Sundial. Fun physical experience at home

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Sundial. Physical experiments

Entertaining experiments in physics

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Observing the shadows cast by opaque objects convinces us that light travels in straight lines. The position of the shadow of a tree, a pillar, and other objects changes throughout the day. The device of a sundial is based on the use of these phenomena. The role of the hour hand here is played by the shadow cast by the pointer rod.

In the summer, and in sunny weather, if objects give shade, it is convenient to use a sundial. Of the existing types, we will describe the device of the simplest, so-called equatorial, clocks with an inclined dial.

The equatorial sundial is quite suitable, if correctly set, for telling the time in sunny weather between March 21 and September 23.

The main parts of the watch: dial board, rod, back wall, base.

Start by making the dial. Draw three circles from a common center on a square board. The center of the circle is at the intersection of the diagonals of the square. Divide each circle into 24 parts. You can do this with a protractor (a semicircle divided by 180°), dividing the circle into parts at 15° intervals. If there is no protractor, you can do otherwise. Draw two lines through the center at a 90° angle to each other. Check with a ruler by connecting the points of intersection of line segments with a circle. With an exact distribution, all four distances will be equal. The circle will be precisely divided into four parts, each containing 90°. Divide each arc into 3 parts. Check again; these will be 30° arcs. Divide these arcs in half again and check. Get 24 15° arcs.

With the correct division of the gaps, the ruler connecting the opposite marks must necessarily pass through the center of the dial.

Intervals less than an hour can be divided, depending on the diameter of the dial, either in half, or into smaller parts, for example, into four (15-minute intervals). With a diameter of 500 mm and above, you can divide the 15-minute intervals into three more parts (5-minute intervals). The rod, the shadow of which shows the time, is made from a nail, wire or pin. Its greatest length is recommended to be 2,5 times less than the radius of the circle, 5 times less than the diameter of the dial.

The rod must be installed strictly perpendicular (vertically) to the plane of the dial. The back wall must maintain a constant inclination of the base with the dial. The angle of inclination depends on the geographical latitude of the given place. The base is a board on which the dial with the back wall is installed.

In order for the watch to accurately show the time, the following conditions are necessary: the dial must be hung directly with the sun's rays so that there is no shadow or reflected light.

Sundial

The angle of inclination is calculated as follows: the latitude of Moscow is about 56°. Subtracting this value from 90°, we get 34°. Consequently, for Moscow, the angle of inclination of the dial to the base is -34°. For St. Petersburg the angle of inclination will be 30°, in Kyiv - 40°, in Baku - 50°, etc. The lower part of the wall rests on the base board. Set the dial to exactly north. Do it this way: set the compass dial to approximately north. At one o'clock in the afternoon, follow the shadow of the rod. At the moment when the shadow is at its shortest, check the time with a normal clock. Now turn the dial of the sundial so that the shadow of the rod shows the same time on them. Finally secure the dial in this position. In the direction of the shadow on the dial, draw the noon line north - south, and at right angles to it, the line east - west.

Clocks are called equatorial because the plane of their dial is parallel to the plane of the equator.

Side view of the watch:

Sundial

View of the clock from the side of the dial:

Sundial

Install a sundial in a flower bed, on a pole and other places so that everyone can use it.

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