Sundial - Sundial

Japanese: 日時計 - ひどけい

An instrument that uses the diurnal motion of the sun to determine time from the change in the sun's hour angle. The sun completes a full 360 degree rotation in 24 hours, so the hour angle changes by 15 degrees per hour.

If you set a vertical stick on the ground and draw radial lines around it dividing the circumference into 24 equal parts, you will have a sundial. This can only be used as a sundial at the North Pole; at mid-latitudes, the stick must be placed parallel to the earth's axis.

Simple sundials, which are simply a stick placed on the ground and marked with marks, were used in Babylonia in 2000 BC. The Book of Isaiah (Chapter 38) of the Old Testament, written around 700 BC, contains the lines, "Behold, I will cast back ten degrees the shadow of the clock that fell on the sundial of Ahaz," which shows that they were commonly used at the time.

In 300 BC, the Chaldean astronomer Berosus created a semicircular or hemispherical sundial. Hemispherical sundials were common in the Greek and Roman eras. A quarter-spherical depression was carved into the marble, and graduated lines were drawn radially from above along the sphere. When graduations were drawn at 15-degree intervals around the central angle, the shadow of the pointer passed over these graduations every hour as the hour angle of the sun changed. This was the ancient sundial.

The oldest surviving sundial is in the collection of the Naples Museum. It was excavated at Herculaneum, an archaeological site on Mount Vesuvius, and is 12 cm long, 8 cm wide, and has a scale marking on a silver plate covering its surface.

Some sundials use a conical or cylindrical surface instead of a sphere.

The one built in 1730 by King Jai Singh II of Jaipur, India, is the world's largest sundial, and a steeple obelisk in Egypt is said to have served as a sundial.

For practical use at mid-latitudes, most satellites are flat and either horizontal or vertical.

[Gorou Ishida]

Horizontal sundial

A triangle whose apex angle is the latitude ∅ of the point (35 degrees for Tokyo) is placed vertically on a flat surface to be used as a pointer. The azimuth angle a of the shadow line corresponding to the solar hour angle t is calculated and written on a horizontal surface. If the pointer is set facing due north, the hour angle can be read from the direction of the shadow, and the time can be determined.

To calculate the azimuth angle a, use the following formula:

tan a = sin∅ tan t
By calculating t with the value of every 225 minutes of angle (3 degrees 45 minutes), you can calculate the azimuth angle of the shadow line for every 15 minutes of the time scale.

[Gorou Ishida]

Vertical sundial

There are also vertical sundials that are hung on walls. The hands are angled from the wall by the complement angle of the latitude of the point, 90°-∅. The time is determined by reading the direction of the shadow cast by the hands on a vertical plane facing south. If the angle from directly below to the direction of the shadow of the hands is α, the calculation is done using the following formula:

tan α = cos ∅ tan t
These values vary with latitude, so the sundial must be designed to suit the latitude of the area where it will be used.

[Gorou Ishida]

Precautions when using a sundial

By making the following adjustments, a sundial can determine the time to within one minute.

(1) Make sure it is pointing correctly toward true north. To use a magnet to determine direction, you need to correct the declination (in Tokyo, move it back 6 degrees to the east).

(2) Correction for longitude difference. Japan's standard time is based on the 135th meridian east (which passes through Akashi City, Hyogo Prefecture), so a correction value is subtracted for areas east of that line and added for areas west of that line. The correction value is four hours and four minutes per degree of longitude difference.

(3) Correction of the equation of time. Standard time is based on the mean sun, so the error (equation of time) from the longitude of the true sun is corrected. This value is determined by each month and day of the year and can exceed 20 minutes at most.

In addition to sundials, water clocks and hourglasses have been used since ancient times, but their accuracy was inferior to that of sundials and they were used only as auxiliary clocks at night or on cloudy days.

[Gorou Ishida]

The principle of sundials
©Shogakukan ">

The principle of sundials

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

太陽の日周運動を利用し、太陽の時角の推移から時刻を定める器械。太陽は24時間で全円周360度の回転を繰り返すので、1時間当りの時角の変化は15度である。

　いま地面に垂直に棒を立て、その周囲に全円周を24等分する放射状の線を引くと、これが日時計になる。これがそのまま日時計として使えるのは北極点に限られ、中間緯度では棒を地軸に平行に置かなければならない。

　単に地面に棒を立て目盛りを描いた簡単な日時計は紀元前2000年にバビロニアにおいて使われている。前700年ごろに書かれた『旧約聖書』の「イザヤ書」（第38章）には、「見よ、わたしはアハズの日時計におりた時計の影を10度あとに戻す」ということばがあり、当時常用されていたことがわかる。

　前300年、カルデアの天文学者ベロススBerosusは半円形または半球形の日時計を製作した。ギリシア・ローマ時代には半球形のものが多い。大理石に4分の1球面のくぼみをあけ、この球面に沿って、上方から放射状に目盛り線を引く。中心角で15度間隔で目盛りを引くと指針の影は太陽の時角の変化に伴い、1時間ごとにこの目盛り線を通過する。これが古代の日時計である。

　現存する最古の日時計はナポリ博物館所蔵のものである。ベスビオ火山の遺跡であるヘルクラネウムから出土したもので、長さ12センチメートル、幅8センチメートル、表面に張った銀板に目盛りの刻みがある。

　球面のかわりに円錐(えんすい)面、あるいは円筒面を使った日時計もある。

　インドのジャイプル王ジャイ・シン2世が1730年に建てたものは世界最大の日時計であり、エジプトにある尖塔(せんとう)オベリスクは日時計の役目を果たしたという。

　中緯度の地点で実用に使うには、平面形で水平式または垂直式のものが多い。

［石田五郎］