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The Great White Spot, also known as Great White Oval, on Saturn, named by analogy to Jupiter's Great Red Spot, are periodic storms that are large enough to be visible by telescope from Earth by their characteristic white appearance. The spots can be several thousands of kilometers wide. Currently, a large band of white clouds called the Northern Electrostatic Disturbance (because of an increase in radio and plasma interference) is enveloping Saturn since 2010, and the ''Cassini'' orbiter is tracking the storm. ''Cassini'' data has revealed a loss of acetylene in the white clouds, an increase of phosphine, and an unusual temperature drop in the center of the storm. As of April 2011, the storm underwent a second eruption.〔(Dissecting Saturn's Big Storm ) - SkyandTelescope.com〕 ==Occurrence== The phenomenon is somewhat periodic at 28.5-year intervals, when Saturn's northern hemisphere tilts most toward the Sun. The following is a list of recorded sightings; years with spots generally considered to be part of the cycle are 1876, 1903, 1933, 1960, and 1990. The next appearance should be in 2016. * 1876 – Observed by Asaph Hall. He used the white spots to determine the planet's period of rotation. * 1903 – Observed by Edward Barnard. * 1933 – Observed by Will Hay, comic actor and amateur astronomer. Until recent times the most celebrated observation. * 1960 – Observed by JH Botham (South Africa). * 1990 – Observed by Stuart Wilber, from 24 September through November. * 1994 – Studied by ground-based observers and the Hubble Space Telescope.〔(HubbleSite - NewsCenter - Hubble Observes A New Saturn Storm (12/21/1994) – Release Text )〕 * 2006 – Observed by Erick Bondoux and Jean-Luc Dauvergne. * 2010 – First observed by Anthony Wesley.〔(Vast Storm Rampages Across Saturn: Discovery News )〕 That none were recorded before 1876 is a mystery, in some ways akin to the long observational gap of the Great Red Spot in the 18th and early 19th centuries; the 1876 GWS was extremely prominent, being visible in apertures as small as 60 mm. Was the earlier record simply poor, or was the 1876 GWS truly a first for the telescopic era? Some believe that neither scenario is likely.〔Kidger (1992) p. 179〕 Mark Kidger (1992) has described three significant GWS patterns: #The GWSs alternate in latitude, with one apparition being limited to the North Temperate Zone (NTZ) or higher, and the following being limited to the Equatorial Zone (EZ). For instance, the 1960 GWS was high-latitude, and the 1990 GWS was equatorial. #The high-latitude GWSs recur at a slightly shorter interval than the equatorial GWSs (~27 versus ~30 years). #The high-latitude GWSs tend to be much less prominent than their equatorial counterparts. Based on these apparent regularities, Kidger forecast that the next GWS will occur in the NTZ in 2016, and will probably be less spectacular than the 1990 GWS.〔Kidger (1992) p. 180〕 抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)』 ■ウィキペディアで「Great White Spot」の詳細全文を読む スポンサード リンク
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