Part 20 (1/2)
Thus there was a transit in June, 1761, then another 8 years after, in June, 1769 The next occurred 113-1/2 years less 8 years, _ie_, 105-1/2 years after the preceding, in December, 1874; the next in December, 1882 The next will be in June, 2004, and June, 2012 At these eagerly anticipated epochs, astronomers watch the transit of Venus across the Sun at two terrestrial stations as far as possible re the two points at which the planet, seen from their respective stations, appears to be projected at the saives the width of an angle for from two diametrically opposite points of the Earth, cross upon Venus, and forle upon the Sun Venus is thus at the apex of two equal triangles, the bases of which rest, respectively, upon the Earth and on the Sun The ives what is called the parallax of the Sun--that is, the angular dimension at which the Earth would be seen at the distance of the Sun
[Illustration: FIG 83--Measurement of the distance of the Sun]
Thus, it has been found that the half-diameter of the Earth viewed from the Sun le of one degree is at a distance of 57 tile of a ree, indicates by the le that it is 60 times more distant, _ie_, 3,438 times
Finally, an object that measures one second, or the sixtieth part of a th
Hence we find that the Earth is at a distance from the Sun of 206,265/882--that is, 23,386 times its half-diameter, that is, 149,000,000 kiloain is as precise and certain as that of the Moon
I hope ulation, the result of which indicates to us with absolute certainty the distance of the two great celestial torches to which e the radiant light of day and the gentle illuhts
The distance of the Sun has, ree perfectly with the preceding The two principal are based on the velocity of light The propagation of light is not instantaneous, and notwithstanding the extreme rapidity of its movements, a certain time is required for its transmission from one point to another On the Earth, this velocity has been measured as 300,000 kilometers (186,000 miles) per second To come from Jupiter to the Earth, it requires thirty to fortyto the distance of the planet Now, in exa the eclipses of Jupiter's satellites, it has been discovered that there is a difference of 16 minutes, 34 seconds in theas Jupiter is on one side or on the other of the Sun, relatively to the Earth, at the ht takes 16 minutes, 34 seconds to traverse the terrestrial orbit, it must take less than that time, or 8 minutes, 17 seconds, to come to us fro the velocity of light, the distance of the Sun is easily found by300,000 by 8 ives about 149,000,000 kilometers (93,000,000 ht again gives a confirine ourselves exposed to a vertical rain; the degree of inclination of our umbrella will depend on the relation between our speed and that of the drops of rain The more quickly we run, the more we need to dip our umbrella in order not to ht The stars, disseht upon the Heavens If the Earth were motionless, the luminous rays would reach us directly But our planet is spinning, racing, with the utmost speed, and in our astronomical observations we are forced to follow its movements, and to incline our telescopes in the direction of its advance This phenoht, is the result of the coht and of the Earth's lobe is equivalent to 1/10000 that of light, _ie_, = about 30 kiloly acco an orbit which she traverses at a speed of 30 kilometers (better 29-1/2) per second, or 1,770 kilometers per minute, or 106,000 kilometers per hour, or 2,592,000 kilometers per day, or 946,080,000 kiloth of the elliptical path described by the Earth in her annual translation
The length of orbit being thus discovered, one can calculate its diameter, the half of which is exactly the distance of the Sun
We may cite one last method, whose data, based upon attraction, are provided by the motions of our satellite The Moon is a little disturbed in the regularity of her course round the Earth by the influence of the powerful Sun As the attraction varies inversely with the square of the distance, the distancethe effect it has upon the Moon
Other e in this summary of the methods employed for determinations, confirm the precisions of these ive us for dwelling at soth upon the distance of the orb of day, since this hest importance; it serves as the base for the valuation of all stellar distances, and may be considered as the meter of the universe
This radiant Sun to which e so much is therefore enthroned in space at a distance of 149,000,000 kilometers (93,000,000 miles) from here
Its vast brazier must indeed be powerful for its influence to be exerted upon us to such athe very condition of our existence, and reaching out as far as Neptune, thirty times more remote than ourselves froreat distance that the Sun appears to us no larger than the Moon, which is only 384,000 kilometers (238,000 miles) from here, and is itself illuminated by the brilliancy of this splendid orb
No terrestrial distance ad of this distance Yet, if we associate the idea of space with the idea of time, as we have already done for the Moon, we may attempt to picture this abyss The train cited just noould, if started at a speed of a kilometer a minute, arrive at the Sun after an uninterrupted course of 283 years, and taking as long to return to the Earth the total would be 566 years
Fourteen generations of stokers would be employed on this celestial excursion before the bold travelers could bring back news of the expedition to us
Sound is transh the air at a velocity of 340 meters (1,115 feet) per second If our atmosphere reached to the Sun, the noise of an explosion sufficiently formidable to be heard here would only reach us at the end of 13 years, 9 raph, would leap across to the orb of day in 8 ination is confounded before this gulf of 93,000,000rays fly rapidly through space in order to reach us
And now let us see how the distances of the planets were determined
We will leave aside the ; that now to be employed is quite different, but equally precise in its results
It is obvious that the revolution of a planet round the Sun will be longer in proportion as the distance is greater, and the orbit that has to be traveled vaster This is sieometric proportion in the relations between the duration of the revolutions of the planets and their distances This proportion was discovered by Kepler, after thirty years of research, and e formula:
”The squares of the times of revolution of the planets round the Sun (the periodic times) are proportional to the cubes of their h to alarm the boldest reader And yet, if we unravel this somewhat incomprehensible phrase, we are struck with its simplicity
What is a square? We all know this ht to children of ten years old But lest it has slipped your memory: a square is simply a number multiplied by itself
Thus: 2 2 = 4; 4 is the square of 2
Four times 4 is 16; 16 is the square of 4