Solar Energy

Key idea: Planetary atmospheres and planetary surfaces are heated by energy from the Sun. This energy is in the form of electromagnetic radiation. Planets which are closer to the Sun receive more radiation than those which are further away. It is primarily this solar radiation which drives weather and climate.

Sun in Extreme Ultraviolet Astronomical tidbit: This photograph of the Sun was taken with a camera which is sensitive to electromagnetic radiation in the extreme ultraviolet portion of the electromagnetic spectrum. By contrast, our eyes are sensitive to radiation in the visible light portion of the electromagnetic spectrum. The disk of the Sun, which appears so bright to us, does not appear bright in the extreme ultraviolet photograph. Also, the bright features in the photograph are not even visible to us. The light that we see with our eyes is radiated by gases in the Sun at a temperature of approximately 5800 Kelvin degrees. The gases in the solar corona which are visible in the extreme ultraviolet photograph are at a temperature of roughly 1 million degrees!!! These hot gases all follow magnetic field lines (and loops!) of the Sun. The photograph was taken with the Extreme Ultraviolet Imaging Telescope aboard the Solar and Heliospheric Observatory (SOHO) spacecraft on August 23, 1996.

Planets are heated by energy from the Sun. This energy is in the form of electromagnetic radiation. Approximately 20% of the incoming solar radiation on Earth heats our atmosphere directly by absorption. For example, ultraviolet energy ("rays") is absorbed by ozone in our atmosphere, and this causes the temperature of that part of the atmosphere to increase. Clouds also absorb some of the incoming radiation from the Sun. Most of the solar radiation which is absorbed in our atmosphere, however, is absorbed by water vapor.

Another 30% of the incoming solar radiation is reflected back to space by gas molecules and particles in the atmosphere, by clouds and by the surface of the Earth. White snow, for example, can reflect as much as 95% of the radiation which is incident upon it, and thick white clouds can be highly reflective as well.

Approximately one half of the incoming solar radiation is absorbed by the surface of our planet. This radiation is, of course, converted to heat energy. We know that much visible light is not absorbed or reflected by Earth's atmosphere because we can see the Sun with our own eyes (caution: don't look directly at the Sun without proper glasses or filters). Not all of the ultraviolet energy is absorbed either, so we need to wear sunglasses and to use sunblock which filters the ultraviolet radiation that is not absorbed in our atmosphere.

Question: Scientists are concerned that man is slowly destroying the ozone in our atmosphere and that holes in the ozone are appearing above the poles of the Earth. Why would this be a problem???

Activity/demonstration: Measure the temperature inside of white and black paper cups which are exposed to sunlight to see how energy from the Sun is reflected and absorbed.

The surface of our planet is heated by the Sun during the hours that sunlight falls upon it. The warm surface radiates infrared energy back toward space around the clock. Some of this infrared energy is absorbed by atmospheric gases such as water vapor and carbon dioxide, and this further heats our atmosphere. In addition, the atmosphere is warmed by conduction and convection of heat from the surface of our planet and by the condensation of water vapor which has evaporated from the surface of the Earth. It is primarily the solar energy which heats our atmosphere and the surface of our planet that drives our weather and climate! You will learn about all of this in later lessons.

We are all well aware that the air temperature near the surface of our planet changes in a somewhat predictable fashion over the course of the day. This is due to the changing balance between the incoming solar radiation and the outgoing infrared radiation from the warm surface of the Earth. The air temperature rises when the energy in the incoming solar radiation exceeds the energy in the outgoing infrared radation. Likewise, the air temperature falls when there is a surplus of outgoing infrared energy over incoming solar energy. The air temperature drops all night when there is no incoming solar radiation and the surface and surface air cool as they radiate infrared energy away.

The coldest air temperature during the course of the day is usually just after sunrise, before the Sun has had a chance to heat the surface of the Earth. The temperature continues to rise during the day because there is a surplus of incoming energy over outgoing energy. The warmest air temperature during the day is generally in the late afternoon, when the decreasing solar energy becomes equal to the outgoing infrared energy radiated from the surface of our planet. The air temperature begins decreasing at this time and continues to do so through the night.

Question: The daytime temperatures are usually highest and the night time temperatures are usually lowest when the sky is clear. Why do you suppose it is that clouds lower the daytime temperatures and increase the night time temperatures???

The Sun is powered by nuclear fusion which converts hydrogen into helium in its core. Fusion has fueled the Sun for 5 billion years and is expected to fuel the Sun for another 5 billion years. Stars which are larger than the Sun (and therefore hotter) actually exhaust their fuel more quickly than stars which are comparable to the Sun in size. Stars which are less massive than the Sun will survive even longer than the Sun. The largest stars are far more massive than the Sun, but most stars are less massive than the Sun. We don't typically see the less massive stars, however, because they are not bright enough.

