Solar Geometry

Question 1

What is the difference between daylight and sunlight?

Answer 1

Daylight:

• The diffused natural light from a completely overcast sky.

Sunlight:

• The direct solar radiation received in clear sky conditions.

Question 2

What are solar gains?

Answer 2

Solar gains are the energy we capture/reject from the sun as and when we need.

Question 3

What is a sun path diagram?

Answer 3

Question 4

What is zenith?

Answer 4

A point which is directly overhead (perpendicular to the Earth’s surface).

Question 5

What is the meridian?

Answer 5

A circle of constant longitude which passes through a given place on Earth’s surface.

Question 6

What is a great circle?

Answer 6

A great circle divides a sphere into two equal halves.

Question 7

What is obliquity?

Answer 7

• Obliquity is the tilt of the Earth with respect to the plane of revolution of the Earth around the sun (23.4^o).
• The obliquity of the Earth causes the seasons depending on whether the Earth is tilted towards/away from the sun.

Question 8

What is declination (D)?

Answer 8

• Declination is the angle made by the Sun’s rays at a given time of the year with the Earth’s equatorial plane.
• It varies over the year from ‘- obliquity (-23.4^o)’ in winter to ‘+ obliquity (+23.4^o)’ in summer. It is 0^o at the equinox.
• It is a measure of where you are in the seasons.

Question 9

What is the equation for declination?

Answer 9

Question 10

What is hour angle (H)?

Answer 10

The angle that the Earth needs to rotate in order to bring the meridian of a place to its position at noon.

Question 11

What is the equation for hour angle?

Answer 11

Question 12

What is latitude (L)?

Answer 12

• Latitude is how far/close you are to the equator, measured using the angle subtended by the place under consideration from the centre of Earth.
• It is important in determining how much solar radiation a place receives.

Question 13

What is altitude (y)?

Answer 13

The altitude is the angle of the sun above the horizon plane. It is zero at sunrise and sunset, and at its maximum value when the sun is due south (in the Northern hemisphere).

Question 14

What is azimuth (z)?

Answer 14

• The azimuth is the compass direction of the Sun.
• This is the angle of the Sun measured from South.
• Azimuth is 0^o when it is due South (at noon).

Question 15

Which solar geometric properties apply to Earth as a whole, and which apply to single places on Earth?

Answer 15

Earth as a whole:

• Hour Angle (H)
• Latitude (L)
• Great Circle
• Obliquity
• Declination (D)

Single place on Earth:

• Altitude (y)
• Azimuth (z)

Question 16

What is the equation for altitude (y)?

Answer 16

Question 17

What is the equation for azimuth (z)?

Answer 17

Question 18

What are the solstices/equinox?

Answer 18

• The obliquity is the tilt of the Earth towards/away from the sun effecting the length of a day.
• Summer solstice = Longest day of the year.
• Winter solstice = Shortest day of the year.
• Equinox = Between the two.

Question 19

What is the declination at equinox?

Answer 19

Zero

Question 20

What are orthographic projections?

Answer 20

Question 21

What are stereographic projections?

Answer 21

Question 22

What are the features of orthographic/stereographic projections?

Answer 22

Question 23

What are the solar features of the tropics and arctic circle?

Answer 23

• In the tropics, the sun is at zenith at least once in a year.
• On the tropics, the sun is at zenith exactly once in a year.
• In the arctic circle, there will be no sunrise or sunset at least once a year.
• On the arctic circle, there will be no sunrise or sunset exactly once a year.

Question 24

How do you derive the altitude at noon equation?

Answer 24

• y*_noon = 90 - ß
  but. .. ß = L - D
  so. .. y_noon = 90 - (L - D)
• y*_noon = 90 - L + D

Question 25

How do you plot a sunpath diagram?

Answer 25

1. Calculate the Azimuth at sunrise (East) and sunset (West), altitude will be 0^o. (Remember, azimuth is measured from South, and so you have to add/subtract the azimuth from 180^o)
2. Calculate the altitude at noon, azimuth will be 180^o.
3. Connect the dots in the arc.
4. Plot the same for summer solstice, winter solstice and the equinoxes.
5. You can then plot hour lines (shown in blue).

Question 26

What is the azimuth at noon?

Answer 26

The azimuth is due South and so 0^o.

Question 27

What are the features of sunpath diagrams?

Answer 27

• Radial lines indicate the solar azimuth.
• Concentric rings indicate the solar altitude.
• Length of arcs show the length of days.
• If there is a complete arc/circle, the sun never sets.
• If there is no arc, the sun never rises.
• For Southern Hemispheres, the arcs appear to be upside down.

Question 28

What are the features of sunrise and sunset azimuth?

Answer 28

• They are equal and opposite.
• At the equinox, the sun always rises at exactly due East, and sets at exactly due West.

Question 29

What is the formula for solar azimuth at sunrise/sunset?

Answer 29

Question 30

What is the equation for glazing characteristics?

Answer 30

Question 31

What factors affect glazing characteristics?

Answer 31

1. The angle of incidence of radiation causes the reflectance/absorption/transmit range of glazing to vary.
2. Therefore, if the sun is lower in the sky (and so perpendicular to the glass pane), there is less reflection, and so more radiation is absorbed/transmitted.
3. If the sun is higher in the sky (and so parallel to the glass pane), there is more reflection, and so less radiation is absorbed/transmitted.

NOTE: The angle of incidence is measured between the incident radiation and the normal to the plane (0^o is perpendicular to the plane).

Question 32

What is the role of shading?

Answer 32

Shading is used to:

• Control solar gains by:

Reducing cooling loads in the summer.

Allowing passive heating in the winter.

• Control glare from the sky.

Question 33

How do you calculate the shading transmittance?

Answer 33