Eclipses of the Sun, when the Moon moves in front of our parent star, are
spectacular events and widely noticed. Transits, on the other hand, when the
inner planets Mercury or Venus cross the face of the Sun, are far rarer and
much more difficult to spot. They are sometimes termed mini-eclipses, but even
this modest description exaggerates their visual impact. Transits of Mercury,
the innermost planet, cannot be seen at all without some form of optical aid,
since Mercury’s tiny silhouette is overwhelmed by the brilliant solar surface. Transits of
Venus are visible to the naked eye given a suitably dark filter to dim the Sun’s radiance, but all that can be seen is a black dot like an oversized sunspot
traversing the face of the Sun over several hours. As with many natural
phenomena, understanding
what you are seeing is the key to appreciating it fully.
Despite being visually unexceptional, transits of Venus have long been of great
scientific importance. In the 18th and 19th centuries they offered the best way
to measure the size of the Solar System. Following a suggestion by Edmond Halley in 1716 that the timing of transits made at widely spaced locations could be
used to find the distance of Venus by simple trigonometry, various countries
sent expeditions around the globe at the transits of 1761, 1769, 1874, and 1882. Nowadays we have more accurate methods involving radar, but those pioneering
efforts, the first example of international scientific cooperation, were
significant early steps in establishing the scale of the Universe around us.
Due to a somewhat complex relationship between the orbital periods of Venus and
Earth, transits of Venus usually come in pairs eight years apart, separated by
a gap of over a century. The last pair was in 2004 June and 2012 June. We must
now wait until 2117 December and 2125 December for the next.
In the table below the stamps are listed alphabetically by country of issue.
Clicking on the stamp will take you to a larger version and further
information.
