Teaching About the Solar System and Solar Energy

5 August 2003

Wm. Robert Johnston
University of Texas at Brownsville



   I. Introduction and issues
  II. Key overlapping concepts
III. Key solar system concepts
IV. Key solar energy concepts
 V. Extended concepts
VI. Conclusion

Be very, very careful what you put into that head, because you will never, ever get it out.

--Thomas Wolsey, 1471-1530

What questions do our students have?

Solar energy concepts from FOSS

Solar energy topics:

Solar energy terms:

absorb, change, energy transfer, heat sink, insulation, orientation, reflect, shadow, solar energy, surface area

5th grade TEKS (selected)

(5.6) Science concepts. The student knows that some change occurs in cycles. The student is expected to:
   (A) identify events and describe changes that occur on a regular basis such as in daily, weekly, lunar, and seasonal cycles;
   (B) identify the significance of the water, carbon, and nitrogen cycles;

(5.7) Science concepts. The student knows that matter has physical properties. The student is expected to:
   (A) classify matter based on its physical properties including magnetism, physical state, and the ability to conduct or insulate heat, electricity, and sound;

(5.8) Science concepts. The student knows that energy occurs in many forms. The student is expected to:
   (A) differentiate among forms of energy including light, heat, electrical, and solar energy;
   (B) identify and demonstrate everyday examples of how light is reflected, such as from tinted windows, and refracted, such as in cameras, telescopes, and eyeglasses;

(5.11) Science concepts. The student knows that certain past events affect present and future events. The student is expected to:
   (C) identify past events that led to the formation of the Earth's renewable, non-renewable, and inexhaustible resources.

(5.12) Science concepts. The student knows that the natural world includes earth materials and objects in the sky. The student is expected to:
   (B) describe processes responsible for the formation of coal, oil, gas, and minerals;
   (C) identify the physical characteristics of the Earth and compare them to the physical characteristics of the moon; and
   (D) identify gravity as the force that keeps planets in orbit around the Sun and the moon in orbit around the Earth.

Solar system--key concepts

(from 3rd grade TEKS)

(3.11) Science concepts. The student knows that the natural world includes earth materials and objects in the sky. The student is expected to:
...(C) identify the planets in our solar system and their position in relation to the Sun;...

Major concept errors in FOSS Solar Energy

Electromagnetic radiation

Electromagnetic radiation--any type of energy transmitted by electric and magnetic field waves. They travel at the speed of light and can cause charged particles (like electrons in atoms) to oscillate and move.

EM radiation is classified by the amount of energy in individual waves:

/-------------higher energy lower energy------------\
gamma rays* x-rays* ultraviolet* visible* infrared* microwaves* radio waves

The very small portion of this electromagnetic spectrum that our eyes are sensitive to is called visible light. Different colors of light represent different energies. From low to high energy, visible light includes red, orange, yellow, green, blue, and violet.

From lower to higher energy:

These categories are based on different ways that these waves interact with matter (which also gives different applications). White paper reflects all colors of visible light. Blue paper reflects blue and absorbs the rest. Visible light does not go through wood and brick walls, but radio waves and gamma rays do, to a degree. Visible light goes through glass windows, but some ultraviolet does not.

Black body radiation

Black body radiation--EM radiation given off by all matter, as a function of that matter's temperature. Hotter objects give off more total EM radiation, and also give off a greater fraction of that radiation as higher energy radiation.

Black body radiation from the Sun: 8% ultraviolet, 42% visible, 50% infrared
  (emitted sunlight peaks in yellow wavelengths, but total sunlight appears white)
Black body radiation from the Earth: virtually 100% infrared

other examples:

Interaction of light with matter:

When EM radiation encounters matter, it can be reflected, transmitted, or absorbed.

Which of these happen in what proportions depends on the energy of the EM radiation and the nature of the material.

Scattering of light occurs passes through dispersed particles (dust particles, water droplets, air molecules) (Why is the sky blue?)

Apparent colors of objects arise from differing reflection of various colors of visible light.

Albedo is the proportion of radiation reflected (either at a specific color or cumulative for a range).

