VIDEO ANIMATION: Sundial

SUNDIALS have been used since antiquity to tell the time by the position of the shadow cast by a gnomon (the sticking out bit of a sundial). Early examples were simply towers whose shadows rotated around them during the day. By tilting the gnomon so that it lies parallel to the axis of rotation of the Earth the hour divisions become reasonably evenly spaced. This simulation shows a sundial at the latitude of Greenwich (London). The upper left animation shows just the sundial with a rotating red clock hand. At midday the arrow points vertically upwards. At midnight it points vertically down. The upper right animation is the same but with the addition of a vertical gnomon. Notice how far the shadow from the vertical gnomon departs from the regular sweep of the clock hand. The lower left animation shows a rotating Earth with tower like projections at various latitudes. The lower right animation shows the shadows cast as viewed from an unmoving point directly above London. See also our garden sundial animation. and our vertical sundial animation.

KEY:

• PURPLE POLE: vertical sundial at the latitude of Greenwich
• ORANGE POLE: vertical sundial at the latitude of the Tropic of Cancer
• YELLOW POLE: vertical sundial at the latitude of the equator
• RED ARROW: sweeps 360 degrees and shows the time.

The Earth is divided into coloured bands:

• WHITE: poles to artic and antarctic circles
• ARCTIC CIRCLE 66.5^o North
• ANTARCTIC CIRCLE 66.5^o South
• GREEN: artic and antarctic circles to tropics
• TROPIC OF CANCER 23.5^o North
• TROPIC OF CAPRICORN 23.5^o South
• BLUE: tropics to equator
• RED BAND: equator