Monday, June 07, 2010
Remember the transit of Venus in 1882? Well, one person alive does. For the rest of us we have old newspapers to look at, and the transit of Venus in 1882 (the last one before the most recent one in 2004) made quite a bit of news. This is a newspaper article from the Boston Globe on 2 December 1882 and it's interesting to compare what was thought about Venus at the time vs. what we know today. The amount of guesswork and extrapolation needed at the time kind of resembles our observation of extrasolar planets today. The next transit of Venus will be in 2012, and then nothing for over a century.
Click on the image to the right to see it yourself, or read it here (I just typed it out):
VENUS IN TRANSIT.
The Shepherds' Star Before Her Fiery Lord.
Science Strains Sight to Scan this Solar Spectacle.
Preparations for Observation -- Value of the Views.
On Wednesday next the planet Venus passes between the earth and the sun, for the first time since 1874, and for the last time before the year 2005. The planet has for some time been brilliant in the evening sky. As she swings round her orbit she is rapidly nearing the line between the earth and the sun, setting closer behind her fiery lord every evening; and when the "transit" occurs she will appear as a small spot upon the sun's bright disc, moving from edge to edge in about six and a half hours -- from 9 a.m. until 3:31 p.m. When in transit she cannot be observed by the unassisted eye, but a good field or opera glass will show the phenomenon. Care must be taken to protect the eye, by means of smoked glass, from unusual glare. The neglect of such precautions cost Galileo his sight and entailed great suffering upon Herschel.
This planet, distant from the earth 24,000,000 miles, is the brightest star in the sky. The Chaldrean shepherds were the first astronomers, and from their fervid paeans on its glowing beauty the East called it the shepherds' star. Ptolemy and Hipparchus named it Venus on account of its matchless lustre and incomparable loveliness. Its brilliancy is, indeed, sometimes so intense that in a very clear sky it is visible by day. Its mean distance from the sun is about 69,000,000 miles. Its orbit is nearer to a perfect circle than any other of the planetary paths. Its day is thirty-five minutes shorter than ours. It revolves on its axis in thirty-five minutes less time than does the earth. Venus traverses its orbit in 225 days. Its speed is next to that of the winged Mercury, being twenty-two miles a second. Its nearest approach to the earth is 24,000,000 miles. Of all the heavenly bodies except the moon, Venus is our nearest neighbor. When beyond the sun it recedes from us 162,000,000 miles. It never departs from the sun beyond 48 degrees, and is thus
Visible but a Short Portion of the Night.
Herschel, Arago, Beer and Madler placed its diameter at 8108 miles. It is about the same size as the earth. It is not as compact as the earth, being little more than nine-tenths its weight.
It is generally conceded by astronomical authorities that if any planet in the solar scheme besides the earth is peopled that planet is Venus. Probably there are not in the universe two bodies more similar and in more respects alike than the earth and Venus. Venus and Terra are truly twin sisters. Herschel tells us that in judging of the habitability of a planet three things are to be principally considered. First, its heat and light; second, the force of gravity or the weight of bodies at its surface; third, the density or consistency of the planet. The amount of heat and light received by Venus from the sun is just double that received by us. But this is in a great measure modified by the enormous density and moisture of this planet's atmosphere. The size of Venus is about equal to that of the arth. The density of Venus is more than nine-tenths of the earth's. The gravity at its surface is about equal to that on the earth's surface. In this particular Venus is far more favorable to habitability than Mars, the gravity at the surface of Mars being only one-half of that of the earth's, and this is the strongest argument of the habitability of Mars, there being no stability on its surface. The strongest argument against the habitableness of Venus is the great obliquity of its equator to the plane of the elliptic and the consequent sudden and extreme change of its seasons. No race of animals nor species of plants that we know of could live in the sudden and appalling severity of Venus' short seasons. It is difficult to determine what conditions are
Absolutely Necessary for Life on a Planet.
It was always thought in the past that no animal life could subsist in the great sea depths, the pressure of the water being so great down there as to crush the mail of the crocodile. But lately it has been discovered that animals do live in these depths, and even have organs of vision. They have been brought to the surface by dredges, but had burst open long before reaching the surface. Still, taking all the facts as we know them, and making a really fair deduction therefrom, it seems probably that if animal life exists in Venus its scale is not higher than the trilobite.
A transit of Venus is due to the simple fact that her orbit about the sun is inside that described by the earth. If her orbit were in the same plane as that of the earth there would be a transit every time Venus completed her revolution in respect to the earth. But sometimes she passes above and sometimes below the sun, and the transits are rare events, occurring in pairs, eight years intervening between those of a pair. The pairs are separated by periods alternately of 105 1/2 and 121 1/2 years. The transit now about to occur is the second of a pair, the first of which occurred in 1874. There is no display of interest to the unscientific eye, in the transit of Venus. But the scientific value of the transit lies in the fact that it can be made to measure the distance from the earth to the sun, which is itself the astronomical unit of measurement in the vast distances of space. The mathematical process by which this is done is very complicated. But it can readily be seen that two observers at distant points on the earth
Would See the Planet at Different Points
on the sun's disc at the same moment. If the exact distance between these two points can be measured, the sun's diameter can be calculated, and its distance from the earth directly follows. The details of this work are so complicated that the results of the transit of 1874 are not yet fully known, and years will pass before anything definite will be known of the deductions from the observations in December. So far the only transits observed are those of 1639, 1761, 1769 and 1874.
