Sundial reading guide and clock-time converter

On a correctly installed sundial, read where the gnomon’s shadow crosses the marked hour lines. A horizontal polar-style dial must be level, designed for its latitude, and aligned with true north or south rather than uncorrected magnetic north. Its polar edge points toward the celestial pole at an angle equal to the absolute latitude. Read intermediate marks according to the dial’s own scale.

A simple dial shows apparent solar time. Enter that reading, date, longitude and the civil UTC offset in effect, including daylight saving. Civil minutes = solar minutes − equation of time − 4 × east longitude + 60 × UTC offset. The daily equation-of-time approximation follows NOAA.

Reference: NOAA solar time equations

The result above uses these exact values. This snapshot is included when the page is printed so the output can be checked against the original measurements.

Date

2024-06-21

Apparent solar time

12 decimal h

Longitude, east positive

0 °

UTC offset including daylight saving

0 h

Primary result: Approximate civil time: 12:01.

• Check the selected mode, units and input values before using the result.
• Read the assumptions and rounding rules; compare the worked example with your intended calculation.
• A numerical result does not supply missing context or change the stated limitations.

This approximate correction does not repair an incorrectly aligned dial or one made for another latitude. Some dials already include longitude or seasonal corrections; do not apply them twice. You must supply the correct UTC offset for the date. No shadow can be read when the Sun is obscured or below the horizon.