How to position a sundial

While several types of sundial are available, each model works in the same way: a metal pointer, also known as a gnomon or style, casts a shadow on a numbered dial. This shadow reflects the rotation of Earth and moves in the opposite direction from the sun, i.e. clockwise.

When it comes to choosing a type of sundial, there are several installation criteria to consider:

• A vertical sundial hangs on a wall while a horizontal, equatorial or polar model goes on the ground.

• The latitude of the sundial's position in relation to the north-south pole axis, and the longitude to position the style according to the meridian.

• Ensuring maximum sun exposure facing south or north depending on the model.

• Once these factors have been verified, you can think of installation requirements such as visibility or protection from weather, trees, bushes or any other obstacle.

In order for your sundial to work, it must be installed correctly depending on the model you have chosen, i.e. on a wall or on the ground.

Wall mounting a vertical sundial

A vertical sundial, as the name suggests, is designed to be mounted to a wall.

Installing a sundial on the ground

Other types of sundials are designed to be installed on the ground, such as:

• Equatorial sundials: the dial plate is parallel to the equatorial plane and the gnomon is perpendicular to the dial plate.

• Polar sundials: The dial plate of a polar sundial is parallel with the earth's axis and the gnomon is parallel to the dial plate.

• Horizontal sundials: the style is parallel to the earth's axis and to the horizontal dial plate.

Knowing the latitude and longitude of the installation location is essential when positioning a sundial. To find out the latitude and longitude of your town, simply look up the information on a map, online, on a GPS or even on your phone.

Latitude and incline of a style or gnomon

Latitude is used to determine the incline of a style. For example, the latitude of London is 51.5° and the latitude of Edinburgh is 55.9°. The style should therefore ideally be set at a 51.5° angle from the dial plate in London and an angle of 55.9° in Edinburgh. Please note, however, that most styles are set to a 45° angle and cannot usually be adjusted. This means you won't get exact times unless you tilt the dial accordingly which will mean you end up with a sundial positioned at a slight angle.

3 ways to find the solar noon using longitude

Longitude allows you to find the local solar noon which is the time when sun passes its highest elevation in the sky which is used to find the north-south line (or meridian) of a location. Solar noon materialises as the style or gnomon casts a shadow over 12pm on the dial. There are three ways to find this:

1. The shadow method

• Put a pole perpendicular in the ground.

• At 10am, mark out the tip of the shadow from the pole and use a string attached to the base of the pole to draw an arc starting from this point.

• Between 13.30 and 14.30 mark the point where the shadow touches the arc of the circle you marked out.

• Then mark out the bisector (the median line between the two points), which should indicate the north-south line. The north is indicated by the direction of the shadow cast and the south is in the opposite direction.

• Then all that is left to do is to place the style and the 12pm mark on the north-south axis, positioned to the north or south depending on the model of sundial you have chosen.

2. The compass method

While relatively straightforward, this technique is less accurate as magnetic north does not match geographic north. For a more accurate indication, you will need to use a topographical map to add or subtract a few degrees depending on the magnetic declination of your geographical position.

3. The watch method

Using the date and the longitude of your position, you can calculate the civil time that corresponds to solar noon.

Find out the equation of time (E) of the day of installation. You can find this information in astronomy books or some calendars (ephemeris).

You can then add or subtract the longitude correction expressed in time units (four minutes of time difference for every degree of longitude).

Add on the longitude correction when it is negative (degrees west) and subtract it when it is positive (degrees east).

Let's take the example of Paris on the 15th July:

• Equation of time = + 6 mins

• Longitude correction: 2°35’ east = (2 x 4 mins) + (35 x 4 s) = meaning - 10 mins 20 secs

• Summer time = + 2 hours

Using these numbers you will get:

• 12 h + 6 mn - (10 mins 20 secs) + 2 h = 13 h 56 mins 20 secs

Therefore, in this example solar noon corresponds to 13 h 56 minutes and 20 seconds civil time. Once you have worked this out, you simply need to position your sundial accordingly.

Please note that these methods are designed to make the sundial as accurate as possible. If you are primarily using the sundial as decoration, opt for the simplest method for you. After all, the aim here usually isn't accuracy, but rather to be able to tell the time in a fun and original way!

While maximum sun exposure is obviously essential for your sundial to work correctly, it can be installed in a range of different spots. A sundial can be installed outdoors (in a garden, on a wall, on a bench, etc.) as well as indoors (in a sunhouse, facing French windows, etc.).

The direction of the sundial depends once again on the model you have chosen:

• If you have a direct south sundial, it should face south.

• If you have a direct north sundial, it should face north.

A few other factors should be taken into consideration when it comes to installing a sundial:

• You will need good visibility in order to read the time easily and to make your sundial a decorative piece in its own right.

• The sundial should be protected from the elements (rain, frost, wind, etc.) to ensure it lasts.

• It should be installed far from plants like climbing plants which can end up covering the dial or any architectural features that could interfere with the shadow.

• Prevent accidents involving the gnomon by installing the sundial up high, especially if you have children.

• Think about how to protect your sundial from theft or damage if it is installed on an exterior wall.