Apparent solar time, measured by the sun’s position, is highly specific to one’s location and was historically used by civilizations worldwide. This differs fundamentally from standard clock time (mean solar time), which is a uniform, averaged measurement dividing the day into 24 equal segments. Standardized time was adopted for modern societal needs like travel and communication. Before accurate mechanical clocks and time zones, estimating time by observing the sun was a necessary skill for navigation and daily life. This ability remains a valuable, non-technical way to connect with natural rhythms.
The Fundamentals of Solar Time
Solar time estimation is based on the Earth’s steady rotation, causing the sun to appear to move across the sky in a predictable arc. The Earth completes a 360-degree rotation in 24 hours, meaning the sun appears to traverse approximately 15 degrees of the sky every hour. This rate is the basis for all sun-based timekeeping methods.
The most significant point in a solar day is local solar noon, which occurs when the sun reaches its highest point in the sky. At this moment, the shadow cast by an upright object is at its shortest. This shortest shadow points due north in the Northern Hemisphere and due south in the Southern Hemisphere. This precise alignment provides the zero-hour reference point from which all other hours are measured.
Estimating Time Using the Shadow Stick Method
The most methodical way to gauge solar time is by creating a rudimentary sundial using a gnomon, or shadow stick. This method relies on marking the movement of a shadow on a level surface throughout the day. Anchor a straight stick vertically into the ground or a flat board in a location that receives full sun.
The foundation of this method is determining local solar noon, the moment when the sun is highest. Monitor the shadow cast by the stick, marking its end point every few minutes around midday. The shadow length decreases until it stops shortening, which is solar noon. A line drawn from the base of the stick through this shortest shadow point establishes the noon line, serving as the 12 o’clock mark.
After establishing the noon mark, use a clock to mark the passage of subsequent hours. One hour later, mark the new position of the shadow’s tip and draw a line from the stick’s base to this point. Repeating this process hourly creates a customized dial specific to that latitude and time of year. Each marked line represents an hour of local apparent solar time.
Once the initial markings are complete, the shadow stick estimates the time by observing where the shadow falls between the marked lines. While this simple horizontal sundial can provide readings accurate to within 15 minutes, its accuracy is dependent on the sun’s changing path. The marked hours will only be precise for the specific day they were created, as the angle between hourly lines varies throughout the year.
Quick Estimation Using Hand Widths
A less precise method for estimating time relies on using one’s hand as a measuring tool, primarily for determining the time remaining until sunset. This technique uses the angular size of a hand held at arm’s length to measure the sun’s altitude above the horizon. The measurement is taken by fully extending one arm and aligning the bottom edge of the hand with the horizon.
The fingers are then stacked horizontally between the horizon and the bottom edge of the sun. Each finger width, measured across the four fingers held together, represents approximately 15 minutes of remaining daylight. A full hand span, or closed fist, therefore covers roughly one hour. Counting the number of stacked fingers or hands needed to bridge the gap calculates the approximate time until the sun disappears.
This estimation works because a typical person’s hand at arm’s length subtends an angle of about 15 degrees, matching the sun’s hourly movement. This method estimates the time until the sun reaches the horizon, not necessarily the time until total darkness, which varies with latitude. The calculation must also follow the sun’s actual path, which is often diagonal to the horizon, especially at higher latitudes.
Converting Solar Time to Standard Clock Time
Local apparent solar time rarely matches standard clock time, requiring two corrections.
Longitudinal Correction
The first adjustment accounts for the difference between the observer’s longitude and the time zone’s central meridian. Since the Earth rotates one degree every four minutes, an observer east of the central meridian will see solar noon earlier than the time zone’s clock noon. This correction is calculated by multiplying the difference in degrees between the two meridians by four minutes.
Equation of Time (EoT)
The second correction involves the Equation of Time (EoT), which accounts for the seasonal irregularity in the sun’s apparent movement. This irregularity is caused by the Earth’s elliptical orbit and the tilt of its axis, making the length of an apparent solar day vary throughout the year. The EoT value, which can be positive or negative, represents the difference in minutes between apparent solar time (sundial time) and mean solar time (clock time) on any given day.
The EoT correction ranges from approximately minus 16 minutes in early November to plus 14 minutes in mid-February, depending on the day. Once both the longitude correction and the EoT correction have been applied, the resulting time is mean solar time for the observer’s location. A final adjustment is necessary if the local jurisdiction is observing Daylight Saving Time, which adds an additional hour.