On Tuesday 8 June, observers throughout Europe, as well as most of Asia and
Africa, will be able to witness a very rare astronomical phenomenon when the
planet Venus lines up directly between Earth and the Sun. Seen as a small
black disk against the bright Sun, Venus will take about 6 hours to
complete its crossing of the Sun’s face – known as a ‘transit’. The whole
event is visible from the UK, weather permitting.

The last transit of Venus took place on 6 December 1882, but the last one
that could have been seen in its entirety from the UK, as on this occasion,
was in 1283 (when no one knew it was happening) and the next will not be
until 2247! (The transit of 6 June 2012 will not be visible from the UK).
The first transit of Venus to be observed was on 24 November 1639 (Julian
Calendar). Transits also occurred in 1761, 1769 and 1874.

Venus and Mercury both orbit the Sun closer than Earth. Both planets
regularly line up roughly between Earth and the Sun (called ‘conjunction’)
but on most occasions they pass above or below the disc of the Sun from
our point of view. Since 1631, transits of Venus have been occurring at
intervals of 8, 121.5, 8 then 105.5 years and this pattern will continue
until the year 2984. Transits of Mercury are more common; there are 13 or
14 each century, the next being in November 2006.

WHEN AND WHERE

The Venus transit of 8 June begins shortly after sunrise at about 6.20 BST,
when the Sun will be about 12 degrees above the eastern horizon. It will
take about 20 minutes from ‘first contact’ until the planet is fully
silhouetted against the Sun, roughly at the ‘8 o’clock’ position’. It will
then cut a diagonal path across the southern part of the Sun. Mid-transit
is at about 9.22 BST.

Venus begins to leave the Sun near the ‘5 o’clock’ position at about
12.04 BST and the transit will be completely over around 12.24. Timings
differ by a few seconds for different latitudes, but clouds permitting,
the transit will be visible from any place where the Sun is up,
including the whole of the UK and almost all of Europe.

For a diagram of Venus’s track across the Sun, see:

HOW TO VIEW

Venus is large enough to be just visible to someone with normal eyesight
without the help of binoculars or a telescope. Its diameter will appear
about 1/32 the diameter of the Sun. However, NO ONE SHOULD EVER
LOOK DIRECTLY AT THE SUN, WITH OR WITHOUT A TELESCOPE OR BINOCULARS
WITHOUT USING A SAFE SOLAR FILTER. TO DO SO IS VERY DANGEROUS AND IS
LIKELY TO RESULT IN PERMANENT BLINDNESS.

For safe viewing of the transit, much the same rules apply as those for
observing an eclipse of the Sun. Eclipse viewers can be used (as long as
they are undamaged), and observing is limited to a few minutes at a time.
(Note that they must NOT be used with binoculars or a telescope.) For an
enlarged view, an image of the Sun can be projected onto a screen by a
small telescope. Pinhole projection, however, will not produce a sharp
enough image to show Venus clearly.

More detailed information on safety from:

http://sunearth.gsfc.nasa.gov/eclipse/SEhelp/safety2.html

http://www.transit-of-venus.org.uk/safety.htm

IMPORTANCE OF THE TRANSIT

In the 18th and 19th centuries, transits of Venus presented rare
opportunities to tackle a fundamental problem – finding an accurate
value for the distance between Earth and the Sun. The unit
astronomers use for distance measurements in the solar system is
based closely on its average value and is called the astronomical
unit (AU). It is approximately 93 million miles, or 150 million
km.

In the end, though observations of transits produced rough answers,
they were never as accurate as originally hoped (see more on this
below). But the quest was the stimulus for unprecedented
international scientific cooperation and for expeditions that
produced discoveries far beyond their original intended scope. Today,
distances in the solar system are known with great precision
through very different means.

In the 21st century, the main interest in the transits of Venus of
2004 and 2012 is their rarity as astronomical phenomena, the
educational opportunities they present, and the sense of a link with
important events in scientific and world history.

However, astronomers are now particularly interested in the general
principle of planet transits as a way of hunting for extrasolar
planetary systems. When a planet crosses in front of its parent star,
there is a minute dip in the star’s apparent brightness. Identifying
such dips will be a useful method of finding planets orbiting other
stars. Some astronomers intend to use the transit of Venus as a test
to help design searches for extrasolar planets.

