After
Copernicus proposed his heliocentric system, with the
Earth in revolution around the Sun, it was possible to build a model of the whole
Solar System without scale. To ascertain the scale, it is necessary only to measure one distance within the Solar System, e.g., the mean distance from the Earth to the Sun (now called an astronomical unit, or AU). When found 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 expansion age[19] of the visible
Universe.
A primitive way to determine the distance to the Sun in terms of the distance to the
Moon was already proposed by
Aristarchus of Samos in his book
On the Sizes and Distances of the
Sun and Moon. He noted that the Sun, Moon, and Earth form a right triangle (right angle at the Moon) at the moment of first or last quarter moon. He then estimated that the Moon, Earth, Sun angle was 87°. Using correct geometry but inaccurate observational data,
Aristarchus concluded that the Sun was slightly less than 20 times farther away than the
Moon. The true value of this angle is close to 89° 50', and the Sun is actually about 390 times farther away.[17] He pointed out that the
Moon and Sun have nearly equal apparent angular sizes and therefore their diameters must be in proportion to their distances from Earth. He thus concluded that the Sun was around 20 times larger than the Moon; this conclusion, although incorrect, follows logically from his incorrect data. It does suggest that the Sun is clearly larger than the Earth, which could be taken to support the heliocentric model.
Although Aristarchus' results were incorrect due to observational errors, they were based on correct geometric principles of parallax, and became the basis for estimates of the size of the Solar System for almost
2000 years, until the transit of
Venus was correctly observed in 1761 and 1769.[17] This method was proposed by
Edmond Halley in 1716, although he did not live to see the results. The use of Venus transits was less successful than had been hoped due to the black drop effect, but the resulting estimate, 153 million kilometers, is just 2% above the currently accepted value, 149.6 million kilometers.
Much later, the Solar System was 'scaled' using the parallax of asteroids, some of which, such as
Eros, pass much closer to Earth than Venus. In a favourable opposition, Eros can approach the Earth to within 22 million kilometres.[21] 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.[22]
Also radar reflections, both off Venus (
1958) and off asteroids, like
Icarus, have been used for solar parallax determination.
Today, use of spacecraft telemetry links has solved this old problem. The currently accepted value of solar parallax is 8".794
143.
Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines.[
1][2] The term is derived from the
Greek word παράλλαξις (parallaxis), meaning "alteration". Nearby objects have a larger parallax than more distant objects when observed from different positions, so parallax can be used to determine distances.
Astronomers use the principle of parallax to measure distances to the closer stars. Here, the term "parallax" is the semi-angle of inclination between two sight-lines to the star, as observed when the Earth is on opposite sides of the Sun in its orbit.[3] These distances form the lowest rung of what is called "the cosmic distance ladder", the first in a succession of methods by which astronomers determine the distances to celestial objects, serving as a basis for other distance measurements in astronomy forming the higher rungs of the ladder.
Parallax also affects optical instruments such as rifle scopes, binoculars, microscopes, and twin-lens reflex cameras that view objects from slightly different angles. Many animals, including humans, have two eyes with overlapping visual fields that use parallax to gain depth perception; this process is known as stereopsis. In computer vision the effect is used for computer stereo vision, and there is a device called a parallax rangefinder that uses it to find range, and in some variations also altitude to a target.
http://en.wikipedia.org/wiki/Parallax
- published: 27 Jul 2015
- views: 361