THE SOLAR SYSTEM

Keywords

• Axis
• Ellipse
• Revolution
• Rotation
• Sphere

By the end of this chapter, you will be able to:

1. Know the relative sizes, positions and motions of the earth, sun and moon
2. Understand how day and night occur and demonstrate the phases of the moon
3. Understand the role of the sun, earth and moon in explaining time, season’s eclipses and ocean tides
4. Know the components of the solar system and their compositions
5. Know the main characteristics of the inner and outer planets in the solar system.
6. Understand the various views about the origin and structure of the universe

8.1 General Structure of the Solar System

Introduction

 The solar system is the gravitation bound system of the sun and the objects that orbit it. Among the objects that orbit the sun are the eight (8) planets. New discoveries are being made by space missions like NASA, and such information is always updated when and as approved.

In this chapter, you will understand the movement of the earth and moon in relation to the sun and explain the consequences of these relative movements to life on the earth.

 THE SOLAR SYSTEM

 The solar system is the gravitationally bound system of the sun and the objects that orbit it, either directly or indirectly.

 Research 8.1

 Using different resources such as the internet or

Physics textbooks carry out research on:

 (a) What our solar system consists of.

(b) The four inner and outer planets.

(c) The difference in composition between the inner and outer planets and what accounts for the difference.

 (d) Why the outer planets have more moons and rings.

Terrestrial planets / inner planets

The terrestrial planets are the four innermost planets in the solar system. They include; Mercury, Venus, earth and Mars. They are called terrestrial because they have a compact, rocky surface like the earths. The planets Venus, earth, and Mars have significant atmospheres while Mercury has almost none.

 Jovian planets / outer planets

Jupiter, Saturn, Uranus, and Neptune are known as the Jovian (Jupiter – like) planets, because they are all gigantic compared to the inner planets, and they are gaseous in nature.

The Jovian planets are also referred to as the gas giants, although some or all of them might have small solid cores.

The table below summarizes the difference between inner and outer planets.

Composition of the Solar System

The sun contains 99.85 % of all the matter in the solar system. The planets, which condensed out of the same disk of material that formed the sun, contain only 0.135 % of the mass of the solar system.

Jupiter contains more than twice the matter of all the other planets combined. Satellites of the planets, comets, asteroids, meteoroids and the interplanetary medium constitute the remaining 0.015 %

Assignment 8.1

 What you need

• Internet and computer

What to do

 (a) In pairs, use the internet to research on the following:

1. The components of the solar system and their mean distance from the sun.
2. The main characteristics of both the inner and outer planets.
3. Why the earth is the only planet that currently supports human life.
4. The asteroid belt and where it is found in the solar system.

(b) Make a PowerPoint presentation and use it to discuss your findings to the whole class.

SUN – MOON – EARTH SYSTEM

 Activity 8.1 Observing rotation and revolution of the earth

 Key Question: What is the effect of the earth’s rotation and revolution?

What you need

• A ball or a globe.

What to do

1. Hold a basketball with one finger at the top and one at the bottom. Have a classmate gently spin the ball.
2. Observe the movement and explain how this models the earth’s rotation.
3. Continue to hold the basketball and walk one complete circle around another learner in your class.
4. How does this model the earth’s revolution?
5. Think critically and write a paragraph in your Physics notebook describing how these movements of the basketball model earth’s revolution.

THE EARTH

You wake up at daybreak to catch the sun “rising “from the dark horizon. Then it begins its daily “journey “from east to west across the sky

 Finally, the sky “sinks “out of view as night falls.

Is the sun moving, or you are?

 Causes of change in seasons

Flowers bloom as the days get warmer. The sun appears higher in the sky and daylight lasts longer. Spring seems like a fresh, new beginning. What causes these wonderful changes?

Another important motion is revolution (which is the earth’s yearly orbit around the sun) and the earth’s tilt about its axis.

The earth’s axis is tilted about 23.50 from a line drawn perpendicular to the plane of its orbit.

It is this tilt that causes the changes in seasons. The number of daylight hours is greater for the hemisphere or half of the earth that is tilted towards the sun. The hemisphere that is tilted towards the sun receives more hours of sunlight each day than the hemisphere that is tilted away from the sun. The longer period of sunlight is one reason why summer (hot season) is warmer than winter but it is not the only reason.

Figure 8.2: Interaction of sun and earth

Research 8.2

This research work aims at discovering the seasons our planet experiences.

What to do

Take a field trip around your school or your family and ask the natives the number of seasons they experience each year.

The following may be guiding questions:

1. How many seasons do you experience in your area each year?
2. What are the characteristics of those seasons?
3. What is the best season for?

(i) Planting?

(ii) Harvesting?

• Make a comprehensive report and present your findings to the whole class.

Tides

 Tides are periodical rises and falls of large water bodies like oceans and seas.

Causes of sew ocean tides

 High tides and low tides are caused by the moon’s gravitational pull. The tidal force causes the earth and its water to bulge out on the side closest to the moon and the side farthest from the moon. These bulges of water are high tides.