The transit will be observed by two solar observatories in space:
TRACE and SOHO. From where SOHO is positioned, it will not see a
transit across the visible disc of the Sun, but it will observe
Venus’s passage across the Sun’s corona (its outer atmosphere).

VENUS TRANSITS OF THE PAST
The first person to predict a transit of Venus was Johannes Kepler, who
calculated that one would take place on 6 December 1631, just a month
after a transit of Mercury on 7 November. Though the transit of
Mercury was observed, the transit of Venus was not visible from Europe
and there is no record of anyone seeing it. Kepler himself died in
1630.

Jeremiah Horrocks (also spelled Horrox), a young English astronomer,
studied Kepler’s planetary tables and discovered with just a month to
go that a transit of Venus would occur on 24 November 1639. Horrocks
observed part of the transit from his home at Much Hoole, near Preston,
Lancashire. His friend William Crabtree also saw it from Manchester,
having been alerted by Horrocks. As far as is known, they were the only
people to witness the transit. Tragically, Horrocks’s promising
scientific career was cut short when he died in 1641, aged about 22.

Edmond Halley (of comet fame) realised that observations of transits of
Venus could in principle be used to find how far the Sun is from Earth.
This was a major problem in astronomy at the time. The method involved
observing and timing a transit from widely spaced latitudes from where
Venus’s track across the Sun would appear slightly different. Halley
died in 1742, but the transits of 1761 and 1769 were observed from many
places around the world. Captain James Cook’s expedition to Tahiti in
1769 is one of the most famous and went on to become a world voyage of
discovery. However, results on the Sun-Earth distance were
disappointing. The observations were plagued by many technical
difficulties.

Nevertheless, 105 years later, optimistic astronomers tried again. The
results were equally disappointing and people began to realise that
the practical problems with Halley’s simple idea were just too great
to overcome. Even so, by the 1882 transit, there was enormous public
interest and it was mentioned on the front page of most newspapers.
Thousands of ordinary people saw it for themselves.

In his 1885 book, “The Story of Astronomy” Professor Sir Robert
Stawell Ball described his own feelings on watching the transit 3
years earlier:

“To have seen even a part of a transit of Venus is an event to
remember for a lifetime, and we felt more delight than can be easily
expressed… Before the phenomenon had ceased, I spared a few
minutes from the somewhat mechanical work at the micrometer to take
a view of the transit in the more picturesque form which the large
field of the finder presents. The sun was already beginning to put
on the ruddy hues of sunset, and there, far in on its face, was the
sharp, round, black disk of Venus. It was then easy to sympathize
with the supreme joy of Horrocks, when, in 1639, he for the first
time witnessed this spectacle. The intrinsic interest of the
phenomenon, its rarity, the fulfilment of the prediction, the noble
problem which the transit of Venus helps us to solve, are all
present to our thoughts when we look at this pleasing picture, a
repetition of which will not occur again until the flowers are
blooming in the June of A.D. 2004.”

For an excellent historical summary, see:
http://sunearth.gsfc.nasa.gov/sunearthday/2004/vt_edu2004_venus_back_his.htm

THE FAMOUS ‘BLACK DROP’ PROBLEM

One of the chief problems visual observers of transits faced was
pinpointing
the exact time when Venus was first fully on the visible face of the
Sun. Astronomers call this point ‘second contact’. In practice, as
Venus crossed onto the Sun, its black disc seemed to remain linked to
the edge of the Sun for a short time by a dark neck, making it appear
almost pear-shaped. The same happened in reverse when Venus began to
leave the Sun. This so-called ‘black drop effect’ was one of the
main reasons why timing the transits failed to produce consistent
accurate results for the Sun-Earth distance. Halley expected second
contact could be timed to within about a second. The black drop
reduced the accuracy of timing to more like a minute.

The black drop effect is often mistakenly attributed to Venus’s atmosphere
but Glenn Schneider, Jay Pasachoff and Leon Golub showed last year that
the problem is due mainly to the way the brightness of the Sun falls off
around its visible ‘edge’. Astronomers call this effect ‘limb darkening’.

More experiments will be done on this phenomenon at the 8 June transit
of Venus using the TRACE solar observatory in space.

