The true shape of the solid Earth—Early ideas—The Earth supposed to be the centre of the Universe—A revolution of ideas—Difficulty in realising that the Earth is flying through space—Astrology—Kepler's discovery—How we discovered the "year"—Why we require leap-years—The whole solar system moving en bloc

Some people imagine that this was a modem discovery, and that all the Ancients believed the Earth to be a flat disc floating upon water. But the astronomers who lived many centuries before Christ pictured this world as a huge sphere, and some of these early astronomers even taught that the Earth was spinning round upon its axis like a top. It is true that this idea was formed before the discovery of any proof that their picture was a true one. The first argument was that the Earth must be round because a sphere is the most perfect form. But while those learned men in the East did not doubt the rotundity of the Earth, the ignorant people who inhabited the British Isles and the West of Europe continued to believe that the world was flat, right on to the time of William the Conqueror.

Quite recently there appeared in one of our daily papers the report of a meeting of a society the members of which still maintain that the Earth is flat. These cranks are only advertising their own ignorance, for if any thoughtful person considers our present knowledge of the subject he cannot fail to be convinced of the roundness of the Earth. It was discovered that a ship on the horizon appears or disappears bit by bit, just as though it were ascending or descending a steep hill. No matter on what part of the world this observation is made, and no matter in what direction the mariner is looking, the result is always the same. Therefore there can be no doubt as to the shape of the Earth. Then we can actually see the shadow of the round earth cast upon the moon.

In these days of modern travel it seems incredible that anyone can imagine the Earth to be flat. A journey round the world is a comparatively common event. As children we were doubtless amused with the idea that the people on the other side of the world must be walking about, like flies on a-ceiling, with the Earth above them instead of beneath them. But later on when we came to consider that there is no up and down except relative to the earth, the mystery disappeared. We realised that no matter on what part of the great ball we happen to be we are held down to its surface by that force which we call gravitation, and the boundless sky is overhead.

It was a long time before man discovered the true position of his planet in the great Universe. He thought of his world as the one thing of greatest importance, and he pictured the starry vault of heaven as a great crystal dome resting upon the Earth, or encircling it. Even when the wise men studied their movements of the Sun and the other heavenly bodies, they believed the Earth to be the centre of the Universe around which all the heavenly bodies travelled. It does look as though the Sun rose in the east, climbed up over the sky, and dropped down in the west. When in a railway station, we ourselves have sometimes been cheated as to whether it was the train we were on board or a neighbouring train that was in motion.

While the most natural picture was that of the solid Earth at rest, there were some ancient philosophers who declared that the Sun was the central body and that the Earth travelled around the Sun. But these philosophers could not bring forward any observed facts to prove their theory, and so it was that their ideas were abandoned by succeeding generations, and men continued to believe the Earth to be the central body right on to the time when Queen Elizabeth ruled over England. It happened to be during her lifetime that the great Italian astronomer Galileo Galilei forced men to believe that the Earth goes round the Sun. The actual proofs of this great fact had been established several generations earlier by the great Polish astronomer Copernicus, but it was Galileo who brought the subject into prominence and caused a complete revolution in man's ideas.

Even now, after this great discovery has been firmly fixed in man's mind for three hundred years, it is no easy matter to realise that we are on the surface of a great planet which is spinning round and at the same time is flying through space at a prodigious speed. As we go about our daily duties it is difficult to realise that we are being carried through space at the speed of one thousand miles per minute, a thousand times faster than the most reckless motor-car, and sixty times faster than a rifle bullet. No wonder that it took man a long time to accept this great discovery!

It is true that the ancient astronomers were apt to let their science drift into "Astrology," which was practically fortune-telling, but we should remember that away back in those far-off days some of the astronomers did make a serious study of the heavenly bodies. If we are apt to think of all Science as being of very modern origin, we should keep before us the fact that when Julius Caesar besieged Alexandria, there was a great university in that town, where for centuries many learned professors had been at work. We should remember that it was there and then that Professor Euclid wrote those books on geometry which have worried many schoolboys, but which are indispensable in modern Science and Engineering.

