Ó 2000-2001 Stephen A. Shepherd
Sundial, Compass Rose & Analemma
From a very early age sundials have fascinated me. The first time I saw a gnomon cast a shadow onto a dial with numbers to indicate the relative time, I was hooked. The first sundial was undoubtedly a stick casting a shadow on the ground and our early ancestors begin to plot the day by the movement of the shadow. When people began to break the day up into hours, hash marks were added to delineate the hours on to some extent the minutes of the hour. The face of the sundial is laid out with marks to indicate the hours, half hours and quarter hours. Further delineation can be done to break it down into minutes if the dial is large enough. The pattern shown is for a sundial with a gnomon that is 1/8” thick or less, thicker gnomons require a different layout to accommodate the thickness of the gnomon. The angle on the top edge of the gnomon is the same angle as the latitude where the sundial is used. If it is a traveling sundial, it must have an adjustable gnomon to adjust the particular latitude. If it is a permanent installation, the gnomon can be fixed. Most commercially made sundials have preset gnomons usually at 40º, as that is the latitude at which most people live. The plans here are for flat sundials however vertical sundials can be mounted on buildings and there are many other types of sundials such as pillar dials, etc. The flat sundial can also be made into a smaller version that is transportable and many old pocket versions are sophisticated tools that can have elaborate decorations and adornments. Traveling or pocket dials almost always have a compass incorporated to be able to accurately orient the dial to get a proper reading.
The dial can be a simple piece of plain wood with a thin veneer gnomon to cast the shadow on the hour marks on the wood. The dial can be carved with incised lines and fancy numbers on figured wood to produce a fine piece of woodwork. The sundial face can be made of veneer carefully fit together with contrasting woods or grains to delineate the hours and halves. Also flat wire inlay can be used to make the hash marks and even the numbers. When laying out the dial make sure that the measurements are accurate to insure that this horological tool keeps proper time. The finer the lines between the hours the more accurate the sundial will be. The left side (of 12 o’clock) and the right side are identical mirror images of each other and the spaces should be the same on both sides. When choosing wood to make a sundial, make sure that it is quartersawn and should be of a species that is stable once dry. This insures that the marks on the sundial will not move when the wood moves. Building up a face with a center joint of two quartersawn pieces of wood, can provide very stable sundial that will always give accurate readings. A sundial made with a veneer face can provide very accurate hour marks with the joints between the various pieces of veneer. This type of face is not necessarily the best if the dial is going to have constant exposure to sunlight. The groundwork for the veneer dial should be of a stable quartersawn material to avoid any problems of movement. The gnomon can be a simple piece of veneer or it can be pierced and decorated to make it quite elaborate. The only important thing is that the gnomon is at the proper angle (latitude) and that the top edge is straight to cast an accurate shadow. The gnomon can also be made of brass, bronze, silver or any non-ferrous metal, especially on pocket dials with a magnet compass. On pocket dials the gnomon usually folds down for ease of transportation. For permanent exterior applications, the wood for the dial should be able to withstand exposure to the elements. Teak, mahogany or any wood that does well in the weather are good choices for exterior dials. The gnomon also needs to be able to take this outdoors application as well.
The compass is necessary to orient the sundial with its gnomon-pointing north. The compass rose was usually made of paper; some were engraved brass and other non-ferrous metals. The roses were marked with the cardinal directions; the semi-cardinal points and many have marks at all 360º points. Many early compasses are made of wood with an iron or steel needle that is magnetized by rubbing it on another magnet. It has a hard center pivot point on the pointing needle that rests on the balance needle also of a very hard material. Some fancy compasses have jeweled pivots for added wear. The pointing needle has to be perfectly balanced to work properly. There is another type of compass that incorporates the compass rose with a magnetic needle properly mounted to the rose and the entire compass rose is attached to a piece of cork. By floating in a bowl of water the compass rose will always point north. This is called a wet card compass. Mounting the rose on a central pivot in a compass case can do the same and this is called a dry card compass. These are probably the earliest forms of compasses whose design remained popular.
The Analemma is a necessary tool for adjusting the time of the sundial to the different locations of the sun on a particular day during the year. Because of the movement of the Earth in its yearly cycle around the sun and the wobble of the Earth the Analemma takes this into account and tells on a given day of the year if the sun is running slow or fast. The sundial is oriented with the compass, a reading is taken and the Analemma is consulted to make the adjustment for the day of the year and you have the current time at your location. Without an Analemma your sundial will read slow by 14 minutes in February and 16 minutes fast in November. The classical shape of an Analemma is that of an elongated figure 8. This is the position of the sun in the sky at noon everyday.
Back to Index