an instrument for determining clock corrections from observations of the sun through the method of corresponding altitudes. A sun ring consists of a metal ring suspended vertically from a pivot; such a suspension ensures that the position of the ring remains vertical. The rim of the ring has a small hole at approximately 45° from the pivot. A scale calibrated in arbitrary units (usually millimeters) is attached to the inner surface of the ring opposite the hole.
When the ring is oriented so that its plane passes through the sun, a dot of sunlight is projected through the hole onto the scale. The time at which the dot passes over some graduation on the scale is read from the clock to be corrected; this observation should be made not later than two hours before noon. The procedure is repeated after noon, and the time at which the dot passes over the same graduation on the scale is noted. The average of the two times gives the clock reading to within half a minute at the apparent noon. The clock reading at the mean solar noon can then be obtained by adding the equation of time. By taking into account the time zone and the geographic longitude of the place of observation, the zone (standard) time and, consequently, the clock correction can be calculated.
The sun ring, as an instrument for the approximate measurement of the zenith distance of the sun, was described as early as the 16th century. As an instrument for reckoning time by means of corresponding altitudes of the sun, the sun ring was first used by S. P. Glazenap, initially in the shape of a triangle, in 1873.