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 is intended 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.
Figure 1: "Correct-A-Dial" (CAD)
you would like to have the "Correct-A-Dial" calculator you can download a pdf file of a
kit that you can put together. It consists of four parts and should be
printed on card stock or glued on to cardboard. The pointer and the
Longitude Correction ring must be able to rotate so they can be adjusted.
Both colour and black and white versions are available. Instructions on the use of the calculator are provided.
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. There are even a series of completed units!