Transit of Venus Totally Explained

Everything about Transit Of Venus totally explained

A transit of Venus across the Sun takes place when the planet Venus passes directly between the Sun and Earth, obscuring a small portion of the Sun's disk. During a transit, Venus can be seen from Earth as a small black disk moving across the face of the Sun. The duration of such transits is usually measured in hours (the transit of 2004 lasted six hours). A transit is similar to a solar eclipse by the Moon, but, although the diameter of Venus is almost 4 times that of the Moon, Venus appears much smaller because it's much farther away from Earth. Before the space age, observations of transits of Venus helped scientists use the parallax method to calculate the distance between the Sun and the Earth.
Transits of Venus are among the rarest of predictable astronomical phenomena and currently occur in a pattern that repeats every 243 years, with pairs of transits eight years apart separated by long gaps of 121.5 years and 105.5 years. Before 2004, the last pair of transits were in December 1874 and December 1882. The first of a pair of transits of Venus in the beginning of the 21st century took place on June 8, 2004 (see Transit of Venus, 2004) and the next will be on June 6, 2012 (see Transit of Venus, 2012). After 2012, the next transits of Venus will be in December 2117 and December 2125.
A transit of Venus can be safely observed by taking the same precautions as when observing the partial phases of a solar eclipse. Staring at the brilliant disk of the Sun (the photosphere) with the unprotected eye can quickly cause serious and often permanent eye damage.

Conjunctions

Normally when the Earth and Venus are in conjunction they're not aligned with the Sun. Venus' orbit is inclined by 3.4° to the Earth's so it appears to pass under (or over) the Sun in the sky. Transits occur when the two planets happen to be in conjunction at (or very near) the line where their orbital planes cross. Although the inclination is only 3.4°, Venus can be as far as 9.6° from the Sun when viewed from the Earth at inferior conjunction. Since the angular diameter of the Sun is about half a degree, Venus may appear to pass above or below the Sun by more than 18 solar diameters during an ordinary conjunction.
The pattern of 105.5, 8, 121.5 and 8 years isn't the only pattern that's possible within the 243-year cycle, due to the slight mismatch between the times when the Earth and Venus arrive at the point of conjunction. Prior to 1518, the pattern of transits was 8, 113.5 and 121.5 years, and the eight inter-transit gaps before the 546 transit were 121.5 years apart. The current pattern will continue until 2846, when it'll be replaced by a pattern of 105.5, 129.5 and 8 years. Thus, the 243-year cycle is relatively stable, but the number of transits and their timing within the cycle will vary over time.

Ancient history

Ancient Greek, Egyptian, Babylonian, and Chinese observers knew of Venus and recorded the planet’s motions. The early Greeks thought that the evening and morning appearances of Venus represented two different objects, Hesperus - the evening star and Phosphorus - the morning star. Pythagoras is credited with realizing they were the same planet. In the 4th century BC, Heraclides Ponticus proposed that both Venus and Mercury orbited the Sun rather than Earth. There is no evidence that any of these cultures knew of the transits. Venus was important to ancient American civilizations, in particular for the Maya, who called it Noh Ek, "the Great Star" or Xux Ek, "the Wasp Star"; they embodied Venus in the form of the god Kukulkán (also known or related to Gukumatz and Quetzalcoatl in other parts of Mexico). In the Dresden Codex, the Maya chart Venus' full cycle, but despite their precise knowledge of its course, there's no mention of the transit.

Modern observations

Aside from its rarity, the original scientific interest in observing a transit of Venus was that it could be used to determine the size of the solar system by employing the parallax method. The technique is to make precise observations of the slight difference in the time of either the start or the end of the transit from widely separated points on the Earth's surface. The distance between the points on the Earth can then be used as a baseline to calculate the distance to Venus and the Sun via triangulation.
   Although by the 17th century astronomers could calculate each planet's relative distance from the Sun in terms of the distance of the Earth from the Sun (an astronomical unit), an accurate absolute value of this distance hadn't been calculated.
   Despite Johannes Kepler being the first to predict a transit of Venus in 1631, no one in Europe observed it because Kepler's predictions were not sufficiently accurate to predict that the transit wouldn't be visible in most of Europe.
The first European scientific observation of a transit of Venus was made by Jeremiah Horrocks from his home in Much Hoole, near Preston in England, on 4 December 1639 (November 24 under the Julian calendar then in use in England). His friend, William Crabtree, also observed this transit from Salford, near Manchester. Kepler had predicted transits in 1631 and 1761 and a near miss in 1639. Horrocks corrected Kepler's calculation for the orbit of Venus and realised that transits of Venus would occur in pairs 8 years apart, and so predicted the transit in 1639. Although he was uncertain of the exact time, he calculated that the transit was to begin at approximately 3:00 pm. Horrocks focused the image of the Sun through a simple telescope onto a piece of card, where the image could be safely observed. After observing for most of the day, he was lucky to see the transit as clouds obscuring the Sun cleared at about 3:15 pm, just half an hour before sunset. Horrocks' observations allowed him to make a well-informed guess as to the size of Venus, as well as to make an estimate of the distance between the Earth and the Sun. He estimated the distance of the Sun from the Earth at 59.4 million miles (95.6 Gm, 0.639 AU) - about half the correct size of 93 million miles (149.6 million km), but a more accurate figure than any suggested up to that time. However, Horrocks' observations were not published until 1661, well after his death.
   Based on his observation of the transit of Venus of 1761 from the Petersburg Observatory, Mikhail Lomonosov predicted the existence of an atmosphere on Venus. Lomonosov detected the refraction of solar rays while observing the transit and inferred that only refraction through an atmosphere could explain the appearance of a light ring around the part of Venus that hadn't yet come into contact with the Sun's disk during the initial phase of transit.
   The transit pair of 1761 and 1769 were used to try to determine the precise value of the astronomical unit (AU) using parallax. This method of determining the AU was first described by James Gregory in Optica Promota in 1663. Following the proposition put forward by Edmond Halley (who had died almost twenty years earlier), Most managed to observe at least part of the transit, but excellent readings were made in particular by Jeremiah Dixon and Charles Mason at the Cape of Good Hope. For the 1769 transit scientists travelled to Hudson Bay, Baja California (then under Spanish control) and Norway, as well as the first voyage of Captain Cook in order to observe the transit from Tahiti. The Czech astronomer Christian Mayer was invited by Catherine the Great to observe the transit in Saint Petersburg, but his observations were mostly obscured by clouds. The unfortunate Guillaume Le Gentil spent eight years travelling in an attempt to observe either of the transits; his unsuccessful journey led to him losing his wife and possessions and being declared dead (his efforts became the basis of the play Transit of Venus by Maureen Hunter).
   In 1771, using the combined 1761 and 1769 transit data, the French astronomer Jérôme Lalande, calculated the astronomical unit to have a value of 153 million kilometers(±1 million km). The precision was less than hoped-for because of the black drop effect, but still a considerable improvement on Horrocks' calculations. Current methods of looking for planets orbiting other stars only work for a few cases: planets that are very large (Jupiter-like, not Earth-like), whose gravity is strong enough to wobble the star sufficiently for us to detect changes in proper motion or Doppler shift changes in radial velocity, Jupiter or Neptune sized planets very close to their parent star, or through gravitational microlensing by planets which pass in front of background stars with the planet-parent star separation comparable to the Einstein ring. Measuring light intensity during the course of a transit, as the planet blocks out some of the light, is potentially much more sensitive, and might be used to find smaller planets.

Past and future transits

Transits can currently occur only in June or December (see table). These dates are slowly getting later; before 1631, they were in May and November.

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