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The sun's path

Understanding the sun's path is essential when designing barriers to UVR, as the position of the sun at any given time affects where trees and structures will cast their shade.

Your designs need to allow for changes in shade patterns which occur because of daily and seasonal changes in:

  • The height of the sun in the sky (the solar altitude)
  • The direction in which shadows fall (the azimuth)
  • The length of the day (longest and shortest days are known as the summer and winter solstice)

You can use sun charts or commercially available computer programs to work out the sun’s path at a given location, at any time of the year if you know:

  • Latitude
  • Longitude
  • Direction of true north.

Shadows

Shadows are always moving and shifting in response to the height and angle of the sun as it passes from east to west. There are 3 daily shade patterns:

  • Morning— shadows fall in a westerly direction away from the object casting the shadow
  • Midday— shadows will be close beneath the object
  • Afternoon— shadows fall in an easterly direction away from the object.

This basic pattern occurs every day of the year, but the areas affected by shadow vary according to the seasonal changes at different times of year:

  • The tilt of the earth’s axis shifts as it orbits the sun.
  • As a result the area of the earth’s surface receiving the maximum solar intensity shifts between the Tropic of Capricorn (latitude 23.50 S) and the Tropic of Cancer (latitude 23.50 N).

Sun angles

The angle of altitude (the angle between the sun and the horizon on a given latitude) is used to determine the length of the shadow cast by a solid object.

A higher solar altitude angle means:

  • The daylight period is longer
  • There is a shorter path of radiation through the atmosphere
  • More UVR reaches the earth’s surface since less radiation is absorbed by ozone, vapours and dust particles in the atmosphere
  • There is a higher intensity of radiation falling on any particular area.

The solar azimuth (the angle in a horizontal plane between true north and the direction of the sun) determines the direction in which the shadow will fall.

You can use a sun calculator to find these figures, and if you enter the height of your object you will see the shadow length for any time, on any day of the year, for your particular location.

Equinox and solstices

Four days are particularly important when marking the sequence of the sun’s path each year:

  • The equinox days—21 March and 23 September—the two days each year when daylight and night time hours are of equal length, each being 12 hours
  • Winter solstice—21 June—the shortest number of daylight hours of the year
  • Summer solstice—22 December—the longest number of daylight hours of the year.
You should plot your shade plan for these days to get a representative shadow patterns for each season.
Last updated: 20 April 2016