Many astronomers and members of the public in Britain will
be getting up early on the morning of June 6, so they can see (using
precautions to avoid permanent eye damage)* the final Transit of Venus
of the 21st century. The Transit, when Venus passes directly between
Earth and the Sun, was last seen in 2004 and will not happen again until
the year 2117.
NEVER look directly at the Sun, with or without a telescope or pair of binoculars, without using a safe solar filter. To do is very dangerous and is likely to result in permanent blindness. Young people watching the transit of Venus in 2004, using approved solar filters. (Credit: Charles Barclay)
To help the public view this rare phenomenon, the Royal Astronomical
Society has created an online resource about the Transit, including a
map of public events around the UK.
In astronomy, transits take place when a smaller body passes in front
of a larger one. Although the Sun, Venus and Earth roughly line up
every 584 days, the orbits of Earth and Venus around the Sun are tilted
with respect to one another. This means that Venus normally appears to
pass above or below the Sun. A transit only takes place on the rare
occasions when the three bodies are almost exactly in line.
On average, Transits of Venus happens only every 80 years or so. This
average figure is however very misleading, because transits occur in a
'pair of pairs' pattern that repeats every 243 years. First, two
transits take place in December (around Dec 8th), 8 years apart. There
follows a wait of 121 years 6 months, after which two June transits
occur (around June 7th), again 8 years apart. After 105 years 6 months,
the pattern repeats.
Prior to the last transit on 8 June 2004, no living person had seen a
Transit of Venus (the previous one was on 6 December 1882). The
forthcoming Transit will take place on 5-6 June 2012, but only the final
stages will be visible from the UK. The entire event will be seen from
eastern Asia and Australasia, the Pacific Ocean and the north-western
parts of North America. The next transit of Venus after that will not
take place until 2117, so 2012 will be the last chance in most of our
lifetimes to see this extraordinary celestial event.
Observing the transit
*NEVER look directly at the Sun, with or without a telescope or pair
of binoculars, without using a safe solar filter. To do is very
dangerous and is likely to result in permanent blindness.
The 2012 Transit of Venus occurs on 5-6 June, with the whole event
lasting slightly under seven hours. The transit starts at 23:04 British
Summer Time (22:04 UTC) on 5 June, after the sun has set in the UK. It
will take about 20 minutes from the point when Venus first encroaches
onto the disk of the Sun ('first contact') until the planet is fully
silhouetted ('second contact'). The planet will then take a curved path
across the northern part of the Sun. Mid-transit is at about 02:30 BST
(01:30 UTC) on 6 June. Venus begins to leave the Sun ('third contact')
at about 05:37 BST (04:37 UTC), and the transit will be over ('fourth
contact') at 05:55 BST (04:55 UTC). Timings differ by a few seconds for
different latitudes, but the transit will be visible from any place
where the Sun is up (clouds permitting).
Unfortunately, this means that only the final stages of the transit
will be visible from the UK. As a guide, on 6 June the Sun rises at
04:30 in Edinburgh, 04:46 in London, 04:51 in Belfast and 04:58 in
Cardiff (all BST), so observers in the UK will be able to see the final
hour or so of the event.
During the Transit, Venus will be visible in silhouette as a dark
disc set against the bright solar surface or photosphere. The planet is
about 1/32nd of the diameter of the Sun, so it will block about 0.1% of
the Sun's light from reaching Earth. Venus will be large enough to be
just visible to someone with normal eyesight, without the help of
binoculars or a telescope (this should NOT be attempted without
appropriate safe solar filters).
For safe viewing of the transit, the same rules apply as those for
observing a partial or annular eclipse of the Sun. Eclipse viewing
glasses 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.
European observers should only use eclipse glasses that are marked CE
under the EU Directive on the safety of Personal Protective Equipment.
Observers in other locations should also use only approved safe eclipse
glasses. These are certified to conform to an agreed and effective
safety standard. Under that specification the glasses or their packaging
must be marked with any applicable obsolescence deadline (colloquially,
the 'best-before' date). The capacity of eclipse glasses to block
harmful radiations from the sun reduces with time. For example, glasses
bought for the total solar eclipse in Britain in 1999 are now nearly
thirteen years old and should not be used. In any case observers should
inspect pre-used glasses for damage (for example scratches, holes or a
weakened mounting for the lenses) and consider replacing them if there
is a risk that their effectiveness in protecting the eyes is reduced.
The best way to see the Transit in the UK is to join one of the 15
public observing events taking place around the country (see the map on
the RAS Transit page: http://www.ras.org.uk/transit2012).
In many cases these are led by amateur astronomy groups who are well
placed to offer access to equipment and to give advice on viewing the
The scientific significance of transits
In the 18th and 19th centuries, transits of Venus presented valuable
opportunities to tackle a fundamental problem of the time -- finding an
accurate value for the distance between Earth and the Sun, called the
'Astronomical Unit' (AU). Modern determinations of the AU fix it at
149,597,870.691 km (and involve a complex technical definition, as
Earth-Sun distance varies slightly with time).
In the 21st century, much of the interest in the transit of Venus of
2012 is its rarity as an astronomical spectacle, the educational
opportunities it presents, and the sense of a link with important events
in scientific and world history.
Astronomers are now particularly interested in the general principle
of planet transits as a way of hunting for and characterizing the nature
of planetary systems around other stars. When a planet crosses in front
of its parent star, there is a minute dip in the star's apparent
brightness. Identifying such dips is a useful method of finding planets
orbiting other stars, and studying their properties. The 2012 transit
will be observed by the Hinode solar observatory, the Hubble Space
Telescope (that will look at the Moon during the Transit), the Venus
Express mission currently in orbit around Venus and by a number of
observatories on the ground.
The Royal Astronomical Society: the Transit of Venus
More detailed information on the 2012 Transit of Venus, historic
observation of Transits, scientific experiments and a map of UK public
observing events can be found on the RAS website at http://www.ras.org.uk/transit2012