See also:

• TIME (0. Eng. Lima, cf. Icel. timi, Swed. timme, hour, Dan. time; from the root also seen in " tide," properly the time of between the flow and ebb of the sea, cf. O. Eng. getidan, to happen, " even-tide," &c.; it is not directly related to Lat. tempus)
• TIME, MEASUREMENT OF
• TIME, STANDARD

For a See also:

• PLACE (through Fr. from Lat. platea, street; Gr. IrAar6s, wide)

F. C. Schjellerup' has shown that the right ascensions assumed by Hipparchus agree within about 15' or one See also:

• MINUTE
• MINUTE (Lat. minutes, small; minuere, to make less)

He sometimes used azimuths, or with the large " armillary See also:

• SPHERES, MUSIC OF THE

2 See his Epistolae astronomicae, p. 73. In the spherical triangle ZPS between the See also:

• ZENITH (from the Arabic)

. ., the mean of all being To, and calling the z corresponding to this Z, we have dZ d27, z'=Z+dt(T1—T0)+zdt2tT~—To)2; 2 ( z2=Z+EkT2To)+2ate`T2.-To)2• and so on, t being the hour-angle answering to To. As E(T—To)=o, these equations give Z=z,+z2+zs+... Id2Z(T,—To)2+(T2—To)2+... n 2 dt2 n al +z2 + zs +... d2Z Z2 sin2z (T — To) n dt2 n But, if in the above-mentioned triangle we designate the angles at Z and S by 18o°—A and p, we have sin z sin A =cos S sin t; sin z cos A = —cos 4' sin S -1- sin 0 cos d cos t; and by differentiation d2Z cos ¢ cos S cos A cos p dt2 sin Z in which A and p are determined by sin A = sin t cos S and sin cos sin Z p sin Z O. With this corrected mean of the observed zenith distances the hour-angle and time are determined, and by comparison with To the error of the timekeeper. The method of equal altitudes gives very simply the clock error equal to the right ascension minus See also:

• HALF

This corrected time of transit, expressed in sidereal time, should then be equal to the right ascension of the object observed, and the difference is the clock error. In observatories the determination of a clock's error (a necessary operation during a night's See also:

• WORK

113 seq.) are given by Chauvenet in his Spherical and See also:

• PRACTICAL

Modern maps and charts generally give the longitude from the observatory of either Paris or Greenwich according to the See also:

• NATIONALITY

Eclipses of the sun and occultations of stars by the moon were also much used for determining longitude before the invention of chronometers and the electric See also:

• TELEGRAPH (Gr. Tike, far, and rypaq5eiv, to write)

4.47' in the case of the See also:

• MADRAS

Transport of Chronometers.—This means of determining longitude was first tried in cases where the chronometers could be brought the whole way by See also:

• SEA (in O. Eng. sae, a common Teutonic word; cf. Ger. See, Dutch Zee, &c.; the ultimate source is uncertain)
• SEA, COMMAND OF THE

The difficulty of the method consists in determining the " travelling rate.” Each time a chronometer leaves the station A and returns to it the error is determined, and consequently the rate for the time occupied by the journeys from A to B and from B to A and by the sojourn at B. Similarly a rate is found by each departure from and return to B, and the time of See also:

• REST (O. Eng. rest, reste, bed, cognate with other Teutonic forms, e.g. Ger. Rast, Riiste, rest, and probably Gothic Rasta, league, i.e. resting or stopping place)

The operations of a telegraphic longitude determination can be arranged in two ways. Either the local time is determined at both stations and the clocks are compared by telegraph, or the time determinations are marked simultaneously on the two chronographs at the two stations, so that further signals for clock comparison are unnecessary. The first method has to be used when the telegraph is only for a limited time each night at the disposal of the observers, or when the See also:

• CLIMATIC

This See also:

• ATTENTION (from Lat. ad-tendo, await, expect; the condition of being " stretched " or " tense ")

And it is a fact that in the former method there have never occurred errors of between half and a whole second such as have not infrequently appeared in the latter method. In transit observations generally this personal error does not cause any inconvenience, so long as only one observer is employed at a time, and unless the amount of the error varies with the magnitude of the star (wh is often the case) but when absolute time has to be determined as in longitude work, the full amount of the personal equation between the two observers must be carefully ascertained and taken into account. And an observer's error has often been found to vary very considerably not only from year to year but even within much shorter intervals; the use of a new instrument, though perhaps not differing in construction from the accustomed one, has also been known to affect the personal error. For a number of years this latter circumstance was coupled with another which seemed perfectly incomprehensible, the personal errorappearing to vary with the reversal of the instrument, that is, with the position of the See also:

• ILLUMINATING

A useful check on the results is afforded by simultaneous experiments with one of the instruments contrived by C. See also:

• WOLF
• WOLF (Canis lupus)
• WOLF, FRIEDRICH AUGUST (1759-1824)
• WOLF, HUGO (1860-1903)
• WOLF, JOSEPH (1820-1899)