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Facts about Sundials For Kids

In common designs such as the horizontal sundial, the sun casts a shadow from its style onto a surface marked with lines indicating the hours of the day. In most designs, the style must point towards true celestial north.

  • It is common for inexpensive decorative sundials to have incorrect hour angles, and these cannot be adjusted to tell correct time.
  • When the sundial reads by shadows, the shadow-casting object — the sundial’s gnomon — may be a thin rod, or any object with a sharp tip or a straight edge.
  • It may be oriented vertically, horizontally, aligned with the Earth’s axis, or oriented in an altogether different direction determined by mathematics.
  • The installation of many dials requires knowing the local latitude, the precise vertical direction, and the direction to true North.
  • Portable dials are self-aligning; for example, it may have two dials that operate on different principles, such as a horizontal and analemmatic dial, mounted together on one plate.
  • The sundial’s indicated solar time thus varies from clock time by small amounts that change throughout the year.
  • This correction — which may be as great as 15 minutes — is described by the equation of time.
  • Second, the solar time must be corrected for the longitude of the sundial relative to the longitude of the official time zone.
  • This correction is often made by rotating the hour-lines by an angle equal to the difference in longitudes.
  • Since the celestial axis is aligned with the axis about which the Earth rotates, the angle of the axis with the local horizontal is the local geographical latitude.
  • Unlike the fixed stars, the Sun changes its position on the celestial sphere, being at a positive declination in summer, at a negative declination in winter, and having exactly zero declination at the equinox.
  • If the shadow-casting gnomon is aligned with the celestial poles, its shadow will revolve at a constant rate, and this rotation will not change with the seasons.
  • The hour-lines will be spaced uniformly if the surface receiving the shadow is either perpendicularor circular about the gnomon (as in the armillary sphere).
  • Although usually a flat plane, the dial face may also be the inner or outer surface of a sphere, cylinder, cone, helix, and various other shapes.
  • Most equiangular sundials have a fixed gnomon style aligned with the Earth’s rotational axis, as well as a shadow-receiving surface that is symmetrical about that axis; examples include the equatorial dial, the equatorial bow, the armillary sphere, the cylindrical dial and the conical dial.
  • The gnomon, set to the correct latitude, has to point to the true South in the Southern hemisphere as in the Northern Hemisphere it has to point to the true North.
  • However, a horizontal sundial is impractical on the Earth’s equator, where λ equals 0°, the style would lie flat in the plane and cast no shadow.
  • For equiangular dials such as the equatorial, spherical or Lambert dials, this correction can be made by rotating the dial surface by an angle equalling the difference in longitude, without changing the gnomon position or orientation.