Earth and Space Sciences

Eclipses: A passing phase

Eclipses provide us with awesome displays – every time the Earth, Moon and Sun line up, which is not very often! They used to give rise to myths, prophecies and panic, but today are more likely to generate worries about power loss.

In this lesson you’ll investigate the following:

• How did the Moon form?

• What causes eclipses?

• How do eclipses relate to the phases of the Moon?

• What are the differences between solar and lunar eclipses?

So let’s shine some light on the spectacular phenomena of eclipses!

This is a print version of an interactive online lesson. To sign up for the real thing or for curriculum details about the lesson go to www.cosmosforschools.com

Introduction: Eclipses

Did you know that the word “eclipse” comes from the ancient Greek word for “abandonment”?

You can understand why people in ancient times felt abandoned when, in the middle of the day and with no warning, the Sunwould disappear from the sky. The world would be thrown into a strange darkness as the temperature dropped and birdsfell silent. The Sun had left them – what could it mean?

Historical records show that humans have been keeping track of eclipses for over 4000 years – probably because they believed theywere evil omens. The ancient Chinese thought that eclipses were due to a dragon swallowing the Sun. They would make a racketwith kitchen pots or anything else they could find – even firing cannons – to chase the dragon away. In 17th century Europe the realcauses of eclipses were known but most people were still so terrified that astronomers had to write booklets to explain that theywere natural events and nothing to be afraid of.

But in our modern age a solar eclipse can be bad news because of our increasing dependence on solar power.

The eclipse that took place in Europe on 20 March, 2015, lasted approximately two hours. In that time it took 35,000 megawatts ofelectricity off the grid. That’s the same as if 50 coal power plants had suddenly shut down!

They may not have been banging pots and readying the cannons this time around, but European energy companies spent over ayear preparing for the eclipse. Now that we know exactly what causes these extraordinary events we can predict exactly where andwhen they will occur.

Read the full Cosmos blog post here.

Question 1

Imagine: Before scientists fully understood the motions of the Sun, Moon and Earth, humans had many different beliefs aboutwhat the Sun and Moon were and what eclipses meant. The Chinese had their dragon and indigenous Australians told stories thatassociated eclipses with death, disease and sorcery.

Imagine you're living in a small group several thousand years ago. One moment it's broad daylight and then suddenly the Sun isextinguished and your whole world falls into eerie darkness. Use your imagination to think of a possible explanation – whatpowerful beings might have caused this? And what could it mean?

Gather: Eclipses

Two time-lapse images of eclipses. Left: A total lunar eclipse seen from San Diego, USA, on 14 April, 2014. Right: A partialsolar eclipse as viewed from NASA's Solar Dynamics Observatory satellite. Different colours have been artificially applied

to each image to show different types of light and heat radiation streaming from the Sun.

We wouldn't see any eclipses on Earth if we didn't have a moon. And for that we have to thank a spectacular event that took placewhen our Solar System was still taking shape.

Why do we have a moon?

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Question 1

Recall: There was a time when the Earth revolved around theSun by itself, but that didn't last long.

The Moon was formed when the Earth was still very young,approximately:

6000 years ago

4.5 million years ago

4.5 billion years ago

14.5 billion years ago

Question 2

Select: The Moon formed from a mass of liquid rock blasted outfrom the early Earth when it collided with a planet.

This planet was about the size of:

Mercury

Mars

Venus

Jupiter

Have you ever seen an eclipse? If you haven't then there's a good reason – they're rare events. They only come about when theEarth, Moon and Sun all line up.

During a lunar eclipse, light from the Sun hits the Earth so that it casts a shadow on the Moon. The Moon (Latin luna) seems todisappear.

During a solar eclipse, light from the Sun hits the Moon so that it casts a shadow on the Earth. The Sun (Latin sol) seems todisappear.

But the Earth is constantly moving in its stable orbit around the Sun while the Moon is moving around the Earth, so perfectalignments of the three objects occur only briefly.

The branch of astronomy that describes these sorts of motion is known as celestial mechanics. "Celestial" is an old word meaning"heavenly" and mechanics is the science of motion.

Celestial mechanics

The phases of the Moon as viewed from Earth during one lunar cycle.

The same celestial motions that cause eclipses also cause the phases of the Moon. At any one time, half of the Moon is lit bysunlight and half lies in shadow. The difference between two phases – such as a full moon and a thin crescent moon – dependson how much of the face that we can see is lit up.

