Brass Nocturnal: Historical Star Clock & Tide Calculator

The brass nocturnal is made primarily of copper and zinc, with the zinc content typically ranging from 5% to 45%. Historically, the device served as a star clock to determine the hours of the night based on the position of certain stars (such as the Big Dipper), and as a tide calculator to determine the tides, which was particularly important for navigation.

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To determine precise local time with a brass sundial, two corrections must be taken into account: - Equation of time: Because the sun does not move across the sky at a constant speed throughout the year, the time shown by a sundial (true local time) deviates from the constant time (mean local time) by up to +16/-14 minutes.

- Longitude adjustment: Local time depends on the exact longitude. Since time zones (such as CET) are usually based on a reference meridian, a correction of 4 minutes per degree of longitude must be applied for locations outside this meridian.

The brass nocturnal features a date scale on the outer disc and an hour or time scale on the inner, rotating disc. To determine sidereal time, the instrument's pointer is first set to the current date. For correct alignment, the nocturnal must be held vertically, with Polaris sighted through the central hole in the handle. The movable arm is then rotated until it aligns with the guiding stars of Ursa Major (or alternatively, Cassiopeia). The point where the arm intersects the hour scale indicates the local sidereal time.