Sundial Furniture

The essential lines on a sundial are those representing the hours of the day, with their accompanying figures. All other lines and symbols constitute the "furniture" of the sundial. The furniture most commonly found on sundials shows:

the difference between apparent and mean time (equation of time)
the sun's declination throughout the year
the time of sunrise and sunset
the Signs of the Zodiac and the date of the sun's entrance into each; and
the points of the compass

sundial furniture Other mathematical and astronomical data may be added, such as meridian lines that show when it is noon at any particular place; the Babylonian hours (reckoned from sunrise to sunset); the Jewish hours (the old, unequal planetary hours); and the Italian hours (beginning at sunset). Such lines increase the interest and usefulness of the sundials displaying them. There seemed to be no limit to the amount of furniture that early dialists were wont to place upon a single sundial, an admirable example of such a sundial, made in the 17th century, which has more lines than most people would want to compute. Aside from the time of day the facts depicted on this sundial are varied and interesting; therefore a short description of them will not be amiss. Upon it are drawn:

Lines of declination, which show the path of the shadow of the nodus when the sun is on the equator and in the two tropics. On the meridian or substyle line is marked the position of the shadow of the nodus for each degree of declination.
Azimuth lines, which show the position of the sun, throughout the day, with respect to the points of the compass; or its angular distance east and west of the meridian.
Lines showing the length of the day; the time of sunrise and sunset.
The sundial is constructed for London, but the time of sunrise and sunset in Constantinople is also shown.
Lines showing the rising and setting of the Signs of the Zodiac (ascending and descending Signs); and the position of the sun with respect to the Signs. These lines were used by astrologers to tell the position of the sun in relation to its Cuspis; and they did not have any astronomical application.
Lines showing the altitude of the sun, its angular height above the horizon.
Date of the sun's entrance into each Sign of the Zodiac, and its declination at that time.
It is evident that the computation of such a sundial would require a good knowledge of celestial mechanics; and also, in the 17th century, the services of an expert engraver.

THE LINES OF DECLINATION AND ZODIACAL SIGNS
Early dialists often placed upon their sundials certain lines, called lines of declination, which recorded the entrance of the sun into the various Signs of the Zodiac. This gave them a measure of time, because it takes the sun about a month to pass from the beginning of one sign to the beginning of the next. Feast days, Holy days, events of importance, and the time of year were also shown by these lines; and if one wished to be facetious he could add lines commemorating birthdays, wedding anniversaries, and so on.
Today, such lines are usually used for ornamentation rather than for the utilitarian purposes of not many centuries ago. Since the location of these lines on the sundial plate depends upon the position of the sun, they have not entirely lost their usefulness, even in this day and age. They have an educational value, for by them one may obtain a clearer conception of the motion of the earth in relation to its all important luminary the sun.

The sun, in its apparent movement among the stars, traces out a path called the ecliptic, the plane of which is inclined to the plane of the celestial equator at an angle of about 23°27'. During one half of the year the sun appears north of the celestial equator and during the other half, south of it. The sun's distance north or south of the equator is called its declination, (expressed in degrees and minutes of arc), which varies from day to day. The amount of this declination for each day in the year, at apparent noon, is given in the accompanying table where the northern declination is preceded by a plus (+) sign and the southern declination by a minus ( ) sign.
Although data have been omitted, which may be easily obtained from any good almanac, this table is inserted because it is not always found in a convenient form for use in the construction of the lines of declination.
These lines are also known as the Arcs of the Signs, and on early sundials the zodiacal symbols were often placed at their extremities. The Zodiac is a zone in the sky 16° wide (8° on each side of the ecliptic), in or near which the planets and sun appear to move. Beginning at the point on the ecliptic, which marks the position of the sun at the Vernal Equinox, this zone or belt is divided into 12 parts of 30° each, called Signs. The Signs derive their names from the constellations with which they coincided, about 2000 years ago.
The Signs meant much to the ancients, who were well acquainted with the meanings and omens attached to each. Even today, the entrance of the sun into the Sign of Aries marks the beginning of spring; and summer begins when it enters the Sign of Cancer.

The following table shows the zodiacal symbol attendant to each Sign and the approximate date of the sun's entrance into each Sign.
Date of Sun's Symbol Name Entrance

Aries Ram March 21
Taurus Bull Spring April 20
Gemini Twins Signs May 21
Cancer Crab June 21
Leo Lion Summer July 23
Virgo Virgin Signs August 13
Libra Balance September 23
Scorpius Scorpion Fall October 24
Sagittarius Archer Signs November 22
Capricornus Goat December 22
Aquarius Water-Bearer Winter January 20
Pisces Fishes Signs February 19

One must not lose sight of the fact that due to the precession (retrograde or backward motion) of the equinoxes along the ecliptic, each Sign has moved backward 300 into the constellation west of it; so that today, the Sign of Aries is in the constellation of Pisces, and so on. The Signs are independent and they have no connection with the apparent position of the sun in the constellations.

Thus the usefulness of a sundial would be increased if the names of the zodiacal constellations are placed upon it as well as the attendant Signs. In order to do this, find the date upon which the sun enters a constellation or Sign. From the table observe the declination of the sun on that day. Then proceed, to plot the line of declination for that day on the sundial plate; and place the symbol or name of the Sign or constellation at the extremities of the line.

We know of one professor of astronomy in a large college, who uses a sundial with its lines of declination as a practical example to show the motions of the sun and earth. The students enjoy the lesson and more often than not they return many times to watch the sundial, and always delight in explaining its use to their friends.

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