Solar Clock: Solar Time and Civil Time
Equatorial Sundial (gallery image 1)
An equatorial sundial is simple, but the time of year must be known when taking a reading. During the spring and summer months, the Sun's path over the horizon becomes increasingly higher causing its rays to illuminate the upper portion of the clock. However, during the autumn and winter, the Sun's position is much lower over the horizon and so its rays will illuminate the lower portion of the clock. In both cases, the time is indicated by the style's shadow on the clock face.
The information obtained from this instrument is known as the solar time, which is not the same as the civil time indicated by a regular clock. To obtain an approximate value for civil time, you need to take the solar time observed on the sundial and add one hour in winter and two hours in summer.
Corrected Equatorial Sundial (main image)
We can use the corrected equatorial sundial to obtain both the solar time and civil time; that is, the time indicated by a regular clock. This correction takes into account the date and the latitude of the place of observation. To do so, the gnomon is substituted by a mobile plate containing a special symbol in the shape of an eight. This symbol is called the analemma, which has been calculated for Valencia. It describes the Sun's position in the sky throughout the year.