Horizontal Dial

As its name indicates it, the horizontal sundial has its horizontal table. The style must be parallel to the axis of the Earth, it thus forms an angle equal to the latitude φ of the place compared to the local horizontal plan. The dial must be directed so that the style is in the plan of the local meridian line. The line of midday thus materializes this local meridian line.

The angle $\ gamma_H$ between the time lines and the line of midday depends on the Latitude as well as time angle H, i.e. the angle which the sun with the local Méridien forms.

$\ gamma_H$ is given by the formula $\ tan \ gamma_H= \ tan H \ sin \ phi \$

Contrary to the case of the equatorial Dial, the angle $\ Delta$ between two successive time lines, corresponding to two time angles H1 and H2, is not constant. The difference $\ Delta$ = $\ theta2$ - $\ theta1$ can be expressed according to H2 and H1 starting from the preceding formula.

See too

Sundial

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