When the earth rotates about its axis, the sun appears to “move” across the sky, causing objects to cast shadows. A sundial contains a gnomon, or a thin rod, that casts a shadow onto a platform etched with different times. As the sun changes relative positions over the course of a day, the rod’s shadows change as well, thus reflecting the change in time.
If a sundial works based upon a rod’s shadow, then why can’t a simple stick in the ground work? As a result of the tilt of the earth’s axis, the visible movement of the sun changes daily.
This can be accounted for in several ways. In a normal horizontal sundial, the base platform is kept steady, while the gnomon is moved to reflect the changes due to the earth’s axis tilt.
Another method achieves the same effect by aligning the platform with the latitude and the gnomon perpendicular; mathematically, this is just the projection of the gnomon onto the platform.
Sundials must be corrected across the span of a time zone. Every zone has a “reference longitude,” and with every degree of longitude away from the reference, the sundial is off by an additional 4 minutes.
Thus, equation of time correction is employed in order to maintain a uniform time across the zone.