-------------------------------------------------------------------------------

Need the Equation of Time right now, then click here

--------------------------------------------------------------------------------

Even when a sundial has been correctly engraved and correctly installed, it seldom seems to tell the same time as one's watch. The sundial tells Local Solar Time. The watch tells Civil Time, which is related to the world's legal standard of Universal Time Coordinated (UTC).

The two time scales are different.

The difference between watch and sundial comes from 2 easily-calculated locally-constant human-made factors and 2 more complex and variable astronomical factors

1 : The difference between one's Longitude and that of the Time Zone meridian.

2. The presence or absence of Summer Time or Daylight Saving Hours.

3. The Ellipticity of the Earth's orbit around the Sun.

4. The Obliquity of the Earth's spinning in relation to the Equatorial plane.

The combined effect of the last two factors is called the Equation of Time.

FORMAL DEFINITIONS

From 1965, the Equation of Time is formally defined as:

The correction to be applied to 12hrs + Universal Time to obtain the Greenwich Hour Angle of the Sun or more generally the correction to be applied 12hrs + Local Mean Time to be obtain the Local Hour Angle of the Sun. It is so tabulated in the almanacs for navigators and surveyors.

Or more recently:

the difference apparent solar time minus mean solar time

Note that the latter definition relates to mean solar time, which is broadly local standard time +/- the longitudinal difference mention above

THE EQUATION OF TIME RIGHT NOW

If you want an accurate value of the Equation of Time or a Table for the Year or a "Victorian Table" for your location, this can be obtained from another of the Author's websites at

THE NAME

The Equation of Time or in its original form - Aequātiō Diērum - is common astronomical terminology. The 'Equation of .... xxx' means the difference between the observed value of xxx and its mean value. In astronomy, there are many examples e.g. the equation of time, the equation of centre, the equation of the equinoxes, the equation of origins, the equation of light.

CAVEAT

While Astronomers use the definition above, Gnomonists tend to use the negative. That is to say the correction to be applied to Solar Time (i.e. the Local Hour Angle of the Sun) to obtain Local Mean Time. Some of the charts on this website use one definition and some the other!

APOLOGY

I have been collecting pictures of sundials for years and years. In many cases, I do not know who owns the image. if I have a photothat is yours, I ask your permission to show them and will acknowledge you. If you want me to remove the image, I will do so.