How astronomers defined time and Earth's rotation 🕛️🌍️ John Flamsteed, the first Astronomer Royal at the Royal Observatory, Greenwich in London, would measure solar time using a sundial and then apply the so-called equation of time to create Greenwich Mean Time, an average that could account for inaccuracies in solar time. He also measured the daily crossing of the bright star Sirius as it crosses the meridian every 23 hours and 56 minutes. This gave him very regular and consistent rates. And