雙語科技百科(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.
日晷又稱“舊規”，中國古代利用日影測得時刻的一種計時儀器。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.
通常由銅製的指針和石制的圓盤組成。銅製的指針叫做“晷針”，垂直地穿過圓盤中心，起着圭表中立竿的作用，因此，晷針又叫“表”，石制的圓盤叫做“晷面”，安放在石臺上，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.
使晷面平行於天赤道面。這樣，晷針的上端正好指向北天極，下端正好指向南天極。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.
在晷面的正反兩面刻劃出12個大格，每個大格代表兩個小時。當太陽光照在日晷上時，晷針的影子就會投向晷面，太陽由東向西移動，投向晷面的晷針影子也慢慢地移動。於是，移動着的晷針影子相當於現代鐘錶的指針，晷面則是鐘錶的表面，以此來指示時間。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.
由於從春分到秋分期間，太陽總是在天赤道的北側運行，因此，晷針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.
從秋分到春分期間，太陽在天赤道的南側運行，因此，晷針的影子投向晷面的下方。所以在觀察日晷時，首先要了解兩個不同時期晷針的投影位置。