A lunar cycle is the time it takes the Moon to complete one orbit of the Earth – about 29.5 days – and in each cycle we see the samesequence of phases.

The phases of the Moon

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Question 3

Match: The sketchpad below shows the orbit of the Moon around the Earth. The phases of the Moon are lined up below thesketchpad in the wrong order. Your job is to match each phase of the Moon to its position in the lunar cycle.

1. Use the dashed white lines to work out how the Moon would appear from Earth at each stage of the lunar cycle. Select thecorrect phase of the moon from the lower image and enter its letter into the white box.

2. Type the name of the phase in the grey box.

The waning gibbous phase has been completed for you.

Question 4

Identify: In which two phases of the lunar cycle are the Moon,Earth and Sun aligned?

New moon

First quarter

Full moon

Third quarter

Question 5

Recall: During a new moon phase, the Moon appears darkbecause:

it loses its ability to reflect light.

it lies between the Sun and the Earth so that we

can only see its shadowed side.

it lies on the opposite side of the Earth to the Sun

so that it falls in our planet's shadow.

Lunar eclipses

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Question 6

Complete: Using the information in the video, select the correct term from the middle column to fill the blanks in each sentenceand enter it into the right hand column.

A lunar eclipse can only occur during a _______ moon phase. full   |   new

The ____________ plane is the plane on which the Earth rotates around theSun.

equatorial   |   ecliptic

The ____________ is the inner part of the Earth's shadow in which all of theSun's light has been blocked.

 umbra   |   penumbra

The ____________ is the outer part of the Earth's shadow in which only partof the Sun's light has been blocked.

 umbra   |   penumbra

During a total lunar eclipse the Moon often has a faint ______ glow due toindirect sunlight that is ___________ through the Earth's atmosphere.

red   |   bluereflected   |   refracted 

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Question 7

Recall: A solar eclipse can only occur when there's a new moonin the lunar cycle.

True

False

Question 8

Select: At any given point on the Earth's surface a total solareclipse occurs once every:

29.5 days

18 months

360 to 410 years

Question 9

Identify: Identify the three types of solar eclipse shown in the sketchpad below. The fourth type – hybrid – appears as a totaleclipse from one location and as an annular eclipse from another.

Solar eclipses

Process: Eclipses

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Question 1

Paraphrase: In your own words, explain why there are only oneor two lunar eclipses per year even though there are about 12full moons per year.

Question 2

Explain: Why do two lunar eclipses in the same year occur 177days apart – for example, 4 April and 28 September, 2015? Whydoesn't the second eclipse occur six calendar months later, on 4October?

Hint: Try dividing 177 by 6!

As explained in the video, you're far more likely to witness a lunar eclipse than a solar eclipse because the Earth is much biggerthan the Moon.

During a lunar eclipse, the whole Moon easily fits within the shadow cast by the Earth – if we include both the umbra and thepenumbra. The eclipse can be viewed from anywhere on the night side of the Earth and can last for several hours.

During a solar eclipse, the shadow of the Moon traces only a narrow path across the Earth's surface so that most people don'tsee an eclipse at all. And even if you're lucky enough to see one, it'll be over in a few minutes!

We can understand solar eclipses better in terms of apparent size. The apparent size of an object is how much of your visual field ittakes up and it depends on two things:

how big the object is, and

how far away it is.

Look out of the window of your classroom and locate a tree. Now close one eye and hold up your thumb so that the top and

Apparent size

bottom of your thumb line up with the top and bottom of the tree – you might need to move your thumb forwards or backwards.Now your thumb and the tree have the same apparent size (or, more exactly, apparent height).

The same principle applies to more distant objects:

If the Moon was either smaller or more distant then it would have a smaller apparent size.

If it was either larger or closer then it would have a larger apparent size.

So what does all of this have to do with eclipses? You've probably noticed that the Moon and Sun have roughly the same apparentsize in the sky – even though the Sun is much larger than the Moon, it's also a lot further away. If the apparent size of the Moon wasmuch smaller than it is, it wouldn't be able to completely cover the Sun to produce a total solar eclipse. 

The apparent size of a round object can be calculated using the formula:

For example, we know that:

the diameter of the Sun is 1,391,684 km

the distance from the Earth to the Sun is 149,600,000 km

So:

apparent size of the Sun (viewed from Earth) = 1,391,684 km / 149,600,000 km = 0.0093 (to four decimal places)

apparent size = diameterdistance

Question 3

Calculate: Using the above formula calculate the apparent sizeof the Moon as viewed from the Earth, given that:

the diameter of the Moon is 3,475 km

the distance from the Earth to the Moon is 363,104 km

Round your answer to four decimal places.

