In the beginning of the 1600's, traveling across continents was quite frequent, it was necessary for the people on-board ships to know their positions on earth. And it turns out from the previous post, sensing the latitude wasn't much of a trouble. On the other hand, the main problems with finding the longitude was the absence of a natural point of reference for its measurement and the difficulty in determining time. Latitude tells us how far north or south of the equator we are located. However, longitude tells us how far east or west we are from?, where? that's the problem.
The measurement of longitude is from an arbitrary line from north to south, the prime meridian. Today, that very line runs from the north pole exactly through the Greenwich Observatory and on to the south pole. In addition to it, to measure longitude, it is necessary that we ought to know the exact time. But, in the seventeenth century there where no accurate clocks, leaving the invention of the of the pendulum clock as an anomaly, as they were comparatively accurate. The heavy and cumbersome nature of these clocks, certainly could not have been used on a ship.
Nonetheless, the reason behind recognizing the exact time to determine the longitude on a ship was because, when a clock in Greenwich shows six o'clock in the morning, that's when the sunrises in Greenwich. As a result, that part of the Greenwich observatory is in the process of rotating out of the shadow and towards the sunlight. However, behind, in the west, it is still dark for a ship that is just on its trans-Atlantic route. A few hours later the sun will rise higher and reach its maximum point at Greenwich. Whereas, the captain on the ship will observe the sun as it still climbs, and it isn't yet mid-day for him at his location that is, in the west. The earth has to rotate a little more before it reaches to its peak. When two people at both of these locations, observe the sun, they obviously come up with different results. Since, one of them is further west. Now, because of this the solar noon's are different at the two instances. A solar noon is basically, the point of time when the sun has reached its Zenith. This occurs at different times, at different places, that is the reason why we have time zones.
Dialing it back to the two instances, when the sun is observed from the two positions they appear at different levels in the sky. If the people on the ship know the time in Greenwich, then they could use it to calculate their position. They could see how high the sun is in the sky, and also how far it is until it reaches its highest point. They know how long this will take, given that they recognize the time in Greenwich, where the sun has so far reached its zenith. As a consequence, they can find the difference to reckon how far west of Greenwich they are located. The "meridian" that goes through the place where Flamsteed worked, is used as the zero point. Now, the calculation of finding your position gets more convenient, the distance from the prime meridian is measured in degrees. Subsequently, a complete rotation around the earth would be 360 degrees, also we know the earth rotates around its axis in twenty four hours, dividing 360 by 24 we arrive at 15. As a result, the earth rotates by fifteen degree every hour. With that said, if the sun reaches its zenith exactly one hour earlier than than in the Atlantic, then it is safe to declare that you're fifteen degrees from the prime meridian at Greenwich.
The other problem that remains is determining the time from anywhere at sea. One could probably use a cosmological event to determine time, like a solar eclipse. One could use the eclipse to determine at what day and at what local time, the eclipse could be seen from Greenwich and then pass on the details to the people on-board a ship before they leave for their voyage. When the eclipse occurs the people on-board could take note of the local time at their location, by determining the position of the sun. They'll only need the difference between the local time that they calculate and the previously calculated time when the solar eclipse can be seen from Greenwich, as a consequence they can recognize their position. But, the problem is that solar eclipses aren't as frequent and it isn't a practical solution. And that is when they decided to use the moon as a cosmic clock. That's where Flamsteed got working, as stated in the previous post, about his role of mapping out the positions of the stars to track the motion of the moon with regards to those stars. As a result, the moon is like the clock's hand and the stars are the numbers. And these readings of the positions of stars could be carried out to the people responsible on the ship in a book, which did make the task of navigation through the sea much more convenient in the 17th century.