Accurate Standard Time Sundials

A sundial is a fascinating device that uses the sun’s position to tell the time. It’s a simple and elegant way to measure time which has been used for thousands of years.

From a scientific perspective, sundials are invaluable tools for understanding the Earth’s rotation and the position of the sun in the sky. Studying them helps us understand the concept of solar time and how it differs from the timekeeping methods we commonly use.

This site offers a variety of sundial resources, including:

• Guides to understand sundial components and calibration techniques.
• Tool to create sundials (time of day and calendar sundials).
• Utility for tracking the sun’s position in the sky throughout the year.
• Simulation tool to understand sundial performance.

Types of Sundials

Although there are many types of sundials, this site focuses on horizontal sundials, both for tracking time of day and calendar dates.

Time of Day Sundial

Sundial Base	Base where the hour lines are imprinted and the gnomon is fixed.
Gnomon	Part that casts the shadow onto the sundial base.
Hour Line	Line marking the shadow position at specific hours of the day.
Shadow Edge	Edge of the gnomon shadow that marks the current time. It moves along the hour lines as time progresses.
Sundial Center	Point in the sundial base where all the hour lines converge and the gnomon is fixed.
North-South Gnomon Slot	Slot in the sundial base where the gnomon fits. Also used to align the base with the polar north/south.
Gnomon Inclination	Angle between the sundial base and the gnomon, typically set to the latitude of the sundial’s location.

Calendar Sundial

Nodus	Pointed part that casts a shadow onto the sundial base.
Nodus Shadow	Shadow cast by the nodus. Its tip is used to determine the current month.
Declination Line	Line on the sundial base, representing the sun’s path on a specific day of the year.

Sundial Calibrations

Making an accurate sundial is not trivial, the traditional stick in the ground may give the correct time at noon and be off by 2 hours at sunset. It may work well for a few days and be significantly off for the rest of the year.

Here are the main calibration steps to get an accurate sundial.

Gnomon Inclination

The gnomon has to be parallel to the earth’s axis.

Why? A correctly aligned gnomon will cast a shadow that moves with a constant speed throughout the year.

How? Set the gnomon to an angle equal to the latitude of your location.
As an example if you’re at 45º latitude, the gnomon should be inclined 45º from the horizontal.

Hour Line Calibration

The sundial hour lines need to be adjusted according to the latitude.

Why? The closer we get to the equator, the higher the sun rises in the sky during the day, and the circular motion of the gnomon shade becomes more elliptical.

How? To calculate the hour line angle on a solar time horizontal sundial use the formula:

angle = arctan(tan(15º * (hour - 12)) * sin(latitude)).

Where:

• angle = Angle of the hour line
• hour = Hour in 24h format
• latitude = Latitude of your location

You can also use the Sundial Builder to create a standard time sundial for your location.

Standard Time Rotation

To have a standard time sundial, the hour lines must be rotated to adjust for the difference between standard time and solar time in your location.

Why? Typical sundials display apparent solar time which changes according to your longitude. For instance, apparent solar time changes ~20 minutes between Philadelphia and Pittsburgh which are ~5º (~410Km) apart. However, standard time is the same in both cities.

How? To adjust the sundial to local time use the formula:

std_t = solar_t + tz_offset - longitude/15

Where:

• std_t = Standard time in hours
• solar_t = Solar time in hours
• tz_offset = Timezone offset from GMT in hours
• longitude = Longitude in degrees

You can also use the Sundial Builder to create a standard time sundial.

North Alignment

The sundial must be aligned to the true north (or south in the southern hemisphere).

Why? A misaligned sundial will have a constant deviation in the shadow, which will distort the displayed time.

How? There are a few ways:

• Using a compass, however it’s hard to get an accurate enough alignment this way due to the needle length, magnetic north deviation and interference of other magnetic fields.
• Alignment with the north star (Polaris) on the northern hemisphere.
• Probably the easiest way is to rotate the sundial for the shadow match the correct time.

Equation of Time Adjustment

Equation describes the difference between apparent solar time and mean solar time. Adjust the sundial time according to the equation of time.

Why? Even the best calibrated sundial will be off by a few minutes depending on the time of year. This is caused by:

1. Earth has an axial tilt of 23.4º.
2. Earth has an elliptical orbit around the sun.

The effect is that the solar day is sometimes slightly longer or shorter than 24 hours, causing the solar time to be ahead or behind the standard time.

How? Add or subtract the minutes from the Equation of Time to the sundial time.