VENUS – THE PLANETARY EQUIVALENT TO HELL.

At first glance, if Earth had a twin, it would be Venus. The two planets
are similar in size, mass and composition, and both reside in the inner
part of the Solar System. Indeed, Venus comes closer to Earth than any
of the other planets.

Before the advent of the Space Age, astronomers could only speculate over
the nature of its hidden surface. Some thought that Venus might be a
tropical paradise, covered in forests or oceans. Others believed that it
was a totally barren, arid desert. After investigations by numerous
American and Russian spacecraft, we now know that Earth’s planetary
neighbour is the most hellish, hostile world imaginable. Any astronaut
unlucky enough to land there would be simultaneously crushed, roasted,
choked and dissolved.

Unlike Earth, Venus has no ocean, no satellites and no intrinsic magnetic
field. It is covered by thick, yellowish clouds – made of sulphur and
droplets of
sulphuric acid – that act like a blanket to trap surface heat. The upper
cloud layers move faster than hurricane-force winds on Earth, sweeping all
the way
around the planet in just four days. These clouds also reflect most of the
incoming sunlight, helping Venus to outshine everything in the night sky
(apart
from the Moon). At the present time, Venus dominates the western sky after
sunset.

Atmospheric pressure is 90 times that of Earth, so an astronaut standing on
Venus would be crushed by pressure equivalent to that at a depth of 900 m
(more
than half a mile) in the Earth’s oceans. The dense atmosphere consists
mainly
of carbon dioxide (the greenhouse gas that we breathe out every time we
exhale) and virtually no water vapour. Since the atmosphere allows the Sun’s
heat
in but does not allow it to escape, surface temperatures soar to more than
450 deg. C – hot enough to melt lead. Indeed, Venus is hotter than Mercury,
the planet closest to the Sun.

Venus rotates sluggishly on its axis once every 243 Earth days, while it
orbits the Sun every 225 days – so its day is longer than its year! Just as
peculiar is its retrograde, or “backwards” rotation, which means that a
Venusian would see the Sun rise in the west and set in the east.

Earth and Venus are similar in density and chemical composition, and both
have relatively young surfaces, with Venus appearing to have been completely
resurfaced 300 to 500 million years ago.

The surface of Venus comprises about 20 per cent lowland plains, 70 per cent
rolling uplands, and 10 per cent highlands. Volcanic activity, impacts, and
deformation of the crust have shaped the surface. More than 1,000 volcanoes
larger than 20 km (12.5 mls) in diameter dot the surface of Venus. Although
much
of the surface is covered by vast lava flows, no direct evidence of active
volcanoes has been found. Impact craters smaller than 2 km (1 ml) across do
not exist on Venus because most meteorites burn up in the dense atmosphere
before they can reach the surface.

Venus is drier than the driest desert on Earth. Despite the absence of
rainfall, rivers or strong winds, some weathering and erosion does occur.
The surface is brushed by gentle winds, no stronger than a few kilometres
per hour, enough to move grains of sand, and radar images of the surface
show wind streaks
and sand dunes. In addition, the corrosive atmosphere probably chemically
alters rocks.

Radar images sent back by orbiting spacecraft and ground-based telescopes
have revealed
several elevated “continents”. In the north is a region named Ishtar Terra,
a high plateau larger than the continental United States and bounded by
mountains almost twice as high as Everest. Near the equator, the Aphrodite
Terra
highlands, more than half the size of Africa, extend for almost 10,000 km
(6,250 miles). Volcanic lava flows have also produced long, sinuous
channels extending for hundreds of kilometres.

