The accelerating circulation of Jupiter's Great Red Spot

Abstract

Jupiter's Great Red Spot (GRS) has been evolving, with fluctuations, since it was first observed in the 19th century. It has shown trends of decreasing length, decelerating drift rate, and possibly accelerating internal circulation. This paper documents how these trends have progressed since the time of the Voyager encounters in 1979, up to 2006, from ground-based amateur observations. The trends in length and drift rate have continued; the GRS is now smaller than ever before. The internal circulation period was directly measured in 2006 for the first time since the Voyager flybys, and is now 4.5 Earth days, which confirms that the period is shortening. In contrast, the 90-day oscillation of the GRS in longitude continues unchanged, and may be accompanied by a very small oscillation in latitude.