Pocket Sundial

What is one?

Need a Universal Ring Sun Watch or what some call a pocket ring dial? This is a reproduction of a Universal Equinoctial Ring Dial invented by the brilliant English mathematician William Oughtred (1575-1660).

This ring dial is a sundial that in theory can be used universally to determine the time in almost any part of the world. It comprises two rings, namely a meridian ring engraved with scales of latitude, and a pivotal equinoctial hour ring. The key to Oughtred's invention of this particular sundial is the bridge which, lying in the polar axis of the instrument, spans the meridian ring and contains a plate engraved with the scales of the sun's declination and the calendar.

Most pocket sundials only work if you can orientate them correctly, so you need a compass before you can use them to tell time. The universal ring variety, though, is self-orientating. This means you not only can tell time without a compass, but you can also use the sundial to tell you which way is north.

It is easy to use with a simple and elegant instrument, which was in popular use at sea in the 17th and 18th centuries for determining the time aboard a ship. This particular example is well made in brass but with steel sliding parts to facilitate its use. As advertised it is quite small (60mm in diameter) and is able to fold flat for easy storage, but works all the same.

It can be used to tell time whether aboard a ship or trekking across country. Likewise, as an educational aid, it could be a useful means of helping to explain the principles of navigation and astronomy. It might also make an attractive little Christmas present for anyone who is interested in sundials, or in navigation. A great gift for adult and child alike.

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What is a Equinoctial Ring Dial?

This group is known as equinoctial, equatorial, astronomical, or universal ring dials. Four names for the same thing! They usually have an eyelet from which they are suspended but are occasionally fixed to a solid base.

In use, the suspension point is et against the latitude scale, and the sliding cursor on the diametral arm is adjusted for the sun's declination. One side of the arm is graduated in degrees and the other shows the date. Either scale may be used. The cursor has a small hole which allows a spot of light from the sun to fall on the inside edge of the equatorial hour ring which is marked with equiangular hour lines.

The dial is suspended from the eyelet and rotated until the light spot falls on the line engraved along the center of the inside edge of the hour ring. In this position the spot shows the time, the central arm is parallel to the earth's axis, the hour ring is parallel to the earth's equator, and the vertical ring lies in the plane of the meridian. For precise readings the central arm is turned on its pivots to face the sun and so produce an undistorted light spot.

At the equator, as the dial is rotated the light spot will cross the hour ring at right angles, but as the latitude increases the angle becomes progressively more oblique and the setting of the dial progressively more imprecise. At the pole the spot moves parallel to the equatorial ring making it impossible to set the dial by this method. However, being an equinoctial dial, if it is correctly oriented, its accuracy of time measurement is unaffected by latitude; it is only the setting of the dial which becomes more inaccurate as the latitude increases.

The altitude ring dial has to be reoriented every time a reading is taken. After initial setting the equinoctial ring dial would, theoretically, continue to function for the rest of the day, although the cursor would have to be rotated to follow the sun.

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