Editing Sundial (section)

===Empirical hour-line marking===
{{see also|Schema for horizontal dials|Equation of time}}
The formulas shown in the paragraphs below allow the positions of the hour-lines to be calculated for various types of sundial. In some cases, the calculations are simple; in others they are extremely complicated. There is an alternative, simple method of finding the positions of the hour-lines which can be used for many types of sundial, and saves a lot of work in cases where the calculations are complex.<ref name="harvp|Waugh|1973|pp=106–107">{{harvp|Waugh|1973|pp=106–107}}</ref> This is an empirical procedure in which the position of the shadow of the gnomon of a real sundial is marked at hourly intervals. The [[equation of time]] must be taken into account to ensure that the positions of the hour-lines are independent of the time of year when they are marked. An easy way to do this is to set a clock or watch so it shows "sundial time"{{efn|
A clock showing sundial time always agrees with a sundial in the same locality.
}}
which is [[standard time]],{{efn|
Strictly, [[local mean time]] rather than standard time should be used. However, using standard time makes the sundial more useful, since it does not have to be corrected for time zone or longitude.}} plus the equation of time on the day in question.{{efn|The equation of time is considered to be positive when "sundial time" is ahead of "clock time", negative otherwise. See the graph shown in the section [[#Equation of time correction]], above. For example, if the equation of time is -5&nbsp;minutes and the standard time is 9:40, the sundial time is 9:35.<ref>{{harvp|Waugh|1973| p= 205}}</ref>
}}
The hour-lines on the sundial are marked to show the positions of the shadow of the style when this clock shows whole numbers of hours, and are labelled with these numbers of hours. For example, when the clock reads 5:00, the shadow of the style is marked, and labelled "5" (or "V" in [[Roman numerals]]). If the hour-lines are not all marked in a single day, the clock must be adjusted every day or two to take account of the variation of the equation of time.