Figure 8.3: Ocean tides and currents

As the earth rotates your region of earth passes through both bulges each day. When you are in one of the bulges, you experience high tide. When you are not in one of the bulges, you experience low tide. This cycle of two high tides and two low tides occurs most days on most of the coastlines of the world.

ECLIPSES

This is the total or partial obscuring of one celestial body by another.

In astronomy,it is the obscuring of one celestial body by another, particularly that of the sun or a planetary satellite. Two kinds of eclipse involve the earth: those of the moon, or lunar eclipses, and those of the sun, or solar eclipses. A lunar eclipse occurs when the earth is between the sun and the moon and its shadow darkens the moon. A solar eclipse occurs when the moon is between the sun and the earth and its shadow moves across the face of the earth. Transits and occultations are similar astronomical phenomena but are not as spectacular as eclipses because of the small size of these bodies as seen from the earth.

There are two types of eclipse:

 (i) Lunar eclipse / eclipse of the moon.

(ii) Solar eclipse / eclipse of the sun.

Lunar eclipse

This is when the earth is between the sun and the moon. The earth, lit by the sun, casts a long, conical shadow in space. At any point within that cone, the light of the sun is wholly obscured. Surrounding the shadow cone, also called the umbra, is an area of partial shadow called the penumbra

Figure 8.4: Anatomy of lunar eclipse

A total lunar eclipse occurs when the moon passes completely into the umbra. If it moves directly through the centre, it is obscured / blocked for about 2 hours. If it does not pass through the centre, the period of total blockage is less and may last for only an instant if the moon travels through the very edge of the umbra.

 A partial lunar eclipse occurs when only a part of the moon enters the umbra and is obscured. The extent of a partial eclipse can range from near totality, when most of the moon is obscured, to a slight or minor eclipse, when only a small portion of the earth’s shadow is seen on the passing moon. Historically, the view of the earth’s circular shadow advancing across the face of the moon was the first indication of the shape of the earth.

Solar eclipse

This is when the moon is between the earth and the sun. Total solar eclipses occur when the moon’s umbra reaches the earth.

The diameter of the umbra is never greater than 268.7km where it touches the surface of the earth , so that the area in which a total solar eclipse is visible is never wider than that and is usually considerably narrower . The width of the penumbra shadow, or the area of partial eclipse on the surface of the earth, is about 4,828km. At certain times when the moon passes between the earth and the sun, its shadow does not reach the earth. At such times, an annular eclipse occurs in which an annulus or bright ring of the solar disk appears around the black disk of the moon. In areas outside the band swept by the moon’s umbra but within the penumbra, the sun is only partly obscured, and a partial eclipse occurs.

Figure 8.5: Interaction of sun and earth during solar eclipse

 Exercise 8.1

1. The picture below is of group of learners and their teacher from Ntungamo district. They happened to be at school on a day when an eclipse occurred.
2. As noticed from the diagram, explain why you think the learners had to put on glasses in order to look at the sun.

Figure 8.6: Leaners observing eclipse

PHASES OF THE MOON

 One revolution of the moon around the earth takes a little over 27 days, 7 hours. The moon rotates on its axis in this same period of time, so the same face of the moon is always presented to the earth, Over a period a little longer than 29 days 12 hours, the moon goes through a series of phases, in which the amount of the lit half of the moon we see from the earth changes. These phases are caused by the changing angle of sunlight hitting the moon. (The period of phases is longer than the period of revolution of the moon, because the motion of the earth around the sun changes the angle at which the sun’s light hits the moon from night to night.)

Activity 8.2 Demonstrating moon phases and eclipses

In this activity, you will demonstrate the position of the sun, moon and the earth during certain phases and eclipses. You will also see why only a small portion of the people on earth witness a total eclipse during a particular eclipse event

Key Question

Can a model be devised to show the positions of the sun, moon and earth during various phases of eclipse?

What you need

• Light source, globe, polystyrene ball, pencil

What to do

1. Use the light source as a sun model and the polystyrene ball on a pencil as a moon. Move the moon around the globe to duplicate the exact position that would have to occur for a lunar eclipse to take place.
2. Move the moon to the position that would cause the solar eclipse.
3. Place the moon at each of the following phases: first quarter, full moon, third quarter, and new moon. Identify which type of eclipse could occur during each phase.

Record your data in a suitable table.

Moon phase observations

(d) Place the moon at a location where a lunar eclipse could occur. Move it slightly towards the earth and then away from the earth. Note the amount of change in shadow.

(e) Repeat the above step with the moon in a position where the solar eclipse can occur.

 Conclude and apply

1. Identify which phase ( s ) of the moon make ( s ) it possible for an eclipse to occur .
2. Describe the effect of a small change in distance between the earth and the moon on the size of the umbra and penumbra
3. Explain why a lunar and a solar eclipse do not occur every month.
4. Explain why only a few people have experienced a total solar eclipse.
5. Explain why it might be better to call a full moon a half moon.

