With an astrolabe the trained men could tell direction, the hour and minute, the end of twilight

The revival of interest in science brought many changes in Europe, and the craftsmen began to find other uses for the simple instrument. They hollowed out its dial, and filled the space with interchangeable plates that could be used for different latitudes. They covered the faces with lines and figures to serve the purpose of nautical almanacs and slide rules. With an astrolabe the trained men could tell direction, the hour and minute, the end of twilight, the "houses of the heavens" and the "unequal hours" which remained in astrological use many years after the practical man had turned to the even hours of mechanical clocks. All the information required by a traveler, except the altitude of a heavenly body, was given on the instrument, and it helped him to determine even the altitude by means of its alidade and its graduated circle. The ecliptic was shown, and twelve to thirty bright stars were mapped out on the "rete" or net, that covered the plates.

At the earnest request of his son who did not know much Latin, Geoffery Chaucer wrote a treatise in English on the astrolabe:

"Litel Lewis, my sone, I have perceived by certain evidences thyn abilite to lerne sciencez and wel I consider in special to lerne the Tretis of the Astrolabie. Therefore I have given thee a suffisaunt Astrolabie compowned after the latitude of Oxenford; upon which, by mediacion of this litel tretic, I purpose to teach thee a certain nombre of conclusions apertening to the same instrument."

Thereafter the great poet discoursed most charmingly, if rather technically upon the uses of the instrument.

With its many advantages the astrolabe became fairly common, but its beauty increased with its popularity. Between the sixteenth and nineteenth centuries craftsmen expended so much care and ingenuity upon it, that today a fine astrolabe ranks among the highest products of the metalworkers art. Very elaborate examples were constructed for the potentates of Europe. Queen Elizabeth's exquisite astrolabe, which was found in Oxford during 1936, has actual stars carved on the net. The primary purpose of any astrolabe was its use, so the ornamentation never became degraded. With its delicate engraving, its compactness, above all, the required precision by which all its decorations were tempered to necessity, the astrolabe is a wonderful example of beauty in the "applied arts."

In one part of the world or another, the astrolabe continued to be used for over eighteen hundred years until it was displaced by more accurate instruments for measuring altitude. The "forestaff," the "backstaff," the "quadrant" took its place and finally the "sextant" which is always used today on the ocean, as the "theodolite" * and "transit" are used on land. But we have in our possession an astrolabe with which we have taken many observations both by day and night. It is over four hundred years old, and today we can determine the time by it to within five or ten minutes of the correct time.

Both the astrolabe, and the instruments which have taken its place, are used for measuring the altitude of a heavenly body. From the altitude, time and latitude are determined. The observations can be made with equal precision on the Sun by day, and a star or planet by night.

If the observer is telling time by the Sun, he must first find his own latitude from a map or by a meridian observation. The declination of the Sun, he can find in a Nautical Almanac, and he measures the altitude.

When he has these three basic requirements his calculations commence. If he has made his observation in the morning, he must compute how much time will elapse, before the Sun reaches the meridian; in the afternoon he calculates the length of time which has passed, since the Sun crossed the meridian line. The calculations can be made mathematically with the aid of a formula in spherical trigonometry and logarithms; or, if the astronomer prefers, they can be made graphically on an astrolabe or a slide rule.

This distance from the meridian is called the "hour angle of the Sun." Of course the astronomer is observing the visible Sun in the heavens; but the visible Sun does not keep clock time. In all these centuries, while we have been adjusting ourselves to the demands of mechanism, the Sun has continued with its old irregularities. We see the Sun's time on our sundials; and we know that sundial time may be fifteen minutes fast or slow of the clock. The difference between the sundial and the clock is known as the "Equation of Time."

The amounts of these variations have been measured and computed for every hour in the year. Tables in Nautical Almanacs give the corrections necessary for converting Sun time into clock time. We set our clocks not to the visible or "apparent Sun," but to an imaginary or "mean Sun" whose rate is the average rate of the apparent Sun on every day of the year.

The observer makes this correction, and, after applying a further correction for longitude, he can set his watch with "the gratifying feeling that his duty has been done."

If he is observing a star, his procedure is necessarily somewhat different. He calculates its hour angle as before, from his latitude, and the declination and the altitude of the star. Then he uses appropriate tables to ascertain the difference between the hour angle of the star and the hour angle of the mean Sun. Again he must correct for longitude, but since he has already made his connections to the mean Sun, rather than to the Sun itself, the equation of time does not enter into his calculations.

Whether the Sun or a star be used, the observer ends by setting his watch to mean solar time at the standard meridian.

The standard meridian is important. The Standard Meridian to which all times ultimately refer, is the longitude of the Observatory of Greenwich, near London. But for convenience many other standard meridians are used. There are four in the United States. Eastern Time (five hours west of and slower than Greenwich) corresponds roughly to the longitude of Philadelphia; Central Time (six hours west of Greenwich) is about the longitude of St. Louis; Mountain Time (seven hours west of Greenwich) happens to be the exact meridian of Denver, Colorado; and Pacific Time (eight hours west of Greenwich) is about at Reno, Nevada. In order to keep each part of the country as near as possible to its corresponding solar time, the practical divisions are made, not at the longitudes, but midway between; as anyone who has ever missed a train at Detroit or Ogden is only too well aware.