Portable Universal Equinoctial Dial

Astronomy

Accession Number: 2014.ast.25

Description:

This is an instrument that consists of a brass base and a brass stand on which a semi-circular metal dial plate and gnomon are attached. The base is circular and has a felt bottom. It has a compass set into it with a central needle, engraved cardinal directions and scale around the rim graduated in degrees. Next to this, on the vertical edge of the base, there is a small rod emerging from a slot. Sliding this fixes the compass needle in place.

The top portion of the instrument consists of two semi-circular pieces. One of these is oriented vertically in order to hold a thin rod (the gnomon) in place, and the other horizontal half-moon piece is oriented more horizontally and is the dial plate set parallel to the Equator. The dial plate and gnomon portion of the instrument is controlled by a knob affixed to the stand of the instrument, which is attached to a broad needle that indicates on an arc shaped scale graduated from 0 to 90.

The dial plate also contains an engraved scale. The number 12 appears in the middle of the intersection of the vertical and horizontal half moons while the left portion of the dial plate contains the numbers 1-8 and the right contains the numbers 11-4. These numbers represent the hours in a day.

There is a metal maker’s label fitted to the base, and a small paper label affixed to the top of the base.

Alternative Name: Dial, Equinoctial Sundial

Primary Materials: Metal: Copper Alloy, Metal: Iron Alloy

Markings:

On the metal label affixed to the base: “EASTERN SCIENCE SUPPLY CO. BOSTON MASS. U.S.A.”

On the paper sticker affixed to the base: “AST 34”.

On the interior of the compass: “MADE IN FRANCE”

Dimensions (cm):

Height = 13.8 (18.4 max.), Width = 12.4 (16 max), Length = 8.8

Function:

The Portable Universal Equinoctial Dial is a type of sundial that contains a dial plate that lies parallel to the Equator, from which it gets its name. Like most sundials, the equinoctial dial is used to tell the time of day from the position of the Sun. This instrument can also be adjusted for latitude and oriented using the compass. The equinoctial dial is used to show Local Apparent Time (L.A.T), which can then be converted to standard time. One drawback of the equinoctial dial is that no clear shadow is created during the spring and autumn equinoxes due to the fact that the sun moves in a circle nearly the same as the equatorial plane.

Condition:

Excellent: There are some small scratches and corrosion on the metal components of the instrument; there is more corrosion on the rear of the scale and the stand. The enamel coating on the knob is worn away in places and the metal underneath is oxidized. the felt base is in good condition, with some wear damage around the edge. The rod that fixes the compass needle in places is somewhat stiff but functional. All the components of the instrument appear to be present.

Associated Instruments: 2012.ast.17

Manufacturer: Eastern Science Supply Co.

Date of Manufacture: ca. 1930s-1940s

Provenance:

The provenance of this instrument is unknown. It may have been purchased or acquired by a faculty member at the Department of Astronomy & Astrophysics as a teaching item. More recently, it has been on display at the department.

According to information with the object, this was “[m]arketed for teaching physics and astronomy at the undergraduate level.”

Additional Information and References:

Higton, Hester. Sundials: An Illustrated History of Portable Dials. London: Philip Wilson Publishers, 2001.

Waugh, Albert E. Sundials: Their Theory and Construction. New York: Dover Publications, Inc., 1973.