Sundials and the Equation of Time: Why Sun Clocks Do Not Keep Civil Time

Podcast summary

The Rest Is Science explains sundials and the equation of time, showing why sun clocks do not line up with civil time and how a special design called the Schmoyer Sunquest sundial corrects for daily and yearly solar time variations. The episode blends astronomy, history and hands on demonstrations and ends with reflections on time and society.

• sundial versus civil time explained
• the equation of time and its two main causes
• analemma and how the Sunquest gnomon self corrects
• subsolar point, Lahaina Noon, and navigational context

Overview

The Rest Is Science offers a thoughtful examination of sundials, time, and how our measurement of the day is not a perfect match to the sun. The hosts discuss how sundials are anchored to solar time, which can drift relative to civil time that clocks and schedules use every day. The dialogue moves through a concise history of timekeeping, from sunrise based days to noon based solar days, and then to the modern concept of the equation of time that reconciles solar time with mean time. This sets up the main problem with sundials: noon on a sundial is not always exactly noon by civil time, because the length of a solar day changes over the year due to two key solar system factors.

Solar time versus civil time

The episode emphasizes that a sundial measures time by the sun, and the traditional day originally followed the sun as a daily rhythm. Civil time, in contrast, uses a steady cadence of seconds counted by atomic clocks. The mismatch arises because a solar day is not perfectly constant over the year. This is due to the Earths tilt and the elliptical shape of its orbit around the Sun, which together create what is known as the equation of time. The host notes that the concept of a solar day can be visualized by considering Noon as the moment when the sun reaches its highest point in the sky, which can differ for observers located at different longitudes and latitudes.

The equation of time and its two drivers

The two main drivers are the planets orbit geometry and the tilt of the Earths axis. When the Earth is closer to the Sun in winter, it orbits a little faster, making the solar day longer, while in other parts of the year the tilt shifts the Sun's apparent path in the sky. The combination of eccentric orbit and axial tilt creates a nonuniform length of the solar day across the year, so a sundial can be several minutes ahead or behind civil time at different dates. The episode walks through a mental experiment using a straw as the equator and a tilted gnomon to illustrate how tilt drags the sub solar point away from the equator, altering the length of the day. The hosts also discuss sidereal days, which are more constant but not what humans use to structure daily life.

Gnomons, dials and the analemma

A sundial uses a gnomon to cast a shadow that marks time along a dial. The short shadow is designated as noon because it occurs when the sun is at its highest. However, the daily rotation of the Earth and the orbital motion around the Sun cause the sun to appear at slightly different places at solar noon throughout the year. The curve that describes the Sun's position over the year is the analemma, and some sundials are designed to reflect this curve to align with civil time automatically. One example discussed is the Schmoyer Sunquest sundial, which uses a wavy, sine like gnomon with a slit to project a beam that matches the sun's path, effectively correcting for the equation of time without daily adjustments.

The Schmoyer Sunquest sundial

Richard Schmoyer introduced the Sunquest, a two horseshoe like structure with a central pin and a wavy bacon shaped gnomon. The design uses a slit that the Sun light passes through to cast a light ray on a time scale. The width and curvature of the slit correspond to the equation of time, so as the year progresses the projection automatically stays aligned with civil time. The hosts discuss practicalities like needing to adjust the gnomon to place Sunlight in the correct beam and the possible adjustments to work in both northern and southern hemispheres. The Sunquest is highlighted as a remarkable attempt to incorporate complex astronomical corrections into a simple device.

Beyond the sundial: context and curiosities

The conversation touches on broader topics such as the subsolar point and Lahaina Noon, a Hawaiian phenomenon where the Sun is directly overhead and shadows disappear. The hosts note that Hawaii is uniquely positioned for Lahaina Noon due to its southern latitude relative to the rest of the United States, and that a sundial would be useless at the moment of overhead Sun. They reference the subsolar point website to track the daily position of the Sun and discuss the cultural and scientific implications of time, navigation by the Sun and stars, and the changing pace of life in a world that relies on both precise clocks and solar rhythms. The episode closes with reflections on the maturing relationship between timekeeping, astronomy, and everyday life.

Takeaways

• Sundials measure solar time which can diverge from civil time by minutes over the year.
• The equation of time arises from the Earths orbit and axial tilt and can be illustrated with a simple model of a tilted equator and a solar gnomon.
• The Schmoyer Sunquest sundial demonstrates a practical automatic correction for the equation of time via an analemma shaped gnomon aimed at the Sun.
• Cultural moments like Lahaina Noon highlight how real world time depends on location and celestial geometry.