The
photo was taken by Sylvia in her back garden in Old Town, Swindon by attaching
a Nikon Camera to the eyepiece of an old Russian Tal 1 telescope (110mm refractor),
and a Baader Mylar filter. The photograph has been modified using the 'curves'
feature in Photoshop.
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 solar 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 smaller because it is 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
8 June 2004 (see Transit of Venus, 2004) and the next will be on 6 June 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 used 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.
Conjuctions
Venus, with
an orbit inclined by 3.4° relative to the Earth's, usually appears to pass
under (or over) the Sun in the sky at inferior conjunction. A transit occurs when
Venus reaches conjunction with the Sun at or near one of its nodes, the longitude
where Venus passes through the Earth's orbital plane, called the ecliptic. Although
the inclination between these two orbital planes is only 3.4°, Venus can be
as far as 9.6° from the Sun when viewed from the Earth at inferior conjunction.[4]
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. Sequences
of transits occur in a pattern that repeats every 243 years, with transits occurring
eight years apart followed by a gap of 121.5 years, then a gap of eight years
and then another long gap of 105.5 years. The pattern repeats every 243 years
because 243 sidereal orbital periods of the Earth (365.25636 days - slightly longer
than the tropical year) is 88757.3 days, and 395 sidereal orbital periods of Venus
(224.701 days) is 88756.9 days. Thus, after this period both Venus and Earth have
returned to very nearly the same point in each of their respective orbits. This
period of time corresponds to 152 synodic periods of Venus. The
pattern of 105.5, 8, 121.5 and 8 years is not the only pattern that is 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 CE transit were 121.5 years apart. The current pattern will continue until
2846, when it will 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 planets 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 charted Venus' full
cycle, but despite their precise knowledge of its course, there is no mention
of the transit.
Modern History
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
and Kepler's third law. The technique involved making 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 was then 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 had not been determined. In
1631, Johannes Kepler was the first person to predict a transit of Venus. His
methods were not sufficiently accurate to predict that the transit would not be
visible in most of Europe, and as a consequence, nobody was able to make arrangements
to observe the transit
First
European scientific observation
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 (24 November 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 realized 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 had not yet come into contact with the Sun's
disk during the initial phase of transit.
1761
and 1769
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),
numerous expeditions were made to various parts of the world in order to observe
these transits; an early example of international scientific collaboration. In
an attempt to observe the first transit of the pair, scientists and explorers
from Britain, Austria and France travelled to destinations around the world, including
Siberia, Norway, Newfoundland and Madagascar. 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 traveled to Hudson Bay, Baja California (then under
Spanish control) and Norway. Observations also were made from Tahiti on the first
voyage of Captain Cook, at a location still known as "Point Venus".
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).
Unfortunately, it
was impossible to time the exact moment of the start and end of the transit due
to the phenomenon known as the "black drop effect". The black drop effect
was long thought to be due to Venus' thick atmosphere, and initially it was held
to be the first real evidence that Venus had an atmosphere; however recent studies
demonstrate that it is an optical effect caused by the smearing of the image of
Venus by turbulence in the Earth's atmosphere or imperfections in the viewing
apparatus.
1771
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. Transit observations
in 1874 and 1882 allowed this value to be refined further. Several expeditions
were sent to the Kerguelen Archipelago for the 1874 observations. The American
astronomer, Simon Newcomb, combined the data from the last four transits and derived
a value of 149.59 million kilometers (±0.31 million km). Modern techniques,
such as space probe telemetry and radar observations of solar system objects,
have allowed a precise value for the astronomical unit to be calculated (to an
accuracy of ±30 m), and so negated the need for parallax calculations. 2004
There
was however a good deal of interest in the 2004 transit as scientists attempted
to measure the pattern of light dimming as Venus blocked out some of the Sun's
light, in order to refine techniques that they hope to use in searching for extrasolar
planets. 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.] However, extremely
precise measurement is needed: for example, the transit of Venus causes the Sun's
light to drop by a mere 0.001 magnitude, and the dimming produced by small extrasolar
planets will be similarly tiny.
Past and future transits
Transits can currently occur only in June
or December. These dates are slowly getting later; before 1631, they were in May
and November. Transits usually occur in pairs, on nearly the same date eight years
apart. This is because the length of eight Earth years is almost the same as 13
years on Venus, so every eight years the planets are in roughly the same relative
positions. This approximate conjunction usually results in a pair of transits,
but it is not precise enough to produce a triplet, since Venus arrives 22 hours
earlier each time. The last transit not to be part of a pair was in 1396. The
next will be in 3089; in 2854 (the second of the 2846/2854 pair), although Venus
will just miss the Sun as seen from the Earth's equator, a partial transit will
be visible from some parts of the southern hemisphere.
Text courtesy
of Wikipedia
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