Reading the Sundial

Hotham Park Sundial

Sited 50°47’12” North – 0°40’03” West

How to read the Sundial

  • Look at the shadow created by the straight sloping edge of the triangular projecting plate (called a Gnomon).
  • The shadow slips past each hour line rather like a clock hand. It starts at sunrise, goes past 12 noon in the middle of the day, and continues ‘clockwise’ until sunset.
  • The hour is shown when the shadow of the straight sloping edge is on, or past, one of lines passing through a Roman numeral.
  • The minutes are shown by the shortest lines on the outer circle.
  • The scale just inside the Roman numerals is divided into 5 and 10 minute intervals.
  • But note – Clock Time is constant – Sundials show Solar Time which is variable. Therefore Sundial Time is not the same as Clock Time.
  • This is because the Earth tilts on its axis and does not move at a constant speed during its elliptical orbit around the sun.
  • The difference between Sundial Time & Clock Time is known as The Equation of Time which varies each day – see the explanation and graph below.

The Equation of Time & Longitude Correction

  • Hotham Park is 0° 40’ 03” West of the Greenwich Meridian.
  • This difference in longitude means the Sundial is 2 minutes 40 seconds slower than GMT, in addition to the adjustment for The Equation of Time.
  • The graph below shows the total adjustment after adding the difference from GMT to The Equation of Time. These values vary by a few seconds from year to year.
  • To work out Clock Time from Sundial Time use the graph below to add or subtract the minutes for today’s date.
  • Clocks go forward one hour in British Summer Time but Sundials do not, so in summer the Sundial will be one hour slower, after the adjustments you have made.
  • If you would prefer to see the individual daily values click here for January to June or here for July to December.

For the history of the Sundial and details of how the replacement was made and installed click here.

© Hotham Park Heritage Trust – Registered Charity Number 1117011