Scope, Sequence, and Coordination

A Framework for High School Science Education

Based on the National Science Education Standards


Heat from the Sun

Heat from within Earth and Heat from the Sun
Earth systems have both internal and external sources of energy, both of which create heat. The sun is the major external source of energy.


Further Description:

Since the sun is a variable star, its solar output varies slightly over short time periods. Solar disturbances such as flares, prominences, and sunspots increase the amount of radiation from the sun. However, no long-term variations in the intensity of solar radiation have been measured at altitudes above Earthís atmosphere. Solar radiation, at about 1,400 W/m2, varies by no more than 2%. In contrast, the geothermal heat flow from Earth averages only about 0.061 W/m2. Some theories have related climatic changes to sunspot cycles (11 years) and magnetic cycles (22 years), but no clear connection has yet been established between climate and sunspots or magnetic cycles.

The sun emits energy as electromagnetic radiation. Unlike sound, such radiation requires no medium, therefore this energy is able to travel through the near vacuum of space from the sun to the earth.

Most radiant energy from the sun is concentrated in the visible and near-visible parts of the EM spectrum, and it peaks at about 500 nm, very near the 555 nm visibility peak for Homo sapiens, a result which surely is not a coincidence (and is connected to evolution). Less than 1% of solar radiation is emitted as X-rays, gamma rays, and radio waves.

Only about 25%, or about 350 W/m2, of incoming solar radiation penetrates the transparent atmosphere of the earth. The remainder is either absorbed by the atmosphere or scattered back into space. There are some latitudinal differences found on Earth. These variations are determined by time of year, by the wavelength of the energy being transmitted, and by the depth and nature of the intervening material.

Terrestrial radiation is produced mainly as a result of reradiation of solar radiation. The energy is absorbed from the sun as the sunís spectrum but reradiated from Earth at the much longer wavelengths associated with the temperature of Earth. The Wien displacement law, in which the wavelength is inversely proportional to the absolute temperature, provides a comparison of those peak wavelengths.

Since the sun has a surface temperature 20 times greater than that of Earth, the reradiated energy has a spectrum that peaks at a wavelength 20 times longer than the 500 nm for the sun, or at about 10,000 nm, well into the infrared. Since the atmosphere is more absorptive to such long-wavelength terrestrial radiation, the atmosphere is heated from the ground up instead of vice versa.

In addition, water vapor and carbon dioxide absorb the long wavelengths of radiation from the earth especially well, leading to the so-called "greenhouse" effect. (The temperature in real greenhouses rises mainly because the glass prevents the heated air from rising, thereby increasing the temperature inside.) Ozone, on the other hand, absorbs only very short wavelengths, mainly in the ultraviolet range, and therefore forms a shield of sorts, absorbing much of the ultraviolet radiation before it can reach the earth.

Radiation from the sun follows an inverse square law, so that comparisons of the solar constant can be made for other planets if we know their mean distances from the sun in AU. For example, Mars is 1.93 times further from the sun than is Earth. Its solar constant should therefore be about 27% that of Earth, about 374 W/m2. Because there is little atmosphere to absorb the energy, this is about the same as what we receive at the surface of Earth.

Spacecraft have provided us with temperature measurements for both Mars and Venus. Mars has an average temperature of about 5 oC, whereas Venus has a temperature of about 500 oC. Earth is about 1.4 times further away from the sun than is Venus, and we would expect from the inverse square law that it would receive about twice as much radiation. The fact that its temperature is so much greater can be attributed only to greenhouse warming.

Winter and summer seasons on Earth are a consequence not just of the obliquity factor associated with the sunís rays, but also of the longer days during the summer than during the winter. Of course, both of these factors are a consequence of the 23.5 degree tilt of Earthís axis of rotation with the plane of its orbit about the sun.

The sunís radiation travels at the speed of light, and all wavelengths travel at that speed in a vacuum. When there is dispersion, some wavelengths travel at slightly different speeds than others. That is how we are able to form a spectrum with a glass prism, since blue light slows down in glass slightly more than does red light.

The methods of heat transferCconduction, radiation, and convection (advection)Callow heat to be transferred and to ultimately warm the earthís atmosphere. Convection is particularly important to air heating and consequent movements in the atmosphere.

A balance is maintained between incoming solar radiation and the amount of terrestrial radiation. This is usually referred to as the heat budget. If this balance is disrupted, Earth will become progressively colder or warmer.

The inverse square law can be used to find the total energy per second emitted by the sun, and we can compare that number to how much energy per kilogram we would get from chemical burning. We can show that whatever is happening on the sun, it is not chemical burning. Other considerations are the constancy of the sunís spectral lines and its mass over the span of human existence. Thus, we must explore the source of the sunís energy, which is nuclear in its origins. This leads to a consideration of nuclear fusion, which requires an understanding of nuclear theory and special relativity (Einsteinís energy equation).


Concepts Needed:

Grade 9

Energy, radiation, wavelength, spectrum, convection, conduction, temperature, calorie, seasons, orbit, Earth tilt, scattering, reflection, absorption, prism, dispersion

Grade 10

Latitude, longitude, orbit, altitude, azimuth

Grade 11

Seasons, earth tilt, heat budget

Grade 12

Corona, chromosphere, photosphere, convection zone


Empirical Laws or Observed Relationships:

Inverse square law, Wien displacement law, mechanical equivalent of heat


Theories or Models:

Electromagnetic theory, Stefan-Boltzmann radiation law, atomic and molecular theories, nuclear theories


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Micro-Unit Description:

Heat from the Sun
Students should understand that the sun is the primary external source of energy for Earth. They should examine the effects of the basic processes by which a substance (including Earth) absorbs radiation at one set of wavelengths and reradiates that energy at longer wavelengths, understanding that this process is largely responsible for the heating of the atmosphere and of Earth's surface and surroundings.


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