Unit 1

February 2005

Tutor support materials

GCSE Edexcel GCSE Astronomy Student Workbook

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Edexcel Limited is one of the leading examining and awarding bodies in the UK and throughout the world. It incorporates all the qualifications previously awarded under the Edexcel and BTEC brands. We provide a wide range of qualifications including general (academic), vocational, occupational and specific programmes for employers.

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For further information please call Customer Services on 0870 240 9800, or visit our website at www.edexcel.org.uk

Authorised by Jim Dobson Prepared by Sarah Harrison

All the material in this publication is copyright © Edexcel Limited 2005

Contents

Introduction 1

Symbols, Units and Data Required 3

Basic Science Knowledge 7

Unit 1 — Planet Earth 11

The Earth 11

Days and Seasons 17

Exercise 1 21

Project 1: Making a Model Earth 24

Unit 2 — The Moon and the Sun 25

The Moon 25

The Sun 27

Eclipses 31

Exercise 2 33

Project 2: Observing the Sun and Moon 37

Unit 3 — The Solar System 39

Planets and Asteroids 39

Meteors and Comets 54

Exercise 3 58

Project 3: Researching the Solar System 61

Unit 4 — Stars and Galaxies 63

Constellations 63

Stars 68

Galaxies 78

Exercise 4 87

Project 4: Constructing a Star Map 91

Unit 5 — Observing Techniques and Space Exploration 93

Observing the Universe 93

Exploring the Universe 102

Exercise 5 112

Project 5: Building a Telescope 116

Answers 117

Exercise 1 117

Exercise 2 120

Exercise 3 122

Exercise 4 125

Exercise 5 128

Student’s Workbook – Edexcel GCSE in Astronomy – Issue 1 – February 2005 1

INTRODUCTION

This workbook has been developed to offer support to learners when studying GCSE Astronomy qualification, and to teachers delivering the course.

The aim of this booklet is to offer learners and teachers information that will provide support when starting the course. This booklet also offers support for revision and for learners to enhance your learning, using research and web-based activities, for each unit.

Each unit gives learners the opportunity to support the development of their coursework and shows learners what is required for each task. The specification provides details on the coursework requirements.

An exemplar project has been included at the end of each unit. They have been designed to make the course more interesting and give the learner the chance to use the project work as a way to understand all the astronomy information that you need.

At the end of each unit there is also an exercise to test the learners’ understanding, and answers are provided at the end of the workbook.

Edexcel wishes to acknowledge the help and support received in the completion of this workbook, particularly the Royal Observatory Greenwich for their technical editing and support.


SYMBOLS, UNITS AND DATA REQUIRED

You are expected to understand and use the following terms:

• The 24 hour clock

• The use of hours, minutes and seconds for measuring time

• The celestial co-ordinate system: (explained further in Unit 4 page 68)

− right ascension (RA) → celestial longitude, measured in hours

− declination (dec) → celestial latitude, measured in degrees (°)

• The units for measuring angles:

− degree

− arc minute →1/60th of a degree

− arc second → 1/3 600th of a degree

• The units for temperature:

− kelvin (K) → 0 K = –273°C

− degree Celsius (°C) → 0°C = 273 K

• The unit of power (luminosity):

− Watt (W) → Joule per second

• The units of length:

− astronomical unit (AU) → the mean distance between the Earth and the Sun (150 million km)

− light year (l.y.) → the distance travelled by light in a vacuum in 1 year (63 240 AU)

− parsec (pc) → the distance at which a star would have parallax of 1 second of arc (3.2616 l.y.)

• The speed of light in a vacuum = 300 000 km/s


In exams you are expected to remember and use the following approximate values:

Earth diameter = 13 000 km

Moon diameter = 3500 km

Sun diameter = 1.4 million km

mean Earth-Moon distance = 380 000 km

mean Earth-Sun distance = 150 million km = 1 AU

difference in magnitude of 1 = brightness ratio of 2.5

difference in magnitude of 2 = brightness ratio of 100

(see apparent magnitude on page 69)

The following information may be useful to you.

• Powers of ten

In astronomy the numbers written can be extremely large or extremely small. Therefore, they are often written in scientific notation, as powers of ten.

Eg the diameter of the Earth is approximately 13 000km. This can be written as 1.3 x 104 km.

