You sure can tell time without a clock or a watch! Sundials work by casting a shadow on a disk marked with different periods of time. As the sun moves in the sky, the shadow falls on a different part of the disk, which gives the time of day. Sundials don’t work well at night, though, as you can imagine.

A sundial is designed to read time by the sun. This places a broad limit of two minutes on accurate time because the shadow of the gnomon cast by the sun is not sharp. Looking from earth the sun is ½° across making shadows fuzzy at the edge. The actual construction of a sundial can be very accurate.

In additional to conventional sundials, it is also possible to have moon or lunar dials, usually in the form of a sun and moon dial. In principle, a sundial can also be used during the night, provided that the moon is sufficiently bright and that the lunar age is known.

The reason is that the sundial and the watch have slightly different ideas of what time means. A watch, on the other hand, measures time in seconds, minutes, and hours so that each day has 24 hours and on the average the sun is in the same position each day at a given time.

The formula for calculating the hour lines (theta) on a horizontal sundial is:

1. tan(theta) = tan(HA) x sin(lat)
2. theta = the resulting dial hour angle measured from the noon line (- is left of the noon line, + is right of the noon line)
3. HA = the hour angle of the sun from the noon meridian, expressed in (+/-) degrees.

As a result of this movement (and depending on where you live), the shadow cast by the sun moves approximately 15 degrees each hour so that over the course of 24 hours it travels a full 360 degrees around your sundial.

Sundial, the earliest type of timekeeping device, which indicates the time of day by the position of the shadow of some object exposed to the sun’s rays. As the day progresses, the sun moves across the sky, causing the shadow of the object to move and indicating the passage of time.

Sundials need to point in the direction of True North, and the style (either a sharp straight edge or thin rod, often located at the edge or tip of the gnomon) must be aligned with the Earth’s rotational axis.

Find the center longitude of your time zone. Your sundial can be off by up to an hour compared to clock time depending on your longitude. For every hour that you’re ahead, the center of your time zone will shift by 15° east; for every hour you’re behind, the center of your time zone will be 15° west.

The style must be parallel to the axis of the Earth’s rotation for the sundial to be accurate throughout the year. The style’s angle from horizontal is equal to the sundial’s geographical latitude.

Projection of the equatorial dial to form the ellipse of the vertical dial. The gnomon of the vertical sundial makes an angle of 90°–L with the vertical (that is, an angle L with the horizontal), as shown in the side view in Figure 5. In the southern hemisphere, the vertical dial is north-facing.

The mathematician and astronomer Theodosius of Bithynia ( c. 160 BC to c. 100 BC) is said to have invented a universal sundial that could be used anywhere on Earth. The Romans adopted the Greek sundials, and the first record of a sundial in Rome is 293 BC according to Pliny.

If you have a sundial in your garden, and you want it to tell time accurately, today is one of the best days to set it. Go out at noon (1:00 p.m., if you’re currently observing daylight savings time), and set yours to 12 o’clock. Sundials can be set on four dates each year for an accurate read.

Learn the Parts of a Sundial The pointer, also called a style or gnomon, casts a shadow, indicating the time of day. The numbered dial is the area in which the shadow falls, showing the hour.

A heliochronometer is an extremely precise form of sundial which applies corrections to the time according to the sun to give a very accurate reading of the mean time as shown by ordinary clocks and watches.

Maharaja Jai Singh I .

Although watches and clocks came into popular use in the 18th cent., sundials were long employed for setting and checking them. Although sundials are still used in many areas, including Japan and China, they are regarded today chiefly as adornments.

Clepsydra

ancient Greece

Who decided on these time divisions? THE DIVISION of the hour into 60 minutes and of the minute into 60 seconds comes from the Babylonians who used a sexagesimal (counting in 60s) system for mathematics and astronomy. They derived their number system from the Sumerians who were using it as early as 3500 BC.