The luminosity of the Sun is approximately 4 * 1026 Watts (four followed by 26 zeroes!!!). By comparison, a typical light bulb might have a luminosity of 100 Watts. Just imagine how many light bulbs would be required to burn as brightly as the Sun!!! Radiation from the Sun travels in every direction in space. The amount of the solar power which actually reaches the Earth is about 2 * 1017 Watts.

The temperature of a gas can be measured with a thermometer. There are three different commonly used temperature scales. The one that is most familiar to us is the Fahnrenheit scale. With this scale, the freezing and boiling points of water (at sea level) are 32 and 212 degrees respectively. The next most familiar temperature scale to us is the Celsius scale. In this scale, the freezing and boiling points for water are 0 and 100 degrees respectively. This scale is more natural for scientists than the Fahrenheit scale. The conversion from Celsius to Fahrenheit is:

F = (9/5) * C + 32
where F and C are Fahrenheit and Celsius degrees.

The final temperature scale is the Kelvin scale. On this scale, water freezes at 273 degrees and boils at 373 degrees. This is called an absolute temperature scale because the temperature can never go below 0 degrees on this scale. Zero degrees Kelvin is called absolute zero. At this temperature, molecules do not move at all due to heat energy. The conversion from Kelvin to Celsius is:

C = K - 273
where C and K are Celsius and Kelvin degrees.

Questions: If the temperature is 300 Kelvin degrees, do you need to wear a heavy coat or shorts and a T-shirt? How many degrees Celsius and how many degrees Fahrenheit are equal to 300 Kelvin degrees?

Activity: Obtain the high and low temperatures for your city from the daily newspaper every day for a week. Record the temperatures with all three different temperature scales. Make sure that you are comfortable converting between the different temperature scales.

Solar Eruption Astronomical tidbit: This sequence of photographs was also taken by the Extreme Ultraviolet Imaging Telescope aboard the SOHO spacecraft. The sequence (from right to left) clearly shows the ejection of a huge blob of hot gas over a period of five hours. It is estimated that the blob of gas is 80,000 miles long and was ejected at a speed of 15,000 miles per hour!!! The temperature of the gas is estimated to be 60,000 Kelvin degrees. Such solar eruptions can interfere with communication, navigation and power systems on Earth. The photographs were taken on February 11, 1996.

Planets which are closer to the Sun receive more Solar flux than planets which are farther away from the Sun. This means that more of the Sun's energy is captured in a given area on planets which are closer to the Sun than on planets which are farther away. Planets which are closer to the Sun and receive more Solar flux would be expected to be warmer than those which are farther from the Sun.

The Solar flux which reaches a planet decreases with the square of the distance from the Sun. If the distance from the Sun to Earth were twice as much as it is, then we would receive four (2 * 2) times less energy from the Sun. If the distance from Sun to Earth were tripled, we would receive nine (3 * 3) times less energy from the Sun.

On the average, Pluto is 40 times farther away from the Sun than is the Earth. Pluto therefore receives 40 * 40 = 1600 times less energy (in a given area) than does the Earth. No wonder that Pluto is a frozen world!!!

Demonstration: Shine a flashlight on a blackboard (or wall) from different distances, and observe how the light intensity on the board decreases as the distance from the flashlight to the board increases.

As already mentioned, some of the incoming Solar radiation is reflected by clouds and particles within the atmosphere and some is reflected from the surface of the planet. In addition to distance from the Sun, the amount of Solar flux which is available to heat a planet also depends on how much of the incoming radiation is reflected. On Earth, about 30% of the incoming Solar radiation is reflected. On Venus, about 70% of the sunlight is reflected. Because so much of the Sun's energy is reflected at Venus, the surface temperature there would be even lower than it is on Earth if it were not for the runaway Greenhouse effect on Venus. This is true even though Venus is closer to the Sun than Earth. You will learn more about the Greenhouse effect in a later lesson.

The Mars Global Surveyor spacecraft employs solar panels to produce the power required to operate the spacecraft. These panels collect the solar radiation which is incident on them and convert it to electrical enery. There are four solar panels on the Mars Global Surveyor, and they are able to generate a total power of 980 Watts (just about 1 kiloWatt). That is not a lot of power (about as much as is used by a hand-held hair dryer!) and must be sufficient to drive the spacecraft's computer systems, navigation system, control system, radio system, heating system and instruments. The MGS Spacecraft Team is responsible for keeping the solar panels oriented in the direction of the Sun. Otherwise, sufficient power is not available to perform the desired observations and mapping of Mars.

Spacecraft which travel to the outer planets of the Solar System (Jupiter, Saturn, Uranus, Neptune and Pluto) cannot be powered with solar panels because the Solar flux is too weak at such great distances. These spacecraft are usually powered by RTG's, or Radioisotope Thermoelectric Generators. RTG's use radioactive materials to produce the energy required to power the spacecraft. Great care is taken to ensure that the radioactive substances do not pose an environmental threat in the event of a spacecraft malfunction at or soon after launch.

Last updated: November 17, 1999
Joe Twicken /
Rob Wigand