One Universe at a Time: In 1665 Gian Domenico Cassini observed a feature on Jupiter. It was described as an oval about 1/7 the width of the planet itself, and since it appeared reddish in color it became known as the Great Red Spot. Since then the spot has been observed over the years by both professional and amateur astronomers. It has been Jupiter's most prominent feature for nearly four centuries. The post Spot On appeared first on Brian Koberlein. In 1665 Gian Domenico Cassini observed a feature on Jupiter. It was described as an oval about 1/7 the width of the planet itself, and since it appeared reddish in color it became known as the Great Red Spot. Since then the spot has been observed over the years by both professional and amateur astronomers. It has been Jupiter’s most prominent feature for nearly four centuries. Early on it was thought that the spot was a feature of Jupiter’s surface, and it was used to estimate the rotation of the planet. We now know that Jupiter is a gas giant, and it rotates differentially like the Sun. The polar regions rotate a bit more slowly than the equatorial region, and the red spot moves at ...