101 = 10

102 = 10 x 10 = 100

103 = 10 x 10 x 10 = 1000 k kilo

106 = 10 x 10 x 10 x 10 x 10 x 10 = 1000 000 (one million)

M Mega

109 = one thousand million = one billion G Giga

100 = 1

10–1 = 1/10 = 0.1

10–2 = 1/(10 x 10) = 0.01 c centi

10–3 = 1/(10 x 10 x 10) = 0.001 (one thousandth)

m milli

10–6 = 1/(10 x 10 x 10 x 10 x 10 x 10) = 0.000001 (one millionth)

µ micro

10–9 = one billionth n Nano


• The Greek alphabet

Letters from the Greek alphabet are often used in astronomy, eg when naming stars.

α alpha β beta

γ gamma δ delta

ε epsilon ζ zeta

η eta θ theta

ι iota κ kappa

λ lambda µ mu

ν nu ξ xi

ο omicron π pi

ρ rho σ sigma

τ tau υ upsilon

φ phi χ chi

ψ psi ω omega


BASIC SCIENCE KNOWLEDGE

The following knowledge of some aspects of science is needed to understand parts of this astronomy course.

1 Reflection — when light ‘bounces off’ a shiny or smooth surface.

2 Refraction — when light passes from one medium to another (of a different optical density) it changes direction.


3 Spectrum of colours — when white light passes through a prism it disperses into the seven colours of the spectrum (red, orange, yellow, green, blue, indigo, and violet).

4 Electromagnetic spectrum — this contains electromagnetic (EM) waves. In the EM spectrum they are placed in order of their frequency and wavelength. Wavelength is the distance from one peak of a wave to the next peak. All of the different wavelengths are used in astronomy.


5 Atomic Structure — the structure of atoms is widely used in astronomy, eg for identifying the elements within stars when looking at their spectra. The elements most commonly looked at by astronomers are hydrogen and helium, which are found in stars such as the Sun.

6 Speed — speed can be calculated using the following equation:

timecetandis

speed =

Speed is measured in metres per second (m/s)

Distance is measured in metres (m)

Time is measured in seconds (s)


Unit 1 — Planet Earth

The Earth

The Earth is the third planet from the Sun and it has one natural satellite, called the Moon. The Earth has a mean diameter of 13 000 km and is approximately 150 million km from the Sun. This distance from the Sun is also known as 1 AU (astronomical unit). The Moon is approximately 380 000 km from the Earth and has a mean diameter of 3500 km.

Figure 1.1: The Earth from space (NASA)

Orbit

The Earth’s orbit around the Sun is not quite circular, but elliptical. So at some points it is closer to or further from the Sun. The closest point to the Sun is called perihelion, and the furthest is called aphelion. As the orbit is elliptical, 1 AU is the mean (average) distance from the Earth to the Sun over a year (one orbit). All planets have elliptical orbits.

Figure 1.2: Perihelion and aphelion


Figure 1.2 is not to scale, and shows the orbit as very elliptical. The orbits of the planets around the Sun are more circular, with the exception of Pluto.

Latitude and longitude

The imaginary great circle around the Earth’s surface is the Equator. The geographical equator is equal distance from the north and south poles and is at 90° to the angle of the Earth’s axis.

The Earth’s surface is divided with imaginary lines. Latitude lines are horizontal, and on the celestial sphere they tell the angular distance from an object in space to the ecliptic (line). Lines of Longitude are vertical and mark the time difference around the world from the Greenwich Meridian. Each 15° difference in longitude is one hour’s difference in time. East of Greenwich the time is ahead of Greenwich Mean Time (GMT), west of Greenwich is behind GMT.

Geology

The crust on the Earth consists of several rocky plates that move due to currents in the hot rocky mantle below.

The structure of the Earth has been determined by monitoring seismic waves from earthquakes.

Figure 1.3: The structure of the Earth

The Earth has a dense core, rich in iron and nickel. The outer part of the core extends down to 5150 km, but the inner core is 2460 km in diameter. The inner core is liquid because of the higher pressure there. The core is surrounded by a mantle of silicate rocks. This extends to a depth of 2890 km. The thin outer layer of lighter rock is called the crust. Continental crust can be up to 50 km deep, but oceanic crust has an average depth of 10 km.


Oceans

The oceans cover approximately 70% of the Earth’s surface and are unique in our solar system. No liquid water has been discovered on any other planets and moons though water ice has been discovered. The average depth of the oceans is 3795 m. However, the deepest part of the oceans is the Mariana Trench, which is approximately 11 000 m deep. This is located in the Pacific Ocean, where two plates join and one is subducted under the other to form the trench.

The average temperature of the oceans is 3.9°C. They absorb heat energy from the Sun and store it. The ocean currents distribute this heat energy around the Earth heating the land and air during winter and cooling it during summer.

