No one knows how long this huge feature has been circulating in Jupiter’s atmosphere, but the Great Red Spot was first spotted more than three centuries ago, shortly after the telescope was invented. Since then, it has varied unpredictably in size, shape, color, and motion as it drifts among Jupiter’s clouds. The color changes from a deep brick red to whitish, presumably because of the composition of the planet’s highest clouds. Small clouds moving around the periphery of the Great Red Spot show that it circulates like a giant vortex. Its cloud pattern looks a lot like that of a terrestrial hurricane, but it rotates in the opposite direction, exhibiting anticyclonic rather than cyclonic flow. (Recall from Chapter 9 that anticyclonic flow is indicative of high-pressure systems.) Because of its colorful appearance and unpredictable changes, the Great Red Spot has long been a favorite target among amateur astronomers.