Timeline of astronomy Rectification of Planetary Theory", eliminated the need for an equant by introducing an extra epicycle, departing from the Ptolemaic system in a way very similar to what Copernicus later also did. Ibn al-Shatir proposed a system that was only approximately geocentric, rather than exactly so, having demonstrated trigonometrically that the Earth was not the exact center of the universe. His rectification is later used in the Copernican model. Nicolaus Copernicus publishes "De revolutionibus orbium coelestium" containing his theory that Earth travels around the Sun. However, he complicates his theory by retaining Plato's perfect circular orbits of the planets. A brilliant
Admiralty in the 16th century has organized the fleet into squadrons from at early 13th century and certainly during the 16th century. In 1560 three squadrons were operating in the Channel, Irish Sea and North Sea Until the 16th century, admirals were high officials under kings, and were charged with protecting the realm from sea attack. The English navy's Narrow Seas Squadron also called as the Eastern Squadron mainly operated in the two seas which laying between England and Kingdom of France (the English Channel particularly the Straits of Dover) and England and the Spanish Netherlands later the Dutch Republic (the southern North Sea). Include: