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.
You may also view the history of the Dial.
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 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 (blue curve) 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 eccentricity 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 (green curve) 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 (red curve), 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 a member of this family of 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.
Note that the picture used for Cancer the Crab is sometimes a crayfish, as in the table above.
The technical term for the green hyperbolic lines is lines of constant declination. During the passage of one day, the shadow of the ball follows a line of constant declination, particular to the time of year. The lines of constant declination depicted on the Dial are those which are the twelve zodiac period boundaries, which include the solstices and equinoxes. The lowest green line on the Dial represents the summer solstice (when the sun is highest, so its shadow lowest), the lower border of the gold band represents the winter solstice, and the sloping straight black line represents the equinoxes, both spring and autumn.
Month of year, and estimated Day of Month
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 by interpolation 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 old Julian calendar: you should add 11 days to the estimated date. 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 may 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 may interpolate between the given figures to find the current length of day.
Altitude of the Sun above the Horizon
Note the position of the ball’s shadow amongst the red hyperbolic curves. Each red line is marked with altitude in degrees above the horizon, at intervals of ten degrees: the HORIZON line can be considered to be a member of this family of curves, corresponding to altitude zero. You may interpolate to estimate the altitude to the nearest degree.
Azimuth (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.
Temporary, Unequal, or Planetary Hours
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 once a common method of measuring working hours. 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. The terms Temporary Hours, Unequal Hours, and Planetary Hours all have the same definition. The HORIZON line may be considered as a double member of the set of unequal hour lines: the HORI side representing sunrise at 6 a.m., and the ZON side representing sunset at 6 p.m., reckoned in unequal hours.
Underneath the Dial is a table of numbers in three rows, shown here with some visual cues added:
You are required to know the age of the lunar month in days since the last Full Moon (1–30). For instance, Full Moon is day 15, and New Moon is day 30 (or zero). To ascertain the age of the lunar month, you can either (a) look up the dates of New Moons in a pocket diary or online database, or (b) use the visible shape of the moon, as shown above, to estimate the age. Locate the current age of the lunar month on the top or bottom line, then read off a time from the centre line, in hours and minutes: p.m. for days 1–15, or a.m. for days 16–30. That time is when, on that day of the lunar month, the moon would be due South (in the northern hemisphere, as we are), with the moonlight shadow of the gnomon falling on 12 mid-day. This gives the offset which, at other times of night, needs to be added to the apparent time, as indicated by the shadow of the gnomon 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 offset is zero, but you are advised not to attempt this exercise on that night.
Here is a worked example: Suppose the lunar month is 10 days old, and the shadow of the gnomon in moonlight indicates 5 o’clock on the dial. The table shows that, on day 10, the moon would have been due South at 8 p.m., so 8 hours needs to be added to any moonlight time. 5 plus 8 yields 13 hours after mid-day. So the time of night is 1 a.m.
A purist might argue that the lunar month is not 30 days long: it is closer to 29½. But the errors introduced by assuming a 30-day month are smaller than the errors arising from attempting to estimate the age of the lunar month from the moon’s shape, or quantising the age of the lunar month into integral days.
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, 2016 February 14.
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.