created by Carl Sabanski
Equation of Time (E, EoT): the time difference between Local Apparent Time (apparent solar time) and Mean Solar Time at the same location. Its value varies between extremes of about +14 minutes in February and -16 minutes in October. It arises because of the elliptical orbit of the earth, and the tilt of the earth's axis to the ecliptic. The preferred usage by diallists is:
Mean Solar Time = Apparent Solar Time + EoT
but this convention is by no means universal and the opposite sign is used in modern almanacs. Irrespective of the sign convention adopted, the sundial will appear slow compared to the mean time in February, and fast in October/November.
EoT varies continuously, but is usually tabulated for noon each day at a particular location.
Longitude Correction: is the correction required to local apparent time (L.A.T.) to translate it to the L.A.T. for the central meridian for that time zone.
Standard Time Zone: a geographical region which uses the same civil (clock) time. These are approximately regions between two lines of longitude, set 15° apart, and hence 1 hour time between adjacent zones. The standard time is the mean solar time at the central or standard meridian for the zone. For the UK, which is in Zone 0, the standard meridian is the Prime Meridian at Greenwich, and the zone extends nominally 7½° west to 7½° east.
The "Correct-A-Dial" Calculator was created to help you determine the value that must be added or subtracted from a sundial that indicates local apparent or solar time. This value includes the corrections for the Equation of Time and longitude. It can be used for any longitude. If you want to have this correction for only one location, let's say for your sundial, then you want the "Correct-A-Dial II".
The "Correct-A-Dial II" is a modified graph of the Equation of Time that has blanks where you enter the values for the combined Equation of Time and longitude correction at a single location. Figure 1 illustrates the "Correct-A-Dial II".
Figure 1: "Correct-A-Dial II" (CAD)
The "Correct-A-Dial II" comes with a number of examples that will help you with the simple calculations that are required to find the values to fill in the blanks. You can get a pdf file of the "Correct-A-Dial II" right here.
If you would prefer a completed "Correct-A-Dial II" they are available too! These are available at longitude intervals of 1/4°. This means that the longitude correction is to the nearest minute. To find the "Correct-A-Dial II" you need requires a little calculation as follows:
Longitude of Sundial: ____________ ° (W/E)
It is important that you enter all west longitudes as positive values and east longitudes as negative values. The pdf file for the "Correct-A-Dial II" offered above provides examples of this calculation.
When you obtain the
"Longitude Difference" round the value off to the nearest 1/4°
and get your completed
"Correct-A-Dial II" pdf file from the table below. Positive
values are for locations west of the local time zone meridian and negative
values for locations east. Cut it out and laminate it and it's good to go.