The Earth has ice caps at its north and south poles. These are formed because the temperature is below freezing throughout the year. The north polar ice cap has thinned recently as the temperature has become warmer.

Tides

The oceans have tides which are caused by the gravitational pull of the Moon and the Sun. As the Earth spins on its axis once each day it rotates under the tidal bulges. Hence each place has two high tides and two low tides each day. When the Earth, Moon and Sun are all lined up (as in Figure 1.4 diagram 1) the gravity of the Moon and the Sun combined makes the tide much larger. When the Moon is not lined up with the Earth and the Sun (as in Figure 1.4 diagram 2) the Moon and the Sun’s gravity are not combined so the tide is smaller. The Moon has the larger affect on the tides when it is closer to the Earth. The position of the Moon, relative to the Earth, affects different parts of the Earth’s seas. The tides are also affected by the phases of the Moon.

Figure 1.4: Causes of spring and neap tides


The atmosphere

The atmosphere of the Earth contains approximately 78% nitrogen, 21% oxygen, and 1% of other gases such as carbon dioxide, water vapour and argon which have promoted life. The oceans keep the planet warmer, by retaining some of the Sun’s heat and this has also helped life to develop.

The atmosphere makes the sky appear blue. The dust particles in the atmosphere are approximately the same size as the wavelength of blue light. This means that they scatter (refract) blue light more than any other colour. Therefore, the colour of light that we see is the colour that is scattered the most, which is blue.

When the sun begins to set the light must travel farther through the atmosphere before it gets to you. More of the light is scattered. As less light reaches you directly, the sun appears less bright. The colour of the sun itself appears to change, first to orange and then to red. This is because even more of the blue light is scattered. Only the longer wavelengths are left in the direct beam that reaches your eyes and these are mostly the red and orange colours.

The sky at the time of a sunset may take on many colours. Sunsets with the most colours occur when the air contains many small particles of dust or water. These particles scatter light in all directions. Then, as some of the light heads towards you, different amounts of the shorter wavelength colours are scattered. You see the longer wavelengths and the sky appears red, pink or orange.

This happens due to the dust particles in the atmosphere and is due to the nature of the particles’ random motion. As the particles move, there are naturally some areas of the atmosphere with more particles than others. Therefore, the density of the atmosphere is not uniform. This acts in a similar way to dust particles and scatters the sunlight as it enters the atmosphere. However, this would happen if there were no dust particles in the atmosphere at all. The presence of dust particles just increases this process.

For astronomers there are benefits and drawbacks of the Earth’s atmosphere. Some of these are:

Benefits Drawbacks

Protection against UV rays There are often too many clouds to observe the skies

The mixture of gases and the temperature is good for life

There is often too much pollution for good observations

Astronomers sometimes have difficulty in making astronomical observations, particularly in Britain, as the conditions for observing the sky can be affected by:

• optical (light) pollution, which causes poor visibility of the skies, so obscuring the stars

• chemical pollution, which causes poor visibility due to the pollutants in the air

• interference from radio signals, which is a problem when astronomers are using radio telescopes.


The best place in Britain to observe is in the countryside (ideally the middle of a big field or on a hill). The best place to site an observatory however would be:

• at a high altitude, to give as many cloudless nights as possible

• nearer the equator, for better weather

• away from towns so the level of light pollution is lower.

Magnetic field

The magnetic field of the Earth is similar to that of a bar magnet. The Earth has a magnetic north and a magnetic south pole. It is the interactions between the liquid outer core and the solid inner core that produce the magnetic field of the Earth.

Magnetosphere

This is the area of space around the Earth that is controlled by the Earth’s magnetic field. This reaches out 600 000 km into space, in the direction of the Sun, but much further in the other direction. The solar wind changes the shape of the magnetosphere and also prevents a lot of the particles in the solar wind from reaching the Earth.

Van Allen belts

The Van Allen belts are zones of highly charged particles in the Earth’s magnetic field (magnetosphere). As charged particles in the solar wind are blown off the Sun the Earth’s magnetic field traps them and brings them down towards the Earth.

There are two Van Allen belts. The outer belt contains mostly electrons and the inner belt contains mostly protons.

Figure 1.5: The structure of the Van Allen belts


Aurorae

These are beautiful, flickering illuminations in the night sky that are most often seen where the Van Allen belts are closer to the Earth (the ends of the belts). These lie near the north and south polar regions. Where the solar wind interacts with the charged particles we can observe colourful patterns of light called aurorae. The aurorae are more intense when there is more solar activity, such as a prominence.

