日期線的利用 The use of calendar lines

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Using the calendar lines on the dial face, one can read the time and find the true south without compass. A foldable and portable sundial is made to demonstrate this idea.

Fig. 1 shows the layout of the structure.




Fig. 2 shows the assembled dial. (The part above the horizontal line on the vertical plate should have been cut off.)




If the shadow of the nodus (indicated by the red arrow in Fig. 3) is cast on correct date line, then the time read from hour lines is the correct solar time. Otherwise, the time read would be wrong. Therefore, it is important to let the nodus casts its shadow on the right calendar line as close as possible. Once this is done, the sundial tells not only the time but also the local meridian line, which passes the 12 o'clock line.