双语科技百科(天文) 第8期:日晷
The Sundial
The sundial，an ancient Chinese timekeeping instrument consisting of a bronze needle and a stone dial，measures the time by taking advan-tage of the shadow of the sun.
日晷又称“旧规”，中国古代利用日影测得时刻的一种计时仪器。The needle crosses the center of the dial vertically，just like the pole of the gnomon，and the stone dial is placed onto a stone plat-form，higher in the south and lower in the north.
通常由铜制的指针和石制的圆盘组成。铜制的指针叫做“晷针”，垂直地穿过圆盘中心，起着圭表中立竿的作用，因此，晷针又叫“表”，石制的圆盘叫做“晷面”，安放在石台上，呈南高北低，
The sundial surface is therefore parallel to the equatorial plane. The upper end of the sundial points right to the North Pole，and the lower end to the South.
使晷面平行于天赤道面。这样，晷针的上端正好指向北天极，下端正好指向南天极。Both sides of the dial are divided into 12 parts，each part representing two hours. When the sunlight beams down on the sundial，the shadow of the needle falls on the surface and moves as the sun efore，the needle’s shadow resembles the needle of a modern watch or clock，and the stone dial is like the watch or clock’s dial plate，indicating time.
在晷面的正反两面刻划出12个大格，每个大格代表两个小时。当太阳光照在日晷上时，晷针的影子就会投向晷面，太阳由东向西移动，投向晷面的晷针影子也慢慢地移动。于是，移动着的晷针影子相当于现代钟表的指针，晷面则是钟表的表面，以此来指示时间。From the vernal equinox(about March 20)to the autumn equinox(about September 22)，the sun moves in the northern side of the equator, casting the needle’s shadow on the upper part of the stone dial.
由于从春分到秋分期间，太阳总是在天赤道的北侧运行，因此，晷针的影子投向晷面上方；Likewise，since the sun moves in the southern side of the equator from the autumn equinox to the vernal e-quinox, the shadow is cast on the lower part. Therefore，when observing the sun-dial，it is important to first find out the projection position in the two different peri-ods.
从秋分到春分期间，太阳在天赤道的南侧运行，因此，晷针的影子投向晷面的下方。所以在观察日晷时，首先要了解两个不同时期晷针的投影位置。
