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Saturn Yields More Secrets

Image of Saturn by Mark and Nigel Stronge taken on 17th December 2002 showing the square shadow where the planet's shadow meets the Cassini division.
Local astronomers may have solved a long-standing puzzle concerning the cause of the square shadow sometimes seen when the planet's shadow lands on certain parts of its prominent ring system.

Click on image for larger version
Drawings of the 1769 transit of Venus by Charles Green and James Cook
showing the black drop effect.

The report was published in a recent issue of the magazine 'Astronomy & Geophysics,' entitled 'Extending the Black Drop to Saturn'** by Mark Bailey (Armagh Observatory), David Stewart (Irish Astronomical Association) and Mark Stronge (East Antrim Astronomical Society). They propose that Saturn's square shadow results in a similar fashion to the notorious 'black drop' effect observed during transits of the inferior planets, Mercury and Venus, across the face of the Sun.

Lt. James (later Captain) Cook and Charles Green documented the effect during their trip to Tahiti to observe the transit of Venus in 1769. It seems that the black drop is caused by a combination of telescopic or instrumental effects and the unsteadiness of the Earth's atmosphere, when observing the sharp, dark edge of a planet against the edge of a background source of light (the Sun).

When Saturn's shadow falls on its rings near the principal gap, the Cassini division, the geometry and optical arrangement are such that the situation is the same as for the black drop effect, that is, a sharp planetary shadow meeting the abrupt edge of the bright background rings.

Saturn's tally of known moons has also just increased to 47. Twelve of the small moons - about 2 to 4 miles across - were co-discovered by British-born astronomer David Jewitt using the 8.2-metre Subaru telescope in Hawaii. They all have distant (about 12 million miles), highly elongated orbits around Saturn. Carolyn Porco of the Cassini Imaging CICLOPS Project, Space Science Institute, Boulder, CO, USA also announced the discovery of a thirteenth new satellite. The object was found in the Keeler gap at the outer edge of Saturn's A ring on six images taken on 1st May. The moon orbits the planet in roughly 14.5 hours at a distance of about 85,000 miles.

See also: **Extending the Black Drop to Saturn - PDF Format

FOR FURTHER INFORMATION CONTACT: John McFarland at the Armagh Observatory, College Hill, Armagh, BT61 9DG. Tel. (Direct): 028-3751-2962; Tel.: (Observatory): 028-3752-2928; jmfarm.ac.uk; Website: http://star.arm.ac.uk/


Last Revised: 2005 May 10th
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