The text and graphics on this page is based on the Queen's College Cambridge website by Robin Walker.
Sundials are typically used to tell time. However, by adding other markers on the dials, we can use these dials to provide more information. These markers are known as dial furniture. We shall use the sundial from Queens' College in Cambridge, England to explain dial furniture.
This photo dates from late 1968. The shadow of the gnomon lies exactly on the 2 o'clock line. The shadow of the ball can be seen at bearing SWBS at roughly 25° elevation.
This is how the dial would look like if the lines on the dial plate are perfectly computed. The actual dial (see Figure 58 above) only approximates to this form.
Information can be read from the dial as follows.
Look for the shadow of the gnomon among the lines radiating from top centre to the roman numerals on the border of Figure 58. The roman numerals give the hour of the day, and the minutes between the hours can be estimated. Note that the quarter-hours are marked. The following are points to note when reading sundials, in particular the Queens' College dial shown above:
For all further information, you need to locate the shadow of the ball on the gnomon amongst the pattern of curves and lines on the dial. The ball is indicated in Figure 58.
Look for the shadow of the ball amongst the curves coloured green in Figure 59 above. Then, if you are between midwinter and midsummer, look to the right-hand ends of the green curves; or if you are between midsummer and midwinter, look to the left-hand ends of the green curves.
The two green lines that the ball's shadow lies between will enclose the current sign of the zodiac. On the dial itself (Figure 58), the sign of the zodiac is drawn in full, and accompanied by its symbol (refer to Figure 60). On Figure 59, only the symbol is shown.
Written in Latin outside the signs of the zodiac are the names of the months (too small to reproduce in Figure 59), with the breaks between the months shown. By interpolating the position of the ball's shadow between two green lines, and extending that interpolation to the column of month names, you can tell the month of the year, and estimate the date within the month.
Note the position of the ball's shadow between the green lines, and extend that relationship to the column on the left labeled ORTUS SOLIS. Times of sunrise are marked for each green line, and you have to interpolate between the given times to find the current time of sunrise.
Note the position of the ball's shadow between the green lines, and extend that relationship to the column on the right marked LONGITUDO. The length of daylight is given in hours and minutes for each green line, and you have to interpolate between the given figures to find the current length of day.
Note the position of the ball's shadow amongst the red lines. Each red line is marked with elevation in degrees above the horizon, at intervals of ten degrees. You can interpolate to estimate the elevation to the nearest degree.
We shall go on to explain how we get these red lines.
The relation that connects altitudes with the hour angle (HA) for the Sun for a particular latitude Ø and declination equation is,
sin (alt) = sin Ø sin + cos Ø cos cos (HA)
Using elementary calculus, we will differentiate the above equation to get,
This equation shows how the altitude varies with respect to the hour angle.
Note the position of the ball's shadow amongst the vertical lines. These are shown blue on the diagram above, but are black on the dial itself. Each vertical line is marked with a compass bearing, as shown in Figure 59.
There is one further set of lines, shown purple in the diagram above, but black on the dial itself. These lines subdivide daylight hours into twelve equal parts, whatever the time of year, which was a common method of measuring working hours before the advent of clocks. In Figure 59, the temporary hour lines coincide with the solar time lines (the ones projecting to the roman numerals) at the equinox (on Figure 59, the green line which is straight and sloping). On the actual dial (Figure 58), the agreement is not as good.
Underneath the dial is a table of numbers:
You are required to know the day of the lunar month (1-30). For instance, Full Moon is on day 15. You locate the current day of the lunar month on the top or bottom line, then read off a time from the centre line, in hours and minutes. This gives the time which needs to be added to, or subtracted from, the apparent time as indicated by the shadow of the gnomon as cast by moonlight, in order to yield the time of night.