At least two people are required to operate an observation station--one to observe, the other to record the results. They should exchange duties every hour to avoid undue eye fatigue. Additional personnel are desirable so that the night can be divided into shifts. Essential materials and equipment include: (1) a small telescope; (2) a tripod with pan-tilt or turret head and a mounting cradle; (3) data sheets similar to the one illustrated in Figure 12. Bausch and Lomb or Argus spotting scopes (19.5 ×) and astronomical telescopes up to 30- or 40-power are ideal. Instruments of higher magnification are subject to vibration, unless very firmly mounted, and lead to difficulties in following the progress of the moon, unless powered by clockwork. Cradles usually have to be devised. An adjustable lawn chair is an important factor in comfort in latitudes where the moon reaches a point high overhead. [Illustration: FIG. 12. Facsimile of form used to record data in the field. One sheet of the actual observations obtained at Progreso, Yucatán, on April 24-25, 1948, is reproduced here. The remainder of this set of data, which is to be used throughout the demonstration of procedures, is shown in Table 1.] [Transcription of Figure 12's Data] ORIGINAL DATA SHEET DATE 24-25 April 1948 LOCALITY Progreso, Yucatán OBSERVERS Harold Harry; George H. Lowery WEATHER Moderate to strong "trade" winds along coast, slightly N of E. Moon emerged above low cloud bank at 8:26. INSTRUMENT B. & L. 19.5 Spotting Scope; image erect REMARKS Observation station located 1 mile from land, over Gulf of Mexico, at end of new Progreso wharf -----------+------+-------+---------------------------------------- TIME | IN | OUT | REMARKS -----------+------+-------+---------------------------------------- C.S.T | | | 8:26 | -- | -- | observations begin; H.H. observing 50 | 4:30 | 9 | slow; small 56 | 3 | 10 | medium size 9:00 | 2 | 10:30 | very small 11 | 5 | 9:30 | moderately fast 25 | 5 | 10 | very small; rather slow 26 | 3 | 11 | " " 36 | 5 | 10 | medium size 40 | 3 | 10 | " " 43 | 5:30 | 9 | " " 46 | 3:30 | 10 | small 56 | 4:30 | 10 | medium size 9:58-10:00 | -- | -- | time out to change observers; G.L. at 10:05 | 4:30 | 11:30 | scope small 06 | 3 | 11 | 12 | 5 | 8 | very small 25 | 5 | 12 | very fast; small 30 | 4 | 10 | small 32 | 4 | 11 | " 32 | 2 | 11 | " 33 | 5 | 11 | " 33 | 4 | 1 | " 33 | 5:30 | 11 | " 35 | 4:30 | 10 | swallow-like 36 | 5 | 1:30 |