Sundials

Sundials Sundials come in variety of forms but all sundials measure “Solar Time”

Sundials Each hour on a sundial is 15° wide. But……. Basic Parts: Hours markers Gnomon – A device that cast a shadow on the hour markers. Basic Premise: If we divide the day into 24 hours and the Earth rotates about 360° each day then, Each hour on a sundial is 15° wide. But…….

Sundials The Gnomon must be aligned with the Earth’s rotation axis for the sundial to work properly.

Sundials There is an alternate version of a sundial called an “Analematic Sundial” This style of sundial uses a vertical gnomon. However, the gnomon must be movable and calendar positions must be determined for each day’s gnomon location.

Sundials Analemmatic Sundial

There’s still a problem Sundials There’s still a problem

Sundials The Solar Day is not always 24 hours long due to our changing orbital speed. The Solar Day can be 15 minutes longer or shorter than the clock day depending on the time of year.

Sundials

Sundials The Result: If we want the sundial to read the same time as the clock, we need to add or subtract a correction factor This factor may also include Daylight Savings Time.

Sundials To read the sundial: Orient sundial so that the gnomon points north. 2. Read the hour markers to determine solar time. 3. Add or subtract appropriate correction factor.

Sundials Another use for the Sundial is finding directions. To find your directions: Use a clock to find the time. Perform a reverse correction to determine the correct sundial reading at that time. Turn sundial until it reads the corrected time. You are now facing north.