Sundials and Equation of Time
Sundials | Equation of Time | General Information | a | a
Sundials
The development of the sundial appears to have gone thru several stages
- A Shadow Clock can be formed from a stick stuck vertically in the ground, but this will only be accurate for one particular season at a time. The rate of shadow movement will vary with the declination of the Sun.
- By orientating the pole or style, to use its proper sundial nomenclature, to point at the pole star, this previous difficulty disappears. At a simple level, the dial can be equally graduated but needs to be placed perpendicular to the style. This unfortunately means that for half of the year, when the Sun is low down, its light will not shine across the dial.
- To improve the sundial further, the dial needs to be horizontal. The graduations can no
longer be equally divided but the calibrations required are fairly easy to work out nowadays.
There are still problems
- It will show local time, not the recognized national time. For Britain, where Greenwich is in the East, western regions can be about 20 minutes behind Greenwich time
- Adjustments still need to be made for the Equation of Time (see below). In a nutshell, this adjustment is required because the Earth does not move along its orbit at a constant speed.
Equation of Time
The Earth's orbit is an ellipse and the Earth moves at different speeds along this ellipse, as explained under Kepler's Laws. There is an astronomical concept of a mean Sun, one that does move uniformly as though the Earth's orbit was circular. The real Sum will sometimes be in front of this mean Sun and sometimes behind.
The time we use in everyday life is based on the behavior of this mean Sun, and variations between this 'mean Sun' time and the time measured by the real Sun can be as much as 16 minutes.
Stated as a formula
Equation of time = apparent solar time - mean solar time
Stated in another way
Equation of time = Sundial Time - Clock Time
Analemma
If throughout the period of one year you were to plot the position of the Sun in the Sky at the same sidereal time, then you will produce a figure-of-eight figure - the Analemma.