Time Measurement

Time measurement, being a problem concerning the phase of the earth's rotation, is eminently a matter for international regu lation. It is inconvenient for states to use local time strictly, and a system of zones has been devised and is accepted by most civilized states, whereby within each zone the same mean time rules, but changes abruptly by one whole hour, or half an hour in some cases, at its borders. Thus the local time at Washington observatory is 5h.8m.16s. late on Greenwich, but the mean time kept is 5h. slow precisely. Since mean time is in any case a con structed time, not an observed one, no difficulty is introduced by this convention.

Some other agreements are wanted. Till 1925 Greenwich mean time commenced at noon ; from the beginning of that year the meaning of the term was changed so that the reckoning com menced at the previous midnight and the hours 0-24 agreed with those reckoned for civil time. Others have adopted the term "universal time" to describe exactly the same system, so that there are now three designations current for a single scheme. A further convention regulates the "date line," in travelling across which a calendar day is dropped from the reckoning in passing westward, or is added in passing eastward. The line is arbitrary; it runs for the most part along the i8oth meridian, but deviates to the east to include Fiji and Tonga with Australasia and to the west to include the Aleutian Islands with the continent of North America.

Wireless Signals.

A new era has been introduced into the determination of time by wireless telegraphy. Local time is required for two purposes, first by observatories, in the form of local sidereal time, in order to set the telescopes upon any cata logued object on the sky, and second by an explorer or sailor in order to construct local mean time and to determine the longitude by comparison with Greenwich mean time, which he is supposed to read from his chronometer. Time signals are now used, originating in some well-equipped central observatory, where the full care that is necessary can be given ; they are issued as wireless signals from a large number of stations distributed over the world. Received at any fixed observatory, of known longitude, a short calculation gives the local sidereal time and abolishes the necessity of observing it directly; while for a traveller who de sires to determine his longitude, the signal supplies a continuous check upon the rate of his chronometer and saves the uncertain ties of prolonged extrapolation. Still more important from the point of view of systematic science is the power conveyed of comparing the determinations of time at different observatories continuously with one another, by receiving at any point the signals from different sources, or by receiving at different ob servatories any one arbitrary signal, and afterwards comparing the results. As already remarked, the results are in poor accord with

one another ; it would be an important advance if a large improve ment of them should result. The faults are now visible and that is the first step towards removing them. The gain would appear first merely as a closer co-ordination of instrumental results. taken under different circumstances of place and time, but could not fail to result ultimately in increased certainty of prediction. At the present stage the investigation figures as a co-ordinated attempt to improve the determination of longitudes.

International regulation of astronomical questions is centred in the International Astronomical Union (I.A.U.), a post-war organ ization formed at Brussels in 1919, in which a number of earlier conventions were assembled in connection with the International Research Council and operating through the principal research in stitution of each adherent country. Perhaps the most energetic and fruitful work on special problems is still done by the initiative of individual observatories, but the task is furthered in many ways by the committee created by the I.A.U. Two of these committees have special reference to the subject treated here, the Committee on Longitudes by Wireless Telegraphy, and the International Time Commission. The former has been organized to execute a scheme of world longitudes, by the issue of time signals, in some cases specially designed, from the powerful stations at Bordeaux, Nauen, Saigon (French Indo-China), Annapolis (Maryland), Honolulu, and others, and the determination of their times of re ceipt at as large a number of observing stations as possible. In a preliminary explanation in 1926 upward of 40 observatories took part; the results are voluminous, and are not yet (1928) fully discussed in co-ordination with one another, but enough has appeared to prove that a much increased precision of longitude de termination has been attained; thus, replacing the rather crude discrepancies that used formerly to occur, we find that the time required for the passage of wireless signals across the Atlantic, about .o2sec., appears as a clearly measurable correction in all the better series. The International Time Commission was de signed in a more general way to organize the issue of wireless time signals suitable for all users, to receive those that were issued, and to standardize such work generally. Its bureau is at the ob servatory of Paris, and in the issue of signals it works in con junction with the stations, military and civil, controlled by the French Government.