The time we use every day is a human invention rooted in the natural cycles of the sun. Solar time is a measure of time based on the apparent position of the sun in the sky, with the moment the sun reaches its highest point directly overhead defining local noon. Before mechanical clocks, this was the most reliable way to mark the passage of a day. The standardization of time has created a significant difference between this natural sun time and the uniform time displayed on our clocks.
Apparent Solar Time Versus Mean Solar Time
The most natural way to measure time is with a sundial, which indicates apparent solar time. This measurement is based on the actual, observed position of the sun as it crosses the sky, with the sun’s passage across the local meridian marking 12:00 apparent time. However, the length of the apparent solar day is not constant throughout the year. This daily inconsistency made apparent solar time unusable for the accurate mechanical clocks developed centuries ago.
To solve this problem, astronomers devised a standardized measure called mean solar time. This is a fictional, uniform time based on a hypothetical “mean sun” that moves at a perfectly steady speed along the celestial equator. The mean solar day is defined as the average length of all the apparent solar days over a year, which is exactly 24 hours. Mean solar time became the standard for mechanical clocks because it provided the necessary uniformity for consistent timekeeping.
Understanding the Equation of Time
The difference between the time shown on a sundial (apparent solar time) and the time on a clock (mean solar time) is known as the Equation of Time. This discrepancy fluctuates throughout the year, causing the clock and the sundial to be out of sync by up to a little over 16 minutes. The variation is entirely astronomical, stemming from two independent factors related to Earth’s movement around the sun.
The first factor is the elliptical shape of the Earth’s orbit, which means our planet’s speed changes as it travels around the sun. Earth moves faster when it is closer to the sun and slower when it is farther away. This changing speed causes the true sun’s eastward motion to be non-uniform, meaning the length of the solar day constantly speeds up and slows down relative to the steady pace of the hypothetical mean sun.
The second factor contributing to the Equation of Time is the Earth’s axial tilt, also known as the obliquity of the ecliptic. The sun’s apparent path, called the ecliptic, is tilted by approximately 23.5 degrees relative to the celestial equator. Near the solstices, the sun’s movement is primarily north or south, while near the equinoxes, a larger component of its motion is east or west.
This tilt means that the sun’s apparent motion, when projected onto the celestial equator, is faster at some points in the year and slower at others. The combination of the elliptical orbit and the axial tilt creates a composite curve of variation. This curve defines the Equation of Time, which must be added or subtracted from the sundial reading to match the constant time on a mechanical clock.
From Local Solar Time to Standard Time Zones
Even after the invention of mean solar time, every city and town historically set its clocks based on its own local solar noon, a system called Local Mean Time. Because the sun reaches its highest point four minutes later for every degree of longitude traveled west, clocks in neighboring towns were often set a few minutes apart. This was of little consequence when travel was slow and local commerce was the norm.
The rapid expansion of the railroad network in the mid-19th century created a chaotic situation, as train schedules could not be reliably coordinated across hundreds of different local times. To solve this commercial and safety crisis, railroad companies in the United States and Canada instituted a system of standard time zones on November 18, 1883. These zones standardized time over a wide geographic area, typically spanning 15 degrees of longitude.
This standardization moved the world away from local sun-based time entirely, introducing a large, constant offset for most locations. Clocks in an entire time zone are set to the mean solar time of a single, central meridian, meaning the true solar noon rarely aligns with 12:00 on the clock. While the Equation of Time explains the daily variation between the sun and a clock, the time zone system explains the large, fixed difference, which can cause local solar noon to occur well after 1:00 PM in some places.