地平方位日晷 Horizontal Azimuth Sundial (3)

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Typical azimuth dial uses a vertical needle, which seats at the center of calendar circles, to cast shadow on those circles. Those circles are drawn with hour marks so that time can be found from the position of the cross point of needle's shadow and calendar circle of the right date. Once the dial is set in the right N-S direction, nothing is required to move. Just watch the moving of the needle's shadow. (http://blog.xuite.net/nycl.chiu/blog/11134875) This is all right in middle and high latitude zones. In low latitude zone, a problem occurs: the shadow becomes very short at time near noon in the summer since the sun is then near the zenith. That means there is no cross point unless the dial is equipted with very, very long needle, which is unpractical.

A magnetic azimuth dail has no such problem. It uses a long magnetic pointer which crosses over those calendar circles with its supporting needle at their center. When one wants to find the time, he turns the square dial plate until the painted on 12 o'clock line is pointing at the solar azimuth. The hour under the magnetic pointer is the time ought to be.(http://blog.xuite.net/nycl.chiu/blog/11134815) The diameters of calendar circles for summer time can be made greater to hold reasonable hour resolution without worrying the problem as in the former case because there is no shadow casting needle. So a magnetic azimuth dail is a suitable one in low latitude zone.

Now a days a long magnetic pointer is hard to find in stores. An alternative is represented here for Hsinchu, Taiwan, which is at 24.9N and 120.9E. Fig. 1 shows the parts and tools needed. The hour diagram is a little bit complicate for it intends to give the standard time. Fig. 2 shows the finished dial. To find the time, one sets the black thread in S-N direction first. Then he flips up the slit and turns the dial plate (CD) until the sunshine passing through the slit is right on the azimuth line, as shown in Fig. 3.

Now the hour on the right calendar circle under the black thread is the time he is looking for. For example, as in Fig. 4, if the date is the 19th of Feb., the time is 1:10; if the date is the 21st of June, the time is 12:00.

新竹市清華大學新日晷 New Sundial on NTHU campus, Taiwan

這日晷在新竹市清華大學校園內，2006年為慶祝清大在台復校50週年而建。 其獨特之處有三： 1. 面盤，就是那藍白色像水又像天空的大圓面，向北傾斜5.2度。 2. 投影體的南、北兩端都開有小孔，夜裡從南孔向北孔望去，可見北極星。 3. 面盤上的日期線，除了夏至、春分、秋分和冬至外還有清大校慶日和雙十國慶日。
在面盤東北方地面上有兩塊銅牌，一塊說明建造原委，另一塊是時差修正表。
This sundial was given to the Nat'l Tsing Hua University in April, 2006 as a gift to celebrate its 50th anniversary of re-erecting at Hsinchu. The mosaic dial face, 8m in diameter, is tilted 5.2 degrees down north. The highest point of the gnomon is about 185 cm from the face, which is decorated in blue and white to imply water and sky. The gnomon's east-west width is 30 cm. Near the top edge of north end is there a hole. A smaller hole is found at south end. Through these two holes, one may see the North star in the night. Five calendar lines are given; summer and winter solstices, equinoxes, the National Day (Oct. 10th) and the University anniversary (Apr. 29).
There are two bronze plates on the ground north-east to the dial face. One states why and how this sundial was built, the other gives an 'equation of time' table on a basis of every minute of change. The picture above shows only the latter.
This sundial was designed by Professor Emeritus Chi-Lian Chiu (chemist) and Mr. PoJui Lin (artist).