Just when or by whom the instrument was thus scientifically perfected is not known. The calculations necessary to the projection of the hour lines upon a flat surface could hardly have been performed before Greek times. The Greeks ascribed the invention of the sundial to Anaximander, in the sixth century B. C., but sundials of various types had been known in various parts of the world long before then. On the other hand, the Hemicycle remained the common form of the instrument all through the classic period and even afterwards. The Babylonians were quite capable of understanding the principle of the sloping gnomon. And once this was discovered, it would have been entirely practical to set up the new dial beside a Hemicycle or Clepsydra, and find the angles of the hour lines by experiment. These, once laid out correctly, would be determined once for all. Even at its best the sundial had certain very marked limitations. Scientifically constructed, it would keep accurate time according to the visible sun. But it could not be read accurately unless made inconveniently large. It was inaccurate when removed from its original latitude, or displaced from a true North and South position; so that in any portable form it became a very rough measure indeed. Moreover, it was of course entirely useless at night or in bad weather or in shadow. And finally, it was never absolutely exact under the most ideal conditions, because of what is known as the Equation of Time. The Earth does not, in fact, move around the sun at an absolutely regular rate of speed; it moves a trifle faster during certain parts of the year and slower at others. The sun therefore varies correspondingly his apparent speed along the Ecliptic, so that even from noon to noon the sun is not always precisely on time. He may be as much as fifteen minutes late or early, according to the season. And our modern days are measured according to the sun's average rate, so as to allow for this variation and keep every day exactly twenty-four hours long. This of course no sun-dial can possibly be made to do, since it must follow the actual sun.
The sun-dial has remained in use to the present day. It seems strange to think of a sun-dial being used as a standard for setting clocks and actually to regulate the running of trains. But these things were done in civilized Europe within the last half century. It was only when the railroad and the telegraph had made standard time at once necessary and easy to obtain that the sun-dial altogether lost its position of authority.
Sun-Dials, Descriptions—Classical sun-dials were of many forms. Vitruvius, the Roman engineer, mentions thirteen, some of them portable; and ascribes the invention of the Hemicycle to the Babylonian astronomer and priest, Berosus. There was a famous dial of this type at the base of Cleopatra's Needle in Egypt. It is now at the British Museum. And the Emperor Augustus, returning from his Egyptian wars, brought home to Rome an obelisk which he set up as the gnomon of a huge dial in the Campus Martius. At Athens there was the famous Tower of the Winds; octagonal in shape, with a weather vane above, and below around the tower, the hours and the winds, to each of which the Greeks gave a personality and a name. There is a curious bit of accidental poetry in the marking of the sun-dial in Greece. The Greek numerals, like the Roman, were simply the letters of their alphabet arranged in a certain order. The hot hours of the day from noon to four o'clock were those commonly devoted by the Greeks to rest and recreation. Reckoning the day from sunrise, this period ran from the sixth hour through the ninth. And the numeral letters for Six, Seven, Eight and Nine, which marked those hours upon the dial, spell out the Greek word ΖἩΟΙ, the imperative of the verb to live. The poet Lucian thus points the moral:
Six hours to labor, four to leisure give;
In them—so say the dialled hours—LIVE.
The shepherds of the Pyrenees still consult their pocket dials. And the Turk makes a sun-dial of his two hands by holding them up with the tips of the thumbs joined horizontally and the forefingers extended upward; so that the shadow of one forefinger falls toward the other and by its position roughly indicates the time. But even now, when it has nearly gone from practical use, the sun-dial, as an appropriate adornment of our public parks and our private gardens, is becoming increasingly fashionable in our own generation.
OLD FRENCH WALL DIAL
Sun-dials are common in almost all parts of the world, and not a few of them have in one way or another become famous. The largest is at Jaipur in India, and was erected about 1730. Its gnomon is ninety feet high and one hundred and forty-seven feet long. A flight of stone steps run up the slope of it, and at the top there is a sort of little watch-tower. And the shadow, which falls upon a great stone quadrant instead of upon a flat surface, moves at the rate of two and a half inches a minute. Another great dial is the so-called Calendar Stone of Mexico, which was made by the Aztec priests more than a hundred years before the Spaniards came. It weighs nearly fifty tons, and is not only a sun-dial but a representation of the zodiac and a diagram of the astronomical changes of the year: thus showing that the ancient Mexicans in their own way paralleled the astrology of the Babylonians on the other side of the world. Probably the most expensive and elaborate sun-dial ever built was the one set up in 1669 by King Charles II of England in front of the banqueting house at White Hall in London. It was in the form of a tall pyramid on which were two hundred and seventy-one different dials, giving not only the hour of the day but various astronomical and geographical indications as well. The place called Seven Dials in London takes its name from a tall pillar with sun-dials around its top which used to stand at the junction of seven streets radiating starwise from that spot as a center. The pillar was overthrown in 1773 by a party of vandals digging for buried treasure which they believed to have been hidden beneath its base. Extensive list, descriptions and illustrations, See Book of Sun-dials, Mrs. Alfred Gatty; Sun-dials and Roses, Mrs. Alice Morse Earle.