Time: From The Astronomer’s Standpoint

TIME: FROM THE ASTRONOMER'S STANDPOINT A. VIBERT DouGLAS T IME is one of the most fundamental and common of all conceptions; it is also one of the most illusive. The qualifying words, "from the astronomer 's standpoint" have been added in order to warn off all optimistic philosophers who might otherwise be led to expect that what is to follow will contain learne~ remarks upon abstruse aspects of the subject of time, such as these-What is time? Has it an absolute quality, or is it purely relative in the sense in which the theory of relativity regards both it and space? Is time directional ? How does the mind of man become conscious of the passing of time? Ought we to regard entropy as "time's arrow" in the physical world, as has been suggested by Sir A. S. Eddington? Are time past and time future equally real in the sens~ in which the passing moment which at a certain critical instant we label the present, is real? These are problems which very few people can discuss to the advantage of others. From an astronomer's standpoint, however, the question of time is less metaphysical, and it embraces such matters of practical importance as the selection of units of time, of arbitrary cycles of time to form the basis of a calendar, and the precise measurement of time. The unit of time, which by its very nature has been universally adopted as the basis of time-keeping, is ·the day. If we take the average time whic~ elapses from sunrise to the next sunrise, averaged throughout an entire year, we get the unit known as the mean solar day. Time soB l f _· I. { ' TIME: FROM THE ASTRONOMER'S STANDPOINT measured thus, by the rate of rotation of the earth upon its axis, is called astronomical time. Here, then, is our fundamental unit; but just as in the matter of units of length it .is convenient to have large units as well as small, so also we require a larger practical unit tha·n the day for purposes of chronology. You would not think of measuring the distance from Montreal to Vancouver or from earth to moon in feet; neither would. you ordinarily wish to measure in days the time from the battle of Hastings to the present. We must cast about then for a larger unit of time. Mankind has for the most part passed over the unit provided by the revolution of the moon with its cycle of phases, the synodic month, and has settled upon a very natural, practical unit, the year. Now when we come to consider the question of the number of mean solar days in a year,.we find that there are three equally logical ways of measuring this, but that each gives a different result. The earth's orbit being an ellipse, there is one point on it at which the earth is nearer to the sun than at any other point. Shall we, then, define the year as that interval of time between one perihelion passage and the next? This turns out to be 365.25964 days and is called the Anomalistic Year. It is not, however, a useful unit, for the entire orbit is slowly swinging around relative to the sun and stars, and this causes the perihelion point to move·eastward eleven seconds of arc per year. If we measure the year as the time required by the earth to make one complete circuit of its orbit relative to a point in exact alignment between the sun and some selected star, we·th~n have what may be regarded as the true mechanical year. This is called the Sidereal Year because the earth and sun are regarded as a moving system THE UNIVERSITY OF TORONTO QUARTERLY within and with reference to the stellar system. Its value is 365.25636 days. But it so happens that man, struggling to maintain his existence upon the earth, is interested less in the stars than in the earth itself. It can never be questioned that fundamentally man lives by bread; even though it may be added in parenthesis that he does not...