VENUS – FAST FACTS

  • Namesake: Roman Goddess of Love and Beauty. With few exceptions, features on Venus are named for notable women from all of Earth’s cultures.
  • Mean Distance from Sun: 108.2 million km (67.2 million mls)
  • Orbital Period: 224.695 days
  • Rotational Period: 243 days (retrograde)
  • Diameter: 12,100 km (7,520 mls)
  • Mass: 0.82 of Earth
  • Density: 5.24 g/cu. cm (slightly lower than Earth)
  • Gravity: 0.91 of Earth Atmosphere: 96 per cent carbon dioxide
  • Mean Surface Temperature: 457 degrees C
  • Number of Moons 0
  • Number of Rings 0

EXPLORATION OF VENUS – SIGNIFICANT DATES

  • 1962: Mariner 2 (US) – first successful flyby of Venus; verified high temperatures.
  • 1970: Venera 7 (USSR) – first soft-landing on Venus.
  • 1972: Venera 8 (USSR) – landed on Venus; transmitted nearly an hour of data.
  • 1974: Mariner 10 (US) – flew by Venus en route to Mercury; tracked global atmospheric circulation with visible and ultraviolet imagery.
  • 1975: Venera 9 (USSR) – sent back the first surface pictures of Venus.
  • 1978: Pioneer Venus Orbiter (US) – radar mapped Venus; Pioneer Venus Multiprobe (US) dropped four probes through Venusian clouds.
  • 1982: Venera 13 and 14 (USSR) – sent back first colour pictures of the surface.
  • 1983: Venera 15 and 16 (USSR) – provided high-resolution radar maps and atmospheric analyses.
  • 1984: Vega 1 and 2 (USSR) – released landers and balloons at Venus en route to Halley’s comet.
  • 1990-94: Magellan (US) – mapped 98 per cent of the surface of Venus using radar.
  • 2005: Venus Express (ESA) – scheduled to be first European spacecraft to orbit Venus.

TRANSIT EVENTS IN THE UK AND BEYOND

The University of Central Lancashire has been designated the UK hub for
Transit of Venus events. On their website (www.transit-of-venus.org.uk), an
events
calendar can be found. This provides individuals with the opportunity of
searching for transit of Venus activities in their local area as well as
allowing amateur astronomers, museums, science centres, school groups etc
to publicise their own transit event.

The web site also enables interested individuals (school pupils, amateur
astronomers etc) to do some simple observations of the transit and then
calculate
the value of the Astronomical Unit (the Sun-Earth distance) for themselves.
By
measuring the time at which the black dot of Venus reaches certain positions
along its path across the Sun’s bright disk and entering these times on an
interactive web page, anyone can make their own estimate of the Astronomical
Unit.

CONTACTS

Astronomers

Professor Gordon Bromage (University of Central Lancashire)
Tel: +44 (0)1772-893568 or (mobile) +44 (0)7905-308340
E-mail: gebromage@uclan.ac.uk
Special expertise: Transit of Venus; Stellar Physics

Dr Robert Walsh (University of Central Lancashire)
Tel: +44 (0)1772-893557 or (mobile) +44 (0)771-2189336
E-mail: rwwalsh@uclan.ac.uk
Special expertise: Transit of Venus; the Sun, SOHO, TRACE

Dr Andrew Coates (Mullard Space Science Laboratory, University College
London)
Tel: +44 (0)1483-204145 or (mobile) +44 (0)7788-448318
E-mail: ajc@mssl.ucl.ac.uk
Special expertise: Venus the planet, Venus Express mission, transit method
for extrasolar planet search

Historian

Dr. Allan Chapman (University of Oxford)
Tel: +44 (0)1865-251577
Fax: +44 (0)1865-277937

WEB LINKS

1. General and comprehensive sites about the transit

University of Central Lancashire, the UK Hub for Transit of Venus
activities.
Includes a press pack with images and other information.
http://www.transit-of-venus.org.uk/

European Southern Observatory: coordinating transit activities across
Europe http://www.vt-2004.org

Orpington Astronomical Society:
http://www.chocky.demon.co.uk/oas/venus.html

The Society for Popular Astronomy:
http://www.popastro.com/sections/planet/venustransit.htm

NASA Sun-Earth Day:
http://sunearth.gsfc.nasa.gov/sunearthday/2004/index_vthome.htm

Fred Espenak’s Transit of Venus site:
http://sunearth.gsfc.nasa.gov/eclipse/transit/venus0412.html

2. The planet Venus

http://www.nineplanets.org/venus.html
http://www.solarviews.com/eng/venus.htm
http://pds.jpl.nasa.gov/planets/choices/venus1.htm

3. Space missions

Venus Express

http://sci.esa.int/venusexpress/

TRACE

http://vestige.lmsal.com/TRACE/

SOHO

http://sohowww.estec.esa.nl/