is a real interval. News of the burn has to be telegraphed from the finger, through the nerves of the arm, up to the brain; and it occupies time in transmission, though so small a fraction of a second that we can not be conscious of it.