How to Make a Sundial Clock

Summary

Why guides-you we often the style of a sundial along the axis of the poles of the Earth? What are the benefits of the different orientations of the dial plan? This article explains how to guide simply a sundial.

Table of contents

  • Preliminary definitions
  • Explanation: Let’s show that a polar style allows you to have a simple Sundial
  • Choice of the position of the dial plan
  • Conclusion

Why guides-you we often the style of a sundial along the axis of the poles of the Earth? What are the benefits of the different orientations of the dial plan? Orient style along the axis of the poles and the dial according to the equatorial plan allows to have a very simple Sundial. We will try to explain why.

Preliminary definitions

  • Sundial: surface with divisions corresponding to the hours of the day and who, after the projection of a shadow of a style illuminated by the Sun, indicates the time (according to Larousse);
  • Style: Rod whose shadow mark time on a sundial (according to Larousse);
  • Solar noon: the highest point of the trajectory of the Sun during the day.At solar noon, the Sun belongs to the Meridian the location plan.

The time given by a sundial is a priori solar time, that can be possibly translated in terms of legal time.

Explanation: Let’s show that a polar style allows you to have a simple Sundial

We will assume all subsequently considered dial is a style and a flat surface. Look at then several possible choices of orientation of the style, and the consequences that this choice has on the graduations of the dial. To fix ideas, one will first consider a single hour, solar noon.

Choice: the shadows corresponding to solar noon have always the same direction during the year.

We have represented here the dial on two dates: the summer solstice and the winter solstice:

On this drawing, shown simultaneously light rays for the two dates. Simply draw a line corresponding to the direction of the shadow at solar noon at a time of the year to have this direction for all the days of the year on the dial. Read the time on the dial will be accurate because all of the shadow of the style (either an entire segment), now under review, should be confused with graduation.

Choice: the direction of the corresponding shadow at solar noon is different depending on the day of the year.
It is impossible to identify solar noon by drawing a scale corresponding to the direction of the shadows at this time, because this direction varies. We then have to draw on the dial a segment corresponding to the different positions of the shadow of the Tip of the style this year. It will read the time on the dial by locating the position of a point from the shadows only (the end), rather than identify the position of a segment as in the previous case.

What direction should the style be if one chooses to be placed where, for every hour of the day, the direction of the shadow of the style is fixed during the year (this corresponds to the first of the two options discussed above)?

You see, if we take the Sundial schema corresponding to the choice, simultaneously representing the two rays of light, it takes that style belongs to the plane defined by the rays 1 and 2 (and by all directions from the Sun at the same time in the course of the year-they all belong to the same plan).

This plan is, for the time being considered here (solar noon), the Meridian plan of the place where the dial is installed. Style must so belong to the Meridian plan of this place.

Reiterate the reasoning for all the hours of the day.

One hour after solar noon, it is more solar noon at A place where the dial… but it’s noon solar (see drawing below) for a B place located west of the location of the dial, specifically for a place that is 360 degrees * 1/24 = 15 degrees of longitude West of the location of the dial (360 ° * 1/24 is 1/24)th the angle which the earth rotates in one day). The direction of the Sun’s rays is therefore up to the Meridian B site plan. If the shadow of the style is to always have the same direction, this year, at a time after solar noon, then (review if necessary drawing represented above) that the style belongs to the plan in which are the rays of the Sun hitting the dial at this time, namely a plan parallel to the Meridian B site plan and passing by the style. It is shown that the style must be not only in Meridian of the place where the dial is installed, but also to the plane parallel to the Meridian site plan located at 15 ° of longitude West of the location of the dial: it cannot then be parallel to the axis of the poles.

Choice of the position of the dial plan

The choice of the position of the dial plan did not intervene in the previous discussion. It was therefore considered a dial plan has a direction, however, we can deduce from the above traits corresponding to the hours on the dial plan are the intersection of this last with each of the plans through the style and parallel to the corresponding Meridian plans. As a result, if the dial is parallel to the equatorial plane, the graduations are very simple: they are regular and spaced 15 ° per hour (see image below).

Conclusion

The simplest dial to make is the equatorial sundial like a wall clock. However, this is not always the most convenient: If you want to place the dial on the wall of a House, for example, we have to build a vertical Sundial. Most sundials (be they Equatorial, vertical…)-and especially those represented on many manuals-have a polar style. The objective of this text was to explain why you may wish to choose this type of dials, however there are also many dials that do not follow this rule.

To achieve the most simple of sundials, the equatorial Sundial, you can print this model of an equatorial Sundial (file format pdf). The style must be oriented along the axis of the poles and the dial plan is perpendicular to the style (parallel to the plan of the Equator). The shadow of the style moves from 15 ° (360 ° / 24) per hour.

To learn more about the different types of sundials, you can consult the site dials, proposed by the ENS of Lyon and the IUFM de Lyon.

Gallery

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