The aurorae have different names depending on where they reach the Earth.

• The Aurora Borealis — The Northern Lights.

• The Aurora Australis — The Southern Lights.

Figure 1.6: An example of an aurora (NASA)


DAYS AND SEASONS

Celestial Sphere

The celestial equator is equal distance from the celestial poles, on the celestial sphere, and at 90° to the angle of the Earth’s axis.

The Earth rotates from west to east so the Sun appears to move across the sky from east to west. As this happens parts of the Earth are in daylight whilst other parts are still in night-time.

A local meridian is the line that passes from the north pole, under your feet (nadir) to the south pole.

The ecliptic is the line that the Sun appears to travel along. It is actually the plane of the Earth’s orbit around the Sun.

Seasons

The Earth’s axis is tilted at 23.5° from perpendicular to the plane of the ecliptic, and this causes the seasons.

Figure 1.7: The Earth’s tilted axis

During the seasons some parts of the Earth point towards the Sun, whilst other parts point away from the Sun. This means that their temperatures are different.


Figure 1.8: The changing seasons of the Earth

At the equator the temperature does not change very much during the seasons. This is because this area receives more direct sunlight throughout the year.

In the temperate regions the seasons show a bigger change in temperature, as the regions point towards or away from the Sun and receive less direct sunlight.

In the polar regions there is a big difference between the amount of sunlight in summer and winter. This is because these regions do not receive very much sunlight for six months and continuous sunlight for six months. Therefore six months of the year is daylight, whilst the other six months is night-time.

Equinoxes

These are the times of year when day and night are of equal length on all parts of the Earth. There are two equinoxes.

• Spring equinox — this marks the beginning of spring and is often called the vernal equinox. It occurs on 21st March. This is where the ecliptic crosses the celestial equator with the Sun apparently travelling north.

• Autumnal equinox — this marks the beginning of autumn. It occurs on 22nd or 23rd September. This is where the ecliptic crosses the celestial equator with the Sun apparently travelling south.

Solstices

These are the times of year when the Sun appears to be furthest north or south of the equator. There are two solstices.

• Summer solstice — in the northern hemisphere this is the longest day of the year. It occurs on 21st June.

• Winter solstice — in the northern hemisphere this is the shortest day of the year. It occurs on 21st December.


Days

We can use a shadow stick to measure how the angle of the Sun has changed. If you put a stick in the ground it will have a shadow on a sunny day. At noon the shadow will be shortest and at sunset or sunrise it will be longest. In summer at noon the shadow will be shorter than in winter, because the Sun is higher in the sky in summer. By measuring the length of the shadow you can observe how the altitude of the Sun changes during the seasons. This can help you to calculate the time of local noon and the longitude of where you are observing.

According to astronomy a day is the time it takes the Earth to rotate once on its axis. This takes 23 hours and 56 minutes (23 h 56’). A year is the time it takes the Earth to orbit the Sun once. This takes 365.25 days. Also in astronomy there are different types of days. An apparent solar day is the time between two successive local noons (ie 2 successive meridian transits of the Sun). However, the Sun’s apparent motion throughout the year varies. A mean solar day is 24 hours (24 h) long. This is the average time calculated for a day, assuming that the Sun ‘travels’ along the celestial equator at a uniform rate.

This means that there is a need for time zones as different parts of the Earth have noon (when the Sun is directly overhead at your zenith) at different times. The time zones help when people travel all over the world so they are not constantly changing their clocks to the local time.

Solar terms

There are some specific terms needed in astronomy for describing the Sun.

• Mean Sun — this is an ‘average’ time for an imaginary Sun moving along the celestial equator. The true Sun travels at a variable rate, so we cannot use this as a measure of time.

• Mean solar time — this is the time based on the mean Sun, which travels at a uniform rate along the celestial equator.

• Apparent solar time — this is the time as observed on a sundial.

• Equation of time — this is the difference between the solar time on a clock and the apparent solar time on a sundial. The maximum difference is 16 and occurs in early November. The difference is zero on four occasions. These are 15th April, 14th June, 1st September and 25th December.

equation of time = apparent solar time – mean solar time

Sundials

Another simple way to observe the changes in the Sun’s position over a year is to use a sundial. Sundials have been used since ancient times as a method of telling the time. The main types of sundials are horizontal sundials, that lie flat on a stand, or vertical sundials that are fixed to walls. They all use the same principles. In equatorial sundials the graduations for the dial are 15° apart, as the Earth rotates by this distance in one hour. The gnomon (part that casts the shadow) should be at the


angle of latitude for the place that the sundial is to be used (eg 51.5° N for Greenwich). This means that sundials are specific to one area. The gnomon should be placed north-south, with the raised end pointing north.

