By Kevin Beck
Updated Aug 30, 2022

Andreas-Joachim Lins / EyeEm/GettyImages

Enter the Sundial: The Basics

A sundial is a simple chronometer that uses the sun’s shadow cast by a gnomon to indicate local time. The gnomon, a vertical rod or pinhole, must be aligned with Earth’s axis and point toward the celestial north pole (CNP). At a given latitude, it is inclined to the horizon by an angle equal to that latitude. For example, at 40° N (Boulder, Colorado), the gnomon is angled 40° above the northern horizon, halfway toward the zenith.

• Note: This article is written for readers in the Northern Hemisphere; reverse north–south directions where appropriate for southern locations.

Learning About Sundials

Understanding a sundial requires familiarizing yourself with a few key terms. The discipline blends astronomy with craftsmanship, offering insight into ancient timekeeping and the celestial mechanics that make it possible.

The Celestial Equator

Imagine Earth’s latitude and longitude system projected onto an imaginary sphere surrounding the planet. A great circle passing through the celestial poles—called the celestial equator—represents Earth’s equator extended into the sky. It is the reference for many celestial coordinates.

The Ecliptic

The ecliptic is the apparent path the Sun traces over the course of a year, lying in the plane of Earth’s orbit. It is tilted 23.4° relative to the celestial equator, causing the Sun to appear north or south of the equator throughout the year. The two points where the ecliptic intersects the celestial equator are the vernal and autumnal equinoxes.

Other Standard Astronomical Terms

Latitude on Earth corresponds to declination in the sky, while longitude corresponds to right ascension. The meridian, a line from north through the zenith to south, divides the celestial sphere into eastern and western halves and is crucial for aligning a sundial.

Sundial History

Early sundials date back to around 1500 BCE in Egypt. Some were pocket‑sized, using a pinhole gnomon, allowing travelers to keep time on the go. Sundials remained in use well into the 1800s, even as mechanical clocks became reliable, often serving as a benchmark for clock accuracy.

Parts and Operation of a Sundial

• Gnomon: The component that casts the shadow; it must point toward the CNP and be inclined by the observer’s latitude.
• Dial Plate: The surface—flat or cylindrical—onto which the shadow falls. It is marked with hour lines that correspond to specific times.
• Hour Lines: Indicate the times of day as the Sun moves across the sky.
• Nodus: A notch or point on the gnomon that sharpens the shadow’s position for precise time reading.

Types of Sundials

• Altitude Dials: Use the Sun’s elevation above the horizon to determine time. Types include plane, cylinder, scaphe, and ring dials.
• Directional Dials: Rely on azimuth (compass direction) and the Sun’s angle at meridian transit. Subtypes include horizontal, polar vertical, azimuthal, and equinoctal dials.

Do‑It‑Yourself Sundial

Building a sundial is an accessible way to apply the physics and geometry we’ve discussed. Choose a clear, sunny day—avoid rain—to see the shadow’s motion and confirm your design’s accuracy.