If a surface is not perpendicular to the incident radiation, less energy is intercepted. Polar regions of the Earth are cooler because the low angle of sunlight disperses energy across a larger area.

The Sun

The Sun is a star. Most stars visible in the night sky are much brighter intrinsically than the Sun, but most stars in the universe are dimmer than the Sun.

The Sun is a sphere of hydrogen (78%), helium (20%), other elements (2%), all plasma or gas. Its tendency to collapse inward from its own gravity is balanced by energy from thermonuclear reactions in core: pressure and temperature in the center are high enough that hydrogen atom nuclei fuse (through several steps) into helium nuclei, releasing much energy.

Energy release is related to the lower mass of the resulting helium nuclei by E=mc2. About 0.5% of the mass is converted to energy in the reactions, so every second 4.3 million (metric) tons of the Sun's mass is converted into EM radiation.

Energy is carried 70% of the distance towards the Sun's surface by radiation, most of the remaining distance by convection, and from the Sun's surface by radiation.

The visible surface of the Sun is the photosphere (temperature 5600° C). Sunspots here are transient regions of cooler (3000° C) gas, which appear dark because they give off less light.

The Sun’s magnetic field is complex, since the fluid Sun distorts this field. Results include sunspots, prominences, flares.

Above the surface is the corona, which is very low density very hot gas. Charged particles stream away from the Sun as the solar wind; this interacts with the Earth's magnetic field.

Diameter of Sun: 1,392,500 km (109 times the Earth)
Mass: 332,900 times the Earth's mass
Distance: 149,600,000 km average

Motion of Sun in sky

Earth rotates about axis once every 23 hours 56 minutes.

Earth orbits Sun once every 365.26 days.

Thus, Sun is in the same position in the sky after 24 hours--day and night.
For a fixed observer over the North Pole, the Earth's rotation about its axis and the Earth's motion around the Sun both are counter-clockwise. (Thus, the apparent motion of the Sun makes the shadow on a sundial in the Northern hemisphere move clockwise.)

Since days don't divide evenly into years, we have leap years.


The Moon

The Moon is the Earth’s only natural satellite. It is about one-fourth the diameter of the Earth and made of rock with a small metal core.

Its surface is dominated by craters, circular depressions blasted out by the impacts of comets, asteroids, and smaller meteoroids. Older craters are covered or obliterated by younger ones. The youngest ones have visible rays, or bright radial patterns of ejected material.

The Moon has no atmosphere and no liquid water. Thus there is no erosion by water or wind. The surface is dust and broken rock, broken up by many impacts.

Maria (singular “mare”), basins filled with lava rock, cover parts of the Moon’s surface. Other evidence of past lava flows include “sinuous valleys” or collapsed lava tubes.

Apollo missions 11, 12, 14-17 each landed two men on the Moon for up to a few days each, 1969-1972. Apollo missions 8, 10, 13 came near the Moon but did not land. This is the furthest humans have ever been from Earth.

Diameter of Moon: 3,476 km (one-fourth of the Earth’s diameter)
Mass: 1.3 % of the Earth's mass
Surface gravity: one-sixth as much as at the Earth’s surface
Distance: 384,400 km average

Lunar motion

The Moon takes 27.3 days to complete an orbit around the Earth, on average. The Moon’s orbit varies slightly since it is perturbed by the Sun’s gravity.

The Moon always keeps the same face towards the Earth—this face is called the nearside. From the Moon’s nearside, the Earth always appears in nearly the same location in the sky; from the farside, it is never visible.

Since the Earth and Moon are orbiting the Sun at the same time the Moon is orbiting the Earth, it takes about 29.5 days for the Sun to return to the same position in the sky—this is the day-night cycle for the Moon.

The Moon’s phases represent the portion of the Moon visible to us illuminated by the Sun. Therefore they cycle with the Moon’s day-night cycle, every 29.5 days. This is a month. (Calendar months vary.)


Ocean tides on Earth are due to the gravity of the Moon (mostly) and the Sun. There are high tides on opposite sides of the Earth (12 hours apart, as the Earth rotates relative to the Moon and Sun), one facing the Moon/Sun and one opposite it. If the Moon and Sun are aligned, their tides add to a higher neap tide.

