This is an idealised diagram of the Dial in Old Court: the lines of the Dial are shown as they ought to be, if perfectly computed. The actual Dial only approximates to this form.
To read the Dial, first identify the gnomon: a metal rod projecting from the centre of the Sun in his Glory at the top of the Dial (other metal rods are just supports for the gnomon). Information can be read from the Dial as follows.
Time of day
Look for the shadow of the gnomon among the straight black lines radiating from the Sun in his Glory to the roman numerals in the blue border. The roman numerals give the hour of day in Cambridge solar time, and the minutes between the hours can be estimated (the quarter-hours are marked). Remember these things when reading sundials, and our Dial in particular:
- Our watches and clocks are set (during winter) to a mean time based on time at the Greenwich Meridian, called Greenwich Mean Time. Sundials at a longitude different from the Greenwich Meridian will display the solar time appropriate to their longitude. Cambridge is close enough to the Greenwich Meridian for time in Cambridge to be almost the same as time at Greenwich, so no correction on account of longitude is required when reading our Dial.
- Between the last Sunday of March and the day before the last Sunday of October, clocks and watches are set to British Summer Time, which is one hour ahead of Greenwich Mean Time, and therefore about one hour ahead of mean time in Cambridge. So do not be surprised if the Dial shows 1 p.m. when the clock shows 2 p.m.
- Solar time (as given by the Dial) may differ from mean time (as shown by clocks and watches) by up to 16 minutes in either direction, according to the time of year. This divergence is given by the Equation of Time. The lengths of solar days are not exactly 24 hours of our synthetic mean time. This arises because:
- the earth’s axis of rotation is not perpendicular to the plane of the earth’s orbit around the sun. This obliquity is responsible for the seasons of the year. It also causes solar time (as given by sundials) to differ from mean time according to season. The divergence arising from this factor taken alone is zero at four times in the year: the solstices (around June 20/21 and December 21/22) and the equinoxes (around March 20 and September 22/23).
- the earth’s orbit around the sun is elliptical rather than perfectly circular, so the earth’s angular velocity around the sun is variable. A solar day consists of one absolute rotation of the earth plus a little bit extra to make up for one day’s worth of progression of the earth around the sun, and the little bit extra varies according to the earth’s angular velocity about the sun at that time of year. This causes another variation to the length of solar day, and therefore a second divergence between solar time and mean time. The divergence arising from this factor taken alone is zero twice a year: at aphelion (sometime in the first week of July) and perihelion (sometime in the first week of January).
- When the two factors are combined, the Equation of Time yields zero divergence around April 16, June 15, September 1, and December 25. Around these dates, sundials should show a time very close to the mean time given by clocks and watches. Sundials are at their most slow (14.3 mins) around February 12, and at their most fast (16.4 mins) around November 4.
- The last time the Queens’ Dial was repainted, the pattern was located several inches too far to the left. If painted correctly, the vertical line representing midday should, if projected, pass through the intersection of the line of the gnomon with the plane of the wall. Instead, the artist appears to have aligned the midday line with a strut supporting the gnomon. This error will cause the Queens’ Dial to read time about 10 minutes fast in the middle of the day.
For all further information, you need to locate the shadow of the gold ball on the gnomon amongst the pattern of curves and lines on the Dial. The ball is fixed to the gnomon level with the horizontal line marked HORIZON on the Dial.
Time of year (Sign of Zodiac)
Look for the shadow of the ball amongst the green hyperbolic curves. On the Dial itself, the straight sloping line is part of this family of green curves, but is coloured black: it is the line followed by the shadow of the ball at the equinoxes. Then:
- if the date is between midwinter and midsummer, look to the right-hand ends of the green curves; or
- if the date is 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, the sign of the zodiac is drawn in full, and accompanied by its own zodiac symbol and the symbol of its ruling planet. On the diagram above, only the zodiac symbol is shown.
Month of year
Written in Latin outside the signs of the zodiac are the names of the months, 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. The Dial was drawn before Britain adopted the current Gregorian calendar in 1752, so the month breaks shown are probably those of the Julian calendar. In fact, because of inaccuracies in the painting of the Dial, you are unlikely to get a good estimate of date even on the Julian system.
Time of Sunrise
Note the position of the ball’s shadow between the green lines, and extend that relationship to the column on the left labelled 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.
Length of Daylight Hours
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.
Elevation of the Sun above the Horizon
Note the position of the ball’s shadow amongst the red parabolic 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.
Compass Bearing of the Sun
Note the position of the ball’s shadow amongst the straight vertical black lines. Each vertical line is marked with a compass bearing, as shown on the diagram.
There is one further set of black lines (radiating from the gold band under HORIZON and apparently straight, but correctly slightly curved) which subdivide daylight hours into twelve equal parts, whatever the time of year. This was a common method of measuring working hours before the advent of clocks. In the idealised diagram above, the Temporary Hour lines coincide with the solar time lines (the ones projecting to the roman numerals) at the equinox (the straight sloping line). On the actual Dial, the agreement is not so 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 day 15, and New Moon is day 30. You locate the current day of the lunar month on the top or bottom line, then read off a time-correction 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. You will be fortunate if you get anything close to the real time. On the night of the New Moon, the correction factor is zero, but you are advised not to attempt this exercise on that night.
Links to other sundial pages:
- Illustrated History of the Queens’ College Sundial
- Sundials on the Internet
- Sundial links
- British Sundial Society
- North American Sundial Society
- Starry Messenger
Text by Dr Robin Walker, 1997 April 15, revised 1998 May 19, 2014 August 31, 2015 at the Summer Solstice.
With thanks and acknowledgements to Dr Frank King, who made computations for the diagram at the head of this page.
Queens' College Dial by Queens' College Cambridge is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.