So basically the annual average time that the sun crossed directly over the Prime Meridian at noon. Greenwich Mean Time or GMT was created to observe the average solar time at the location of the Prime Meridian. OK – so before we move on, if you are a bit rusty on Meridians of Longitude and Lines of Latitude you can read more about them in this blog post about some basic navigation terminology. All points on the globe are measured from here in degrees either East or West of Greenwich and North or South of the equator. In 1851 Greenwich was adopted as the location for the Prime Meridian – The basis of Longitude, and this was used in combination with lines of Latitude to create a grid over the globe for mapping and time keeping by Nations around the world. The purpose of the observatory was ‘to apply with the most exact care and diligence to the rectifying of the tables of the motions of the heavens, and the places of the fixed stars, so as to find out the so much desired longitude of places for the perfecting of the art of navigation’ Way back in the 1675, King Charles II built an observatory in Greenwich, England. Ok lets look at the time and why this is so important to working out our position. I have just also added a quick quiz at the bottom of that post, so if you want to skip back and read that first to check your learnings, you can do so by clicking here. Hopefully you have already read the post about your Sextant. Thanks again for reading along on my Celestial Navigation notes as I study for the Ocean Yachtmaster Exam.