The professors of that ancient university believed the heavenly bodies to move in circles, because the circle gave the most perfect motion, but the great German astronomer Johann Kepler discovered that the planets moved in ellipses or oval-shaped orbits. Kepler made this discovery in a rather curious way. He kept guessing different ways in which the planets might move, and then testing how each particular path would account for the observed motions of the planets. Formerly the astronomers had been forced to make a most elaborate arrangement of circles, one moving within another, in order to try and explain the motions of the planets. But Kepler's simple ellipses did away with all these intricate theories; he had discovered one of the great secrets of Nature.

Now we picture ourselves on board the Earth making a continuous journey in a great elliptical path around the Sun. We know that this journey takes us one year of 365 days; it will be of interest to consider how we discovered this great fact.

It was an easy matter for the Ancients to divide time into days, as each day is marked off clearly by the accompanying night. Then it was obvious to them that there was a regularly recurring cycle of the phases of the moon, and they noted that twenty-nine days elapsed between one "new moon" and the succeeding one. We have discovered since these early days that the month, the time of the moon's journey around the Earth, is not exactly twenty-nine days. It is almost twenty-nine and a half days, the actual time being 29 days 12 hours 44 minutes and 2.86 seconds.

The discovery of the "year" was not so easy, for the completion of our circuit around the Sun is not so apparent. However, it was obvious that there were regularly recurring seasons, and that the Sun was much higher in the heavens at noon in summer than in winter. This fact would cause the shadow of an object to be much shorter at noon in summer than in winter. By watching the shadow cast by some fixed object, it became clear which was the longest and which was the shortest day; mid-summer and midwinter. Then again, the shadows cast at sunrise and sundown would keep changing gradually their direction day by day, so that by noting carefully its positions at sunrise and sunset on any day, and counting the number of days until the shadows again reached the very same positions, it was possible to determine the length of the year. Indeed, the Egyptians discovered about five thousand years ago that the year consisted of three hundred and sixty-five days. This discovery was based upon the seasons alone. The Greeks had a year of three hundred and fifty-four days, and other nations had a different number of days.

Some centuries later the Egyptians discovered that the actual journey around the Sun took about a quarter of a day more than three hundred and sixty-five days, and now we know that the exact time of the journey is 365 days 5 hours 48 minutes and 48i seconds.

It is generally known that a "leap year" is introduced in order to get rid of this awkward fraction of a day coming into each year. It was our old friend Julius Caesar who introduced the leap year of 366 days in every fourth year. This would have cleared away the disturbing fraction if the figures had been exactly 365 ¼ days, but as already stated, the exact fraction is a little less than a quarter. At a much later date it was agreed to omit a leap year at every hundredth excepting the four hundredth year. The reason for all this juggling is very simple.

We start at a particular point in our orbit at the beginning of the year, but at the close of the last day of the year we have not quite reached that starting-point. We require an additional 5 hours 48 minutes and 48 seconds to reach that point. We get later by that amount each year, so that in four years we are about 231 hours behind time. We put an extra day in the fourth year to give our planet time to reach its proper starting-place before the next year opens. The extra day means 24 hours, whereas the planet will reach its goal in 281 hours. The Earth has set off f hour in advance of its proper time, and it will get ahead by that amount every fourth year, so that in one hundred years it will be about 19 hours in advance. We therefore agree to omit the leap year at every hundredth year, thus taking 24 hours off that year. This not only disposes of the hours which we had to spare but robs us of other 5 hours, making us late again to that extent. As we shall lose another 5 hours at every hundredth year, we shall be 20 hours late in four hundred years, and therefore we agree not to omit the leap year at that point. By keeping in the extra day we allow the Earth to reach its proper starting-place in good time. Indeed, we shall have a fraction to spare at every four-hundredth year, but it will take some thousands of years before this occasional addition reaches the total of one day, so that we can, leave some far distant descendants to put matters right then.

We picture ourselves on board this planet Earth, with our attendant moon, waltzing round this great elliptical path around the Sun. We picture the other planets making similar movements, some nearer and some farther from the Sun. Even then we must not picture the solar system as being fixed in space, for in these modern times we have discovered that the whole solar system is moving through space, and that even the far-distant "fixed" stars are in motion.

Having formed a picture of the Earth's movements in space, it will be of interest to consider some of the things we have discovered concerning our planet itself.