How do Sundials work?
Sundials is the device that tell the time of day by the position of the Sun. It works when an object casts a shadow onto the dial where hour-lines are placed. An object that casts a shadow is called a gnomon. You can tell the Sun's altitude by the shadow.
There are various types of gnomons: It may be a long thin rod or other object with a sharp tip or a straight edge. The Sun changes its position throughout the day: it revolves around the Earth at 15° per hour. It takes the sun 4 minutes to move one degree of longitude from east to west. The gnomon shadow moves alongside with the movement of the Sun, gnomon casts a shadow onto the dial.
Sundials are confronted with the axis of the Earth's rotation. The shape of the device has to point to the true North - this is not the North or South Poles. The shape of the horizontally oriented clock should correspond to geographical latitude.
Types of Sundials
There are three main type of Sundials: horizontal, equatorial and vertical. People used to make spherical, conical and cylindrical clocks.
Horizontal sundial consists of dial plate and gnomon. You already know what gnomon is, and dial plate is a plane with hour-lines. On a horizontal clock the dial plate is set parallel to the horizon plane. A gnomon is usually represented by a triangle that is perpendicular to the plane of the dial plate. One of the side of the gnomon is tilted at an angle equal to the geographical latitude of the clocks location point.
The line that crosses gnomon and dial plate is directed parallel to the meridian line. At apparent noon the shadow of the rod is directed alongside this line.
Apparent noon is the time in the middle of the day between sunrise and sunset.
As well as horizontal sundials, Equatorial ones also consist of gnomon and dial plate. The hour-lines are marked on the dial plate by equal angular intervals - time is represented the same way on the dial of regular clocks. Gnomon, as a rule, is a metal rod, which is mounted on the dial plate perpendicular to its surface. The dial plate is in the horizontal plane so that the line that connects gnomon and hour-lines corresponding to noon is directed parallel to the noon line. Towards the south - for the Northern Hemisphere, towards the north - for the Southern Hemisphere. Likewise the line that connects the gnomon base and hour-lines is tilted concerning the horizon plane.
The main drawback of the equatorial clocks is that they work during one particular period: from the day of the spring equinox to the autumn equinox.
Vertical sundials can be found on the walls of various buildings. Their dial plates are located vertically and are perpendicular concerning the horizon plane. Meanwhile the dial plate can face any direction changing the position of hour-lines. In order for the hour-lines to be symmetrical with respect to the midday division, the dial plate should face south in the northern hemisphere and north in the southern.
In Russia vertical sundials can be found in the Moscow Planetarium. They show the current date and time.