Figure 1.9: Sundials and the angle of the gnomon

Sidereal day

A sidereal day is measured with respect to the stars. It is the time taken between two successive crossings of the star across the observer’s meridian. It is also the time between two successive crossings of the spring equinox across the observer’s meridian. A sidereal day takes 23 h 56’4’’.

Figure 1.10: A sidereal day and a mean solar day


EXERCISE 1

Questions

1 Describe the size and position of the Earth.

2 Describe the shape of the Earth’s orbit around the Sun.

3 Explain the following terms:

a equator

b ecliptic

c perihelion

d aphelion

e latitude

f longitude

g pole

h meridian

i zenith

j horizon

k astronomical unit

l mean Sun

m mean solar time

n apparent solar time

o a year

p a day

q a solar day

r a sidereal day

s equinox

t solstice

u a horizontal sundial

v a vertical sundial

w a gnomon.

4 One Astronomical Unit (1 AU) assumes that the Earth has a circular orbit around the Sun.

a Why is this wrong?

b Why do we still use 1 AU if it is inaccurate?

5 Explain how the Earth is distinguishable from the other planets in our Solar System.


6 Explain how the Moon and the Sun affect the seas and oceans on Earth.

7 Explain why the sky is:

a blue (most of the time)

b red at sunrise and sunset.

8 Describe two benefits and two drawbacks of the Earth’s atmosphere for the following people:

a the general population

b astronomers.

9 Explain what these types of pollution are:

a chemical pollution

b optical pollution

c radio interference.

10 Where are the Van Allen belts and state their composition?

11 Explain where and how Aurorae are formed.

12 Does the Earth spin clockwise or anticlockwise on its axis?

13 Does the Earth orbit the Sun in a clockwise or anticlockwise direction?

14 Why are time zones needed across the Earth?

15 Explain how the tilt of the Earth and its orbit around the Sun gives us seasons.

16 What do astronomers measure with a shadow stick and what can these measurements show throughout a year?

17 On a sundial what angle does the gnomon correspond to?

18 Why does the Sun rise and set at different times throughout the year?

19 What is the equation of time?


Research

1 Find out what the different layers are in the Earth’s atmosphere (from the internet or textbooks).

2 Find out about the Campaign for Dark Skies and the International Dark Skies Association.

3 Find out what the desirable conditions are for siting an observatory.

4 Find diagrams (from the internet or textbooks) to show the location of the Van Allen belts.

5 Find pictures of the Aurorae and find the difference between the Aurora Borealis and the Aurora Australis.

6 Carry out a shadow stick experiment, on two different sunny days, and on each day observe and measure how the shadow length varies. This can become part of your coursework. Use the results from this to determine the following:

a the length of the shadow at noon

b the time of local noon

c the longitude at which you carried out the experiment.

7 Draw a template to make a sundial (face and gnomon). Construct the sundial and test its accuracy on at least three widely separate occasions. This could be used as part of your coursework.

8 Find a graph for the equation of time showing how the solar time differs during the year. Identify on the graph the times of the year that the equation of time equals zero, and which time of the year it is at its maximum.


PROJECT 1: MAKING A MODEL EARTH

Introduction

This project is to make a model of the planet Earth in order to help you understand the ideas that are covered within this unit.

Resources

A papier maché model would be most suitable as this would be light enough to use in demonstrations and could be decorated to show all of the features of the Earth. Suggested resources include:

• balloons

• paper (newspaper is suitable)

• wall paper paste (or similar adhesive)

• paint

• brushes (for paint and paste)

• a variety of arts and crafts materials to decorate the model

• pictures of the Earth showing all its features.

Method

Construct the papier maché model of the Earth and paint it to resemble the land and oceans. You may wish to extend your model by adding atmosphere, clouds and aurora with a little creative thinking. The models can then be used to simulate the motion of the Earth around the Sun, rotation of the Earth and the seasons.

Links to the specification

This project covers the Days and Seasons part of this unit (1.16–1.21) and areas of The Earth part of this unit (1.1–1.13).

When to start

This project could be started at the beginning of Unit 1, and you could use and improve on your model as you learn about the various aspects of Unit 1: Planet Earth.

Unit 1

Documents

Transcript of Unit 1