August 25th, another double anniversary

August the 25th is quite an ostentatious date for the telescope. Two events hit on that date that have repercussions even now.

400 years ago on August 25th 1609, Galileo took Venetian law and policy makers up to the top of the bell tower of St Mark’s square. There he demonstrated use of the telescope (with eight times magnification) and impressed them enough to receive a doubling of his salary and a job for life at the University of Padua. 400 years of the telescope is an International Year of Astronomy, 2009 movie celebrating the events of 1609, which has a planetarium version too. The type of telescope Galileo used is replicated by the Galileoscope, which is available for purchase.

The second anniversary is of something that happened twenty years ago and four light hours away. The Voyager 2 space probe had been travelling in space for twelve years, having taken unparalleled views and data from closer in planets. Now it reached planet number eight, the last of them – Neptune. Since its discovery in 1846, Neptune had shown nothing to us but a tiny blue disc, the distance being so great between it and us that very powerful telescopes are required to see anything of use. But Voyager 2 was a lot closer than we are and pointing its television cameras down, it filled in a lot of detail about this distant world. The blue disc came alive with white crystal methane clouds and giant dark spots – enormous storms larger than the Earth, similar to Jupiter’s great red spot, but much shorter lifespans, follow up observations with Hubble have shown a great dark spot in residence about fifty percent of the time viewed. Voyager 2 discovered six new Moons, saw the rings were complete rings rather than just arcs as had been seen from Earth and measured the massively offset magnetic field. It carried out atmospheric measurements that remain our best to date and took in situ particle data. Even now, twenty years later, the data continues to surprise with one guy eking out an image of a Moon transit and shadow transit across Neptune’s disc, the only recording we have of such an event at this planet (all other planets with satellites have had these things seen). Astronomy Now has a retrospective on the event.


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