Abstract
For the sake of the members obeying the meteorological service at local stations or the isolated islands who have nol astronomcal tables, an easy method of giving the direction of the true north within the accuracy of one minute of arc, without almost any computations, was explained.
If the direction of the polaris at any hour angle was observed with theodolite or likewise inst uments, the correction which must be added to this reading in order to reduce it to the true north has been given in the table on page 357. The table gives correction for tbe region between the latitude 20°N and 70°N, the sign (+) and (-) indicate that the polaris is west or east from the true north at the time of observation. As the table are given with the interval of 10° in latitude, the value at the observer's latitude must be obtained by interpoltion. In the next place, two concentric circular plate disks with an index arm should be made as Fig. 5. At the edge of the outer disk, the hour angle is graduatel, and at the perimeter of the inner disk, the corresponding azimuths are graduated. When the polaris is west from the true north, the shorter index indicates somewhere the left hand half in the inner disk, the absolute value of which is given by that graduation.
The hour angle t of the polaris at the time of observation T (Japanese standard time is always ustd) can be computed by t=(T-T0)+(λ-λ0) where T0 is the time at when the polaris passes the southern meridian, in Tokyo T is the time of observation, λ0 an dλ are east longitude of Tokyo (9n18m58._??_7_??_9n19m) and the oberving station respectively.
The value of T0 at the beginning of the year is known from Fig. 5. At the beginning of any date in the year it is faster 3n56m n from the value at the year's beginning, wheren is the number of days elapsed since the beginning of that year.
