Sundial.

Due to coincidence, ancient people remarked that the shadow of a tree was amputated when the sun moved in the sky. Similarly, when plugging a stick upright on the ground, the stick ball also moves and the length of the ball changes during the day. When the shadow of the stick is short, the ancient people know that it is near noon when the ball is long, they know that the day begins or is about to end. By using the stones, ancient people marked the location of this shade. So the basic device used to measure time has formed more than 4,000 years ago and people call it a sundial. Chaldée astronomer named Berossus described the sundial in the 3rd century BC.

Sundial has many types, many shapes. The simplest thing is a vertical stick with the name "hour column" (gnomon). This column has a small, big thing like the Kim Cléopatre (Cleopatra's Needle) that is currently built in New York City's park and city. The pillar now has many shortcomings: for one year and staying at the same time of day, the shadow of the column changes both in length and direction.

Although the stick was changed by the 'shadow watches' (obélisque) buried in the ground, all these shortcomings still exist.

In order to correct the disadvantages, the ancients thought of 'fundraising' (cadran solaire). Japan is a sundial, consisting of a horizontal square piece of wood and a cross-cut piece of wood close perpendicular to the horizontal surface. The diagonal piece of wood has a diagonal edge parallel to the axis of the earth, which means towards the North Star. Thanks to the latter, the shadows do not change in the direction anymore and the ancient people have certain degrees of division. How to use the daily fund is very simple: when putting the fund in the sun, if the fund of the fund is close to the 9th, then it is 9 o'clock. Japan fund is not an accurate metering device and can only be used on sunny days. Japan fund needs to be manufactured in accordance with each locality. Someone stole a fund in Egypt and brought it to Greece. At this place, that fund was wrongly making the thief question.

Japan has been used since ancient times. The oldest surviving fundraiser in Egypt dates back to the 15th century BC. The Egyptians as well as the Greeks loved this type of sundial. By the 19th century, watchmakers also used appropriate funds to get hours. The Romans, on the other hand, did not pay attention to this time measuring instrument because it was not until the year 491 that they also used a column that only plugged in before the Hall to limit speeches of orators.

Pham Van Tuan

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