HC Russell’s observations at the end of the transit as seen from Sydney Observatory. Photo lithograph from Observations of the Transit of Venus, 9 December, 1874. Powerhouse Museum Research Library

In the previous post I considered the preparations of Henry Chamberlain Russell, the director of Sydney Observatory, for the 1874 transit and the magnificent illustrated book that he published on the event. Here I show a couple of the illustrations from the book.

Russell observed with the new 29-cm or 11½-inch refractor or lens telescope from Hugo Schroeder of Hamburg, Germany. To reduce the heat from the Sun he used an aperture to reduce the main lens to 5 inches (12.5 cm) in width and coloured glass filters in front of and behind the eyepiece. From Sydney the transit began just before local noon and within a few minutes he could see the aureole on the part of the planet still outside the disc of the Sun. He described what he could see as, ‘It was very remarkable and beautiful, like a fringe of green light, through which the faintest tinge of red could be seen’. Since Russell was looking through coloured filters the colours that he describes may not be real.

Four hours later Venus was again at the edge of the Sun prior to egress. In his illustration of the egress Russell presents a sequence of five images with time increasing to the left. Though Russell emphasises that he did not see ‘the black drop’, in the first drawing of the sequence, made just after internal contact, we can see some haziness that is clearly due to similar or the same atmospheric effects as the black drop. Two minutes later the aureole that he called the ‘halo’ was clearly visible on the part of Venus off the Sun. Another 15 minutes later he says, ‘the halo was for the first time seen irregular–in diameter it seemed considerably broader at the north pole of the planet as shown’. For the last few minutes before the planet completely left the Sun, Russell was struggling with poor definition due to approaching clouds, but a white patch could be seen near the north pole of the planet.

Amateur astronomer Mr Alfred Fairfax’s drawing of the aureole through a 4¾-inch (12-cm) lens telescope. The scale of the aureole is greatly exaggerated to allow details to be shown. Photo lithograph from Observations of the Transit of Venus, 9 December, 1874. Powerhouse Museum Research Library

The aureole is due to sunlight refracted through the atmosphere of Venus, but why was there a brightening near the pole of the planet? This went unexplained for 130 years until the 2004 transit. In an article in the Astronomical Journal (141:112 (9pp), 2011 April) Jay M. Pasachoff, Glenn Schneider, and Thomas Widemann indicate that they saw the same effect with the TRACE spacecraft, this time with a brightening near Venus’ south pole. They explain their observations and those of Russell by appealing to previous spacecraft observations of the structure of the planet’s atmosphere. The observations indicate a ring or torus of cold air surrounding each of the poles of the planet that lower the average cloud top height by about 10 km. Extra sunlight can thus stream through regions surrounding the poles of the planet to create the polar spots.

Russell would have been thrilled to have his observations explained. Maybe the 2012 transit will lead to an explanation of those by Mr Fairfax!

This blog post is simultaneously published on the Transit of Venus website