Interested in Reading the latest World Sundial News?
Click on the Image to the right...
Corrections need
to be made in order to obtain Mean (that, in the UK, is to say 'GMT Clock') time from a sundial.
These are:
Correction for the Equation of Time
Correction for Longitude
Correction for Summer Time (Otherwise known as
Daylight Saving Time)
These are considered in turn below:
Why not get the
Estimator? Want a
little card to carry with you to help you check a sundial's
displayed time at ANY Longitude or Latitude in the British Isles? Hurry if you want one of the few remaining printed
versions...
Click the picture for more details!
The equation of time describes the
discrepancy between two kinds of solar time. These are Apparent Solar
Time, which directly tracks the motion of the sun, and Mean Solar Time,
which tracks a fictitious "mean" sun with noons 24 hours apart.
Apparent
(or true) solar time is that indicated by a sundial.
Mean solar time, for
the same place, would be the time indicated by a steady clock set so that
over the year its differences from apparent solar time average to zero.
More about the components of the
Equation of Time HERE
Why not get The Recorder's Reference? It provides anyone
interested in dialling with a way to estimate by how much a
non-direct-south vertical dial declines and also finds the design
latitude for any horizontal dial.
The figures in the table below show more accurate
figures in mins and secs by which a sundial should be corrected during the
whole of 2024 and further down for 2025 too. A link is also included to a printable page of the
same current EoT figures for 2024. The figures in the tables represent the Equation of Time at
12Hrs (Noon) GMT
on the date stated and are here calculated using the method after Smart (1956)
and Meeus which is accurate at worst to about 4 secs - frequently to one second..
Interpolation will of course, be necessary if an EoT is needed for times other than Noon.
Note that in non-leap-years there is still an entry in the tables for Feb 29th
but its value is then the same as that for Mar 1st.
NB Don't forget the two
other corrections for longitude and daylight saving time that are needed to
convert solar time to clock time...
Negative values in the tables
below indicate
Dial Slow.
Thibaud
Taudin Chabot has prepared a two sheet print out of
average values of the
sun's declination and the equation of time at 12:00 GMT for the years
2000 - 2099. This can be found and printed off by clicking the image
here.
Accuracy - Just how accurate are these estimates of EoT?
The above EoT figures are calculated using Meeus's 'simpler' formula (after Smart) and are generally accurate to within 4 seconds and to less than 1 second for most of the year.
To illustrate this, the graph below shows the differences in seconds between Smart's EoT figures for 2012 and those of the ''Great Circle'' data when a detailed comparison was last made. NB A further Excel comparison made over the year 2014 (that is two years later in the centre of that leap year cycle) may be found Here(Needs Excel)
II - Correction for Longitude
When a dial is not on the local Time Meridian,
for example for a dial in the UK that is not on the actual Greenwich Meridian,
then the sun is overhead at the dial at a time earlier or later than at Greenwich.
The correction to be made is 4 minutes of time for every degree of longitude
difference from Greenwich. This figure should be subtracted from the dial
reading for dials East of Greenwich (such dials are Fast) and added for dials
West of Greenwich. (such dials are Slow).
III - Correction for Summer Time
Lastly during the period when Summer Time
(Daylight Saving Time) is in force one hour should be added to the corrected
sundial reading to obtain clock time.
Finally, how about a simple in-the-pocket card that allows you to check a dial
at any Latitude or Longitude in the British Isles, by converting its Solar time to Clock time?
More about the components of the
Equation of Time
HERE
Thibaud
Taudin Chabot has prepared a two sheet print out of
average values of the
sun's declination and the equation of time at 12:00 GMT for the years
2000 - 2099. This can be found and printed off by clicking the image
here. NB A further Excel comparison made over the year 2014 (that is two years later in the centre of that leap year cycle) may be found Here (Needs Excel)
