Transit slice.jpg
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.