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The Planet Mercury by Andrew James, ASNSW

Friday 10th February 2017 - 8:00pm
Topic: The Planet Mercury
Speaker: Andrew James, ASNSW
Venue: Epping Creative Centre - 26 Stanley Road, Epping
Abstract: Although known since ancient times, our overall knowledge of the small rose-coloured planet, Mercury, has until recently been quite incomplete. This nearest planet to the Sun, is often visually difficult to see, as it is usually placed near to the east or west horizon, regularly becoming immersed in the morning or evening mirky glow of twilight. Such proximity is not just inconvenient in observing the planet, but has greatly hindered the discovery of its orbit and understanding its general appearance and nature. This talk will principally focus on several long-term historical problems assigned in determining Mercury's orbit and its predicted ephemerides, whose substantial troubles mostly stem from gaining accurate astrometric positions. Earliest orbital elements were first derived by infrequent Mercury transits across the Sun, but this proved mostly unsuccessful due to its higher inclination to the ecliptic and its eccentricity. Improvements only occurred once additional positions were achieved by new transit observations across the meridian. This awkward task fell on the quite extraordinary but poorly known French astronomer, Citizen (Jacques) Vidal who made these observations at Mirepoix between 1790 and 1805 - some made when Mercury was within one-quarter the apparent diameter of the Sun. By combining these observations with the Mercury transits later led to Jerome Lalande's newer provisional orbital elements followed by those made by Urbain La Verrier. However, these solutions remained problematic, especially regarding discrepancies withprecessional changes of orbital nodes on successive perihelia. In turn, this led to competition between French and English astronomers, who wrongly had assigned the cause of the perturbations to either smaller asteroid bodies or an intra-orbiting planet to be named Vulcan. These deviations were later only fixed during the early 20th century by applying the successful ideas of Albert Einstein's 'Theory of General Relativity', therefore enabling Mercury's orbit to be properly established. Other briefer highlights during this talk include; Mercury's telescopic appearance, planetary rotation, and some of the very recent discoveries found by the NASA interplanetary spacecraft, Messenger.

Biography: Andrew James is a long-time member of ASNSW and is currently the Deep Sky and Planetary Nebulae Section Leader.

Event Date: 
Friday, 10 February, 2017 - 20:00
Epping Creative Centre, 26 Stanley Road, Epping NSW

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