Herschel is a huge crater in the leading hemisphere of the Saturnian moon Mimas, on the equator at 100° longitude. It is named after the eighteenth century astronomer William Herschel, who discovered Mimas in 1789. Herschel is the largest crater relative to its parent body of any equilibrium planetary moon in the Solar System. It is so large that astronomers have expressed surprise that Mimas was not shattered by the impact that caused it. It measures 139 kilometres (86 miles) across, almost one third the diameter of Mimas. Its walls are approximately 5 km (3.1 mi) high, parts of its floor are 10–12 km (6.2–7.5 mi) deep, and its central peak rises 6–8 km (3.7–5.0 mi) above the crater floor. If there were a crater of an equivalent scale on Earth it would be over 4,000 km (2,500 mi) in diameter and wider than Canada, with walls over 200 km (120 mi) high. The impact that formed Herschel must have nearly disrupted Mimas entirely. Chasmata that may be stress fractures due to shock waves from the impact traveling through it and focusing there can be seen on the opposite side of Mimas. The impact is also suspected of having something to do with the current 'Pac-Man'–shaped temperature pattern on Mimas. Herschel has an estimated age of around 4.1 billion years.