The dates of transits of Venus for four centuries preceding our time are as follows: 1518, June 2; 1526, June 1; 1631, December 7; 1639, December 4; 1761, June 5; 1769, June 3; 1874, December 9. The dates of transits for the four centuries following our time are as follows: 1882, December 6; 2004, June 8; 2012, June 12; 2117, December 11; 2125, December 8; 2247, June 11; 2255, June 9.
Kepler, the friend and pupil of Tycho Brahe, announced that a transit of Venus would occur in 1631. The prediction excited the greatest interest among astronomers. No one had observed a transit, because none had occurred since the invention of the telescope. At that time there lived near Liverpool a gentleman named Horox, who had just attained his majority. He had given much attention to astronomy and enjoyed a local celebrity. His ardent study of science was
Only Equalled by His Religious Zeal.
In verifying the calculations of Kepler he discovered an error which, when corrected, gave a far different result. Fearing that possibly he had made the mistake he carefully revised his work, but the result invariably indicated 1639 as the year and December 4 as the day of the transit. His modesty prevented him from publishing the result of his calculations, but prudence suggested that he should deposit a copy of his work with some one for safety. He imparted his knowledge to a single friend, and together they waited. The year 1631 came and went without a transit. Eight years passed without an exhibition of the phenomenon. The 4th of December, 1639, fell upon Sunday. As the table of elements was not absolutely perfect the precise hour of the occurrence could only be ascertained by observation. But Horrox would not permit a "pastime," as he called it, to prevent him from performing his customary devotions. At the sound of the church-going bell he left his observatory and repaired to the chapel. After attending the two services appointed for the day he resumed his place at the telescope, and was finally rewarded by a sight of the first transit observed by human eyes. His tables thus verified and subsequently published were accepted as the basis of future calculation.
After the transit of 1769 the distance from earth to sun was reckoned at from 92,000,000 to 95,000,000 miles. This was so wide a margin that over $1,000,000 were spent in observing the transit of 1874, and the calculations so far made vary from 92,000,000 to 93,500,000 miles. It is hoped that the observations of next week will reduce this immense margin. Observations are made in three ways. The simplest is merely to observe the moments
When the Edge of the Planet Just Touches the Edge of the Sun--
the "contacts," as they are technically called. Another method is to measure as often as possible the apparent distance of the planet from the edge of the sun. A third method is to make as many photographs of the sun with Venus upon it as can be obtained during the transit. These pictures are afterward carefully measured. The photographs of 1874 were for the most part failures, but the American photographs were exceptionally good. It is difficult to avoid distortion in photographs, and, moreover, the scale of the plates must be so accurately fixed that errors shall not exceed 1-100th of an inch on a four-inch picture. The difficulty in the observation of "contacts" is caused by the atmosphere of Venus, which causes a band of darkness which seems to join the planet to the edge of the disc for several seconds after it has really passed. This atmosphere was first observed by David Rittenhouse of Philadelphia at the transit of 1769, when Venus was half on the sun, he saw a ring of light surrounding the rest of the planet, so that its whole outline could be seen. This statement was doubted until confirmed by the observations of 1874. Another point of interest to observers will be the possibility of discovering a satellite to Venus (which would probably be named Cupid). Supposed discoveries of a moon to Venus have been often made, but more than a century has passed since the last announcement. If the discovery is again made the lost satellite will appear as a small black speck near the planet.
The Transit of Wednesday Next Will be Widely Observed.
Congress at the last session appropriated $75,000, which will put eight parties in the field, provided with the same apparatus used in 1874. The transit will be visible from the beginning to end all over South America and the Atlantic slope of North America. Professor Newcomb's party has sailed for the Cape of Good Hope; Professor Boss has gone to Santiago, Chili; Assistant Smith of the coast survey, to New Zealand, and Lieutenant Very, of the navy, to Santa Cruz, Pantagonia; Assistant Davidson of the coast survey will observe at Fort Thorn, New Mexico, and Professors Hall, Harkness and Eastman will observe, respectively, at San Antonia, Tex.; Washington, D.C., and Cedar Keys, Fla. France, Germany and England have also sent out parties; and besides all these, observations will be made at all the observatories which command the eclipse, and the results collated at Washington.
It is hardly likely that the best results will do more than lessen uncertainly. Errors of a second in an observation mean errors of millions of miles in a calculation; and so elusive and diverse are the phenomena, so different the circumstances of air and instruments, that comparisons of observations are difficult. Fortunately, there are other means to solve the problem, based on the velocity of light; but the opportunity of a century will not be allowed to pass untried.