Grades 3–5 · Everyday Life
How does a sundial connect a shadow with solar time?
A fixed rod's shadow shifts across sundial marks because sunlight reaches it from changing directions during the day. The projecting shadow-caster, called a gnomon, blocks rays and leaves a shadow on the opposite side. Its moving shadow connects sunlight with marked time.
Earth's rotation changes the direction from which sunlight reaches a place during the day. The gnomon remains attached to the dial, but its shadow changes position. This is why one fixed object can indicate different times without a motor moving its pointer.
The hour marks must fit the dial's location, shape and orientation. In the pictured schematic, that calibration has already been done. When the shadow reaches the line marked 9, the dial indicates 9:00 local apparent solar time. Changing the direction the dial faces without changing its marks would spoil that relationship.
The gnomon's thick shadow reaches the 09 line in this site-calibrated schematic. That alignment indicates 9:00 local apparent solar time because these marks fit this dial's location and orientation.
In the diagramA sundial uses a calibrated shadowSchematic for one place · local apparent solar time091215Rod = gnomon · thick line = shadowThe shadow aligns with 9:00 solar time.
A sundial follows the Sun at its own location, while a clock follows an agreed time-zone reference. Daylight-saving rules and seasonal differences in the Sun's apparent timing can widen the gap. Its shadow can mark local solar time without matching a nearby clock.
About this explanation
Starting knowledge: Understand that an opaque object can cast a shadow and recognize labeled hours on a simple time scale.
Sources
- Royal Museums Greenwich — Sundials