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How does a sundial work?

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Quick answer

A sundial uses a gnomon to cast a shadow onto a marked surface. As Earth rotates, the Sun appears to move, shifting the shadow across hour lines. The gnomon aligns with Earth's axis, so the shadow's position indicates solar time, while its length varies with the seasons.

A sundial works because Earth's rotation makes the Sun appear to move across the sky, so a fixed gnomon casts a shadow that sweeps across a marked plate. The shadow's position and length indicate the time of day and the season.

Step by step

  1. Sunlight strikes the gnomon, an upright edge or rod, casting a shadow onto the dial's surface.
  2. Earth's rotation causes the Sun's position in the sky to change gradually throughout the day.
  3. The shadow moves across the dial's hour lines, which are marked to correspond to solar time.
  4. The gnomon is angled to point at the celestial pole, aligning with Earth's rotational axis.
  5. The shadow's length also changes with the Sun's altitude, indicating the season.

Key things to understand

  1. The gnomon is a raised edge or rod that casts a shadow onto a marked surface.
  2. As Earth rotates, the Sun's apparent position shifts, moving the shadow across the dial.
  3. The Sun appears to move 15 degrees per hour; only an equatorial dial spaces its hour lines evenly to match.
  4. The gnomon must point toward the celestial pole, aligning with Earth's axis, for accurate time.
  5. Seasonal changes in the Sun's height alter the shadow's length, indicating the time of year.

Labelled diagram

  1. Gnomon

    The raised edge or rod that casts a shadow; aligned with Earth's axis.

  2. Dial Plate

    Flat surface with hour lines, where the shadow falls and time is read.

  3. Hour Lines

    Markings spaced to show solar time as the shadow moves.

  4. Shadow

    Dark area cast by the gnomon; its position indicates the time.

  5. Base

    Supports the dial and may be leveled for accurate alignment.

Parts of a Sundial

Real-world examples

  • The ancient Romans used sundials in public squares to schedule daily activities and legal proceedings.
  • The Jantar Mantar in Jaipur, India, features giant stone sundials whose shadows can be read to within a few seconds.
  • Many modern gardens include decorative sundials, often with engraved mottos, though they may not be precisely aligned.
  • The Royal Observatory in Greenwich has a sundial that helps visitors understand the difference between solar and clock time.

Common mistake

✗ Myth: A sundial tells time by the Sun's heat or brightness.

✓ Fact: A sundial tells time by the position of a shadow cast by the gnomon. The shadow's location on the dial corresponds to the Sun's angle, which changes as Earth rotates.

People also ask

How does a sundial work?
A sundial works by casting a shadow from a gnomon onto a marked surface. As Earth rotates, the Sun appears to move, shifting the shadow across hour lines. The gnomon is aligned with Earth's axis, so the shadow's position indicates solar time, which can be converted to clock time.
What is the gnomon on a sundial?
The gnomon is the part of a sundial that casts a shadow. It is typically a thin rod or the edge of a plate, angled so it points toward the celestial pole. This alignment ensures the shadow moves at a consistent rate, matching the Sun's apparent motion.
Why do sundials not work at night?
Sundials require direct sunlight to cast a shadow. At night, the Sun is below the horizon, so no shadow is produced. Without a shadow, the dial cannot indicate time. Some sundials have moon dials, but they are less accurate and rarely used.
Why does a sundial's shadow move?
The shadow moves because Earth rotates on its axis, making the Sun appear to travel from east to west across the sky. The gnomon blocks sunlight, and as the Sun's angle changes, the shadow shifts along the dial's hour lines.
Can a sundial tell time at night?
No, a sundial relies on direct sunlight to cast a shadow. At night or under heavy clouds, it cannot indicate time. Some sundials include moon dials or other features, but they are not as accurate.
Why do sundials have different hour line spacings?
Hour lines are spaced according to the dial's orientation and latitude. A horizontal dial has lines that vary in angle to match the Sun's changing path, while an equatorial dial has evenly spaced lines because its plane is parallel to the equator.
How accurate are sundials?
A well-made sundial can be accurate to within a few minutes, but it shows apparent solar time, which varies from clock time due to Earth's elliptical orbit and axial tilt. Converting to standard time requires the equation of time and longitude corrections.

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