Gravity—the cause of orbits and tides

Orbits of planets and moons and comets, the arc of a tossed ball, the straight down plunge of a dropped book—all represent freefall motion under the influence of gravity.

Consider an object thrown horizontally sideways: it curves down until it hits the ground, going further if it is thrown faster. Thrown sufficiently fast, it will take so long to curve down that the Earth’s surface will curve away that fast. The object will then miss the surface, continuing to curve around the Earth in an orbit.

While in freefall, an observer experiences weightlessness. The observer is falling, so there is a gravitational force on them, but there is no apparent gravity in their frame of reference.

The force of gravity decreases with distance. Extended objects in freefall are attracted differently at different points, due to varying distances from the attracting object. At the center of mass of the object one would observe weightlessness, but would observe a stretching force towards and away from the attracting object. These are tidal “forces”.

We might say pseudoforces, since this is only a force to the observer in that moving frame of reference. The same is true for centripetal “force”, the apparent outward gravity experiences by an observer in a spinning frame of reference.

Thus, a space shuttle orbiter orbiting the Earth is in freefall, constantly falling towards the center of the Earth, but always missing. Astronauts aboard in this state of freefall feel weightless. NASA now calls this “microgravity” instead of “zero gravity” to acknowledge the imperceptible but still measurable tidal forces across the orbiter. But keep in mind the Earth is exerting a force on the orbiter and the astronauts, forces about 88% of what they experience while on the Earth’s surface.


Solar eclipse--Moon passes between Earth and Sun, casts a shadow on part of Earth

Lunar eclipse--Earth passes between Moon and Sun, casts a shadow on the Moon

The Moon's orbit is slightly tilted with respect to the Earth's orbit, so the opportunity for eclipses happens at intervals of about six months.

Solar eclipses are special because the Sun is 400 times larger in diameter than the Moon and also 400 times farther away: the apparent sizes in the sky are almost the same. In a total solar eclipse, the Moon exactly covers the Sun's disk, allowing us to see the corona. In a partial eclipse, it partly obscures the Sun. Next total eclipse in Texas is in 2024.

An annular eclipse is a partial solar eclipse in which the Moon is too far away to fully cover the Sun’s disk, leaving a visible ring. Next ones in Texas are in 2012 and 2023.

Since the Earth is larger than the Moon, the Moon may be entirely within the Earth's shadow during a lunar eclipse. Sometimes, sunlight is bent (refracted) by the Earth's atmosphere, so the Moon appears red rather than dark.

It is not inherently more dangerous to look at the Sun during a solar eclipse: it is always dangerous to look directly at the Sun. With eclipses, people are more inclined to keep staring despite the pain in their eyes telling them to stop! A pinhole projection into a shaded spot is an easy way to view a solar eclipse safely.

Lunar eclipses are visible from half the Earth at a time (ignoring clouds). Next one is November 2003.

Scale of things

The universe is mostly empty, with a scattering of hundreds of billions of galaxies...
A galaxy is a collection of stars and other objects (nebula, black holes, etc.), also mostly empty space between these objects...
Our galaxy, the Milky Way Galaxy, is disk-shaped with spiral arms and a central core. It has 400 billion stars, probably many with planets...
The space between neighboring stars is mostly empty, with our solar system very small in comparison...
Within our solar system is a star, nine planets, some other debris, but mostly empty space...
One of the smaller planets is home!

Layout of the Solar System

Unifying concepts

Near-spherical shape of planets and other larger objects (over about 300 km across) a result of self-gravitation (shape may be affected by rotation or tides)


Processes that shape surfaces of icy/rocky objects:


Sample objectives and activities for solar system instruction

Sequence of the planets:

Scale of solar system:

Motion of planets:

Characteristics of the planets:

Explore multi-thematic applications:

Nature of matter

A molecule is the smallest component of a substance that preserves its properties.
An atom is the smallest component of an element preserving its properties.
Molecules are combinations of atoms, joined by electron interactions.
A neutral atom includes protons and (usually) neutrons in a compact nucleus, surrounded by electrons.

