As the power and commerce of Spain declined, two other nations entered into the contest for the sovereignty of the seas, and with that sovereignty predominance in the New World of America—France and England. The problem of the longitude at sea, or, as already pointed out, what amounts to the same thing, the problem how to determine when at sea the local time at some standard place, became, in consequence, of greater necessity to them.
The standard time would be easily known, if a thoroughly good chronometer which did not change its rate, and which was set to the standard time before starting, was carried on board the ship. This plan had been proposed by Gemma Frisius as early as 1526, but at the time was a mere suggestion, as there were no chronometers or watches sufficiently good for the purpose. There was, however, another method of ascertaining the standard time. The moon moves pretty quickly amongst the stars, and at the present time, when its motions are well known, it is possible to draw up a table of its distances from a number of given stars at definite times for long periods in advance. This is actually done to-day in the Nautical Almanac, the moon's distance from certain stars being given for every three hours of Greenwich time. It is possible, then, by measuring these distances, and making, as in the case of the latitude, certain corrections, to find out the time at Greenwich. In short, the whole sky may be considered as a vast clock set to Greenwich time, the stars being the numbers on the dial face, and the moon the hand (for this clock has only one hand) moving amongst them.
The local apparent time—that is, the time at the place at which the ship itself was—is a simpler matter. It is noon at any place when the sun is due south—or, as we may put it a little differently, when it culminates—that is, when it reaches its highest point.
To find the longitude at sea, therefore, it was necessary to be able to predict precisely the apparent position of the moon in the sky for any time throughout the entire year, and it was also necessary that the places of the stars themselves should be very accurately known. It was therefore to gather the materials for a better knowledge of the motions of the moon and the position of the stars that Greenwich Observatory was founded, whilst the Nautical Almanac was instituted to convey this information to mariners in a convenient form.
This proposal was actually made in the reign of Charles II. by a Frenchman, Le Sieur de Saint-Pierre, who, having secured an introduction to the Duchess of Portsmouth, endeavoured to obtain a reward for his scheme. It would appear that he had simply borrowed the idea from a book which an eminent French mathematician brought out forty years before, without having himself any real knowledge of the subject. But when the matter was brought before the king's notice, he desired some of the leading scientific men of the day to report upon its practicability, and the Rev. John Flamsteed was the man selected for the task. He reported that the scheme in itself was a good one, but impracticable in the then state of science. The king, who, in spite of the evil reputation which he has earned for himself, took a real interest in science, was startled when this was reported to him, and commanded the man who had drawn his attention to these deficiencies 'to apply himself,' as the king's astronomer, 'with the most exact care and diligence to the Rectifying the Tables of the Motions of the Heavens and the Places of the Fixed Stars, in order to find out the so much desired Longitude at Sea, for the perfecting the Art of Navigation.'
This man, the Rev. John Flamsteed, was accordingly appointed first Astronomer Royal at the meagre salary of £100 a year, with full permission to provide himself with the instruments he might require, at his own expense. He followed out the task assigned to him with extreme devotion, amidst many difficulties and annoyances, until his death in 1719. He has been succeeded by seven Astronomers Royal, each of whom has made it his first object to carry out the original scheme of the institution; and the chief purpose of Greenwich Observatory to-day, as when it was founded in 1675, is to observe the motions of the sun, moon, and planets, and to issue accurate star catalogues.
It will be seen, therefore, that the establishment of Greenwich Observatory arose from the actual necessity of the nation. It was an essential step in its progress towards its present position as the first commercial nation. No thoughts of abstract science were in the minds of its founders; there was no desire to watch the cloud-changes on Jupiter, or to find out what Sirius was made of. The Observatory was founded for the benefit of the Royal Navy and of the general commerce of the realm; and, in essence, that which was the sole object of its foundation at the beginning has continued to be its first object down to the present time.
It was impossible that the work of the Observatory should be always confined within the above limits, and it will be my purpose, in the pages which follow, to describe when and how the chief expansions of its programme have taken place. But assistance to navigation is now, and has always been, the dominant note in its management.