 |
Business | Industries | Finance | Tax |
| Index: > A B C D E F G H I J K L M N O P Q R S T U V W X Y Z |
|
|||||
| First Prev [ 1 2 3 4 ] Next Last |
had no idea how one could calculate the distance separating the Moon from the Earth. The circumstance was exploited to teach them that this distance was obtained by measuring the parallax of the Moon. If the word parallax appeared to amaze them, they were told that it was the angle subtended by two straight lines running from both ends of the Earth's radius to the Moon. If they had doubts on the perfection of this method, they were immediately shown that not only did this mean distance amount to a whole two hundred thirty-four thousand three hundred and forty-seven miles (94.330 leagues), but also that the astronomers were not in error by more than seventy miles (— 30 leagues)."
A primitive way to determine the lunar parallax from one location is by using a lunar eclipse. The full shadow of the Earth on the Moon has an apparent radius of curvature equal to the difference between the apparent radii of the Earth and the Sun as seen from the Moon. This radius can be seen to be equal to 0.75 degrees, from which (with the solar apparent radius 0.25 degrees) we get an Earth apparent radius of 1 degree. This yields for the Earth-Moon distance 60 Earth radii or 384.000 km.
Another way to use parallax to determine the distance to the moon would be to take two pictures of the moon at exactly the same time from two locations on earth, and compare the position of the moon relative to the visible stars. Using the orientation of the earth, and those two points, and a perpendicular displacement, a distance to the moon can be triangulated.
After Johannes KeplerThis article is about Johannes Kepler the astronomer. For the planned planet-finding space telescope, see Kepler Space Mission. Johannes Kepler ( December 27, 1571 November 15, 1630), a key figure in the scientific revolution, was a German astronomer, mat discovered his Third LawJohannes Kepler's primary contribution to astronomy/ astrophysics were the three laws of planetary motion . Kepler derived these laws, in part, by studying the observations of Brahe. Isaac Newton would later design his laws of motion and universal gravita, it was possible to build a scale model of the whole solar system, but without the scale. To fix the scale, it suffices to measure one distance within the solar system, e.g., the mean distance from the Earth to the SunThe Sun (also called Sol is the star in our solar system. Planet Earth orbits the Sun. Other bodies that orbit the Sun include other planets, asteroids, meteoroids, comets and dust. Not all objects passing through the solar system have been orbitally capt or astronomical unitThe astronomical unit AU is a unit of distance, approximately equal to the mean distance between Earth and Sun. The currently accepted value of the AU is 149,597,870,691+-30 metres (about 150 million kilometres or 93 million miles). Earth's orbit is not a (AU). When done by triangulation, this is referred to as the solar parallax, the difference in position of the Sun as seen from the Earth's centre and a point one Earth radius away, i.e., the angle subtended at the Sun by the Earth's mean radius. Knowing the solar parallax and the mean Earth radius allows one to calculate the AU, the first, small step on the long road of establishing the size — and thus the minimum age — of the visible Universe.
It was proposed by Edmund Halley in 1716, that the transit of Venus over the solar disc be used to derive the solar parallax. And so it was done in 1761 and 1769. There is the famous story of the French astronomer Guillaume Le Gentil, who travelled to India to observe the 1761 event, but didn't reach his destination in time due to war. He stayed on for the 1769 event, but then there were clouds blocking the Sun...
The use of Venus transits was less successful than had been hoped due
to the black drop effect.
Much later, the solar system was 'scaled' using the parallax of
asteroids, some of which, like Eros, pass muchcloser to Earth than Venus. In a favourable opposition, Eros can approach the Earth to within 22 million kilometres. Both the opposition of 1901 and that of 1930/1931 were used for this purpose, the calculations of the latter determination being completed by Astronomer Royal Sir Harold Spencer Jones.
Also radar reflections, both off Venus (1958) and off asteroids, like
Icarus, have been used for solar parallaxdetermination. Today, use of spacecraft telemetry links has solved this old problem completely.
| Politics | Business | Finance |
 |
 |
 |
|