Communicate your answers to other learners.

Figure 8.7: Phases of the moon.

 First quarter

When the moon reaches the first quarter of its phases, it appears as a half – moon. One half of the moon is now lit up by sunlight.

 Waxing gibbous

After the first half – moon, or first quarter, the moon moves towards full moon. As more of it is lit up, the moon is said to be waxing. It is also said to be gibbous (between a semi – circle and a full circle in shape)

Full moon

When the Moon appears as a full moon, all of its side is visible in sunlight.

Waning crescent

After a full moon, the moon begins to decrease again. The moon is now said to be waning. It is also said to be gibbous (between a semi – circle and a full circle in shape).

Last quarter

 The moon continues waning until it reaches its last quarter, also known as half – moon.

Waning gibbous

 After a full moon, the moon begins to decrease again. The moon is now said to be waning. It is also said to be gibbous (between a semi – circle and a full circle in shape).

New moon

 With a new moon, the moon is between the sun and the earth, causing none of the illuminated side of the moon to be visible to the earth.

 Waxing crescent

After a new moon, the moon moves through a crescent phase. The moon is now said to be waxing.

The origin (evolution) of the universe, earth and life

The term “evolution “usually refers to the biological evolution of living things. But the processes by which planets, stars, galaxies and the universe form and change over time are also types of “evolution “. In all of these cases there is change over time, different findings from different astronomers show that the universe is expanding.

Research 8.3

 What you need

• Internet and Manila paper.

What to do

1. Using the internet (reliable sources), carry out research on the origin of the universe.
2. Write your findings on the Manila paper.
3. Present and discuss your findings to the whole class.
4. Note down (in your notebook) the important information for future reference.

BIBLICAL THEORY OF EVOLUTION

According to the Holy Bible (Genesis 1: 1.2 and 3). God created the heavens and the earth. It is believed that the earth was without form and void and darkness was over the face of the deep.

Take time and read this part of the Holy Bible. It is very interesting. It is on the basis of this that Christians believe that the universe was created by God.

Also, according to the Qur’an, “the heavens and the earth were joined together as one unit, before we clove them asunder “(21:30). Following this big explosion, Allah ” turned to the sky, and it had been (as) smoke. He said to it and to the earth: ‘ Come together, willingly or unwillingly. They said: ‘ we come (together) in willing obedience ™ (41:11). Thus the elements and what was to become the planets and stars began to cool , come together , and form into shape , following the natural laws that Allah established in the universe . The Qur’an further states that Allah created the sun, the Moon, and the planets, each with their own individual courses or orbits. ” It is He Who created the night and the day , and the sun and the moon ; all ( the celestial bodies ) swim along , each in its rounded course ” ( 21:33 ) .

 SCIENTIFIC THEORY OF EVOLUTION

Scientists believe that our universe originated from a singularity (an infinitely hot area with infinite pressure and density) but none knows where that singularity originated from!

Note:

• Before the singularity, there was no space, time, matter and energy since they all began inside it.
• The Big Bang is more of a balloon being blown up than a bomb! This theory was first suggested by a Belgian priest named George Lemaitre in 1920. It is called the “Big Bang theory “. He guessed that the universe began from a single atom with high density and temperature. For a not yet known reason that single, hot chaotic atom started to expand rapidly, and cooled as it grew older. Therefore, all the matter, energy and space came from the above expansion.

 As the universe expanded, according to current scientific understanding, matter collected into clouds that began to condense and rotate, forming the forerunners of galaxies. Within galaxies, including our own Milky Way galaxy, changes in pressure caused gas and dust to form distinct clouds. In some of these clouds, where there was enough mass and the right forces, gravitational attraction caused the cloud to collapse. If the mass of material in the cloud was sufficiently compressed, nuclear reactions began, and a star was born.

 Some proportion of stars, including our sun, formed in the middle of a flattened spinning disk of material. In the case of our sun, the gas and dust within this disk collided and aggregated into small grains, and the grains formed into larger bodies called planetesimals (“ very small planets ” ) , some of which reached diameters of several hundred kilometres . In successive stage, these planetesimals coalesced into the nine planets and their numerous satellites. The rocky planets, including the earth, were near the sun, and the gaseous planets were in more distant orbits.

Project work 8.1

 What you need

• Manila paper, internet, coloured markers.

 What to do

In groups, design posters (using Manila paper) and pin them in your class. Your poster up should include the following:

(a) The solar system with all planets, asteroids and comets

(b) The two types of eclipse

 (c) Phases of the moon

Chapter Summary

 In this chapter, you have learnt that:

1. The solar system is the gravitationally bound system of the sun and the objects that orbit it.
2. The objects that move around the sun are called planets.
3. Planets are divided into terrestrial and outer planets.
4. The earth’s rotation causes day and night.
5. Tides are periodical rises and falls of large water bodies.
6. Evolution is the biological change of living things.