Chemical reactions rearrange atoms in molecules, nuclear reactions rearrange particles in nuclei.

Four states of matter:

Heat--internal energy of an object associated with internal kinetic energy of the particles making up the object.

Heat is a form of energy; forms of energy include

Temperature scales

Fahrenheit (° F)Celsius (° C)Kelvin (K)
-459-2730absolute zero
320273water freezes
5814287average Earth temperature
9938311average body temperature
212100373water boils
1020056005900surface of Sun

Temperature measures heat or specifically the kinetic energy of the molecules, atoms, etc., making up an object. A cold enough object has no internal atomic motion; this temperature is absolute zero.

Transfer of heat:

Heat may be transferred by conduction, convection, or radiation.

conduction--flow of heat from a warmer object touching a cooler object. The vibrating/moving molecules in the warmer object bump those in the cooler object, speeding them up.

convection--warmer and cooler fluids move around. With an increase in temperature the density of a given fluid decreases (usually), so if there is effective gravity the warmer fluid rises, the cooler sinks.

radiation--electromagnetic radiation transfers heat from warmer to cooler object. The objects are not touching, but blackbody radiation from the warmer object is absorbed by the cooler object, raising its temperature.

Thermal properties of matter

conductor--transfers (conducts) heat easily (e.g. metals)

insulator--transfers (conducts) heat poorly (e.g. fiberglass, air)

heat sink--object which requires more thermal energy to change temperature by a given amount (e.g. water, ceramic)

thermal expansion--tendency of matter to expand when heated


Our weather is driven largely by two things: the transfer of heat and the presence of water. The Earth’s rotation, landforms, and other factors come into play.

The Earth’s surface absorbs heat more efficiently than the atmosphere. The lower atmosphere is heated by the Earth’s surface, and temperature decreases with height. Temperatures rise again in the stratosphere, where absorption of UV light by ozone produces heating.

Winds, or air motion, result from convection and from the Earth’s rotation.

Water has more thermal inertia than land surfaces, so temperatures over water vary less than over land. One noticeable result is convective winds at the coasts.

Evaporation involves water going from liquid to vapor form at a water-air interface, transferring heat in the process. This transfers water from the oceans and other water bodies back into the atmosphere as part of the water cycle.

Hurricanes acquire energy particularly from ocean heat. The circular motion is the effect of the Earth’s rotation on the convective air motion.

Greenhouse effect 1



Greenhouse effect 2

EM radiation from the Sun reaching the Earth is either reflected back into space or absorbed by the Earth, warming it. The Earth also radiates blackbody radiation but almost entirely as infrared light (since it is much cooler than the Sun).

Certain gases in the Earth's atmosphere are transparent to the Sun's visible light but tend to absorb the outgoing infrared light emitted by the Earth. These are called greenhouse gases and include primarily water vapor and carbon dioxide. By absorbing some of this outgoing EM radiation, the atmosphere becomes warmer and also radiates more blackbody radiation. The atmosphere and the Earth itself becomes warmer than it would be without the greenhouse gases.

The Earth's atmosphere (by volume) is about 77.8% nitrogen, 20.9% oxygen, 0.9% argon, 0.4% water vapor, 0.037% carbon dioxide, and 0.003% other gases. The water vapor and carbon dioxide naturally occurring in the atmosphere produce a greenhouse effect which gives the Earth a temperature 30° C more than it would have otherwise.

Natural sources of carbon dioxide include animals, natural fires, and releases of carbon dioxide stored in the ocean or in minerals. Plants also produce carbon dioxide, but they consume more than they produce. Manmade sources include the burning of wood or fossil fuels (coal, oil, and natural gas), clearing of forests, and cement production. Certain manmade activities (such as agriculture) also remove carbon dioxide from the atmosphere.

Types of solar energy systems

Energy sources in the United States (portion of total consumption)


World energy sources (portion of total raw production)


(Data sources: U.S. Energy Information Administration, World Resources Institute)

On Line Resources 1

Solar system

General astronomy

Space missions

On Line Resources 2

Weather and climate

Other science


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© 2002, 2003 by Wm. Robert Johnston.
Last modified 4 August 2003.
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