Question 4

Calculate: In fact, the Moon's orbit is not perfectly circular andthe distance given in the previous question was its minimum

distance.

Calculate the Moon's apparent size as viewed from the Earthwhen the Moon is at its maximum distance of 406,696 km.Round your answer to four decimal places.

Question 5

Compare: The two diagrams below show the Earth, Moon and Sun (not to scale) when the Moon's distance is a minimum and amaximum. Enter the apparent sizes of the Moon that you've just calculated into the appropriate spaces.

By comparing the apparent sizes of the Moon and the Sun in each case, decide which diagram represents a total solar eclipse andwhich represents an annular solar eclipse.

Question 6

Think: Scientists can now make accurate measurements of thedistance to the Moon by bouncing lasers off reflectors that wereplaced on its surface during the Apollo landings. Thesemeasurements have confirmed that the Moon's averagedistance is increasing at a rate of about 3.8 cm per year.

If this increase continues over millions of years, we can expect:

the apparent size of the Moon to increase.

the apparent size of the Moon to decrease.

the frequency of total solar eclipses to increase.

the frequency of total solar eclipses to decrease.

Question 7

Infer: If you were standing on the Moon, a solar eclipse wouldoccur when the Earth blocks the light from the Sun.

Imagine that you're witnessing a solar eclipse from the surfaceof the Moon. Which of the following statements is false?

Everything around you is dark because you're

standing in the Earth's shadow.

Someone standing on the night side of the Earth

can witness a lunar eclipse at the same time.

Someone standing on the night side of the Earth

can witness a new moon at the same time.

Sun, as viewed from the Martian surface.

Just imagine the eclipses we might see if our planet had multiplemoons of different sizes...

Question 8

Explain: The map on the right shows the calculated paths of theMoon's shadow during all of the solar eclipses between theyears 2008 and 2028.

Explain how it's possible for scientists to predict the precisetimes and locations of future solar eclipses.

Did you know?

Eclipses can take place on other planets too!

In 2013, the NASA rover Curiosity captured a solar eclipse fromthe surface of Mars. The time-lapse photo on the left showsthree images taken three seconds apart as Phobos  – the largerof Mars' two moons – passed directly in front of the Sun.

This was an annular eclipse because the apparent size ofPhobos is quite a bit smaller than the apparent size of the

Apply: Eclipses

Experiment: Create your own eclipses

To create a model of the Earth, Moon and Sun and use thismodel to investigate the phases of the Moon and both solar andlunar eclipses.

Working in groups of about four students, each group will need:

a cardboard tube (e.g. an empty toilet paper roll)

a pair of scissors

sticky tape

a large (60 cm x 20 cm) strip of cardboard

a styrofoam ball the size of a large orange

a ping pong ball

aluminium foil

a sturdy but bendable piece of wire, about 40 cm long

a ruler

a torch or other strong light source

a stack of books

Aim

Materials

2. Fold out the strips you've created at each end of the cardboard tube. Stand the tube vertically so that the strips at the bottomlie flat on the table or floor and the strips at the top fan out like a flower.

3. Using sticky tape, attach the cardboard tube to the strip of cardboard near one end, as shown in the top diagram.

4. Now attach the styrofoam ball to the open "flower" at the top of the tube using sticky tape. This ball represents the Earth.

5. Wrap the ping pong ball with a sheet of aluminium foil, shiny side out. This ball represents the Moon.

6. Insert one end of the wire into the top of the Earth so that the wire is vertical.

7. Measure 6 cm along the wire and bend the wire at a right angle to give a horizontal arm.

8. Insert the other end of the wire into the Moon.

9. Measure 8 cm along the wire from the Moon and make another right-angled bend so that the Moon hangs vertically. Thecentre of the Moon should be at the same height as the centre of the Earth, as shown in the top diagram. You can adjust theheight of the Moon by raising or lowering the segment of the wire that sticks into the styrofoam "Earth".

10. Balance the torch on a stack of books at the other end of the cardboard strip from the Earth. The torch represents theSun. Adjust the height of the stack of books so that the torch beam is at the same height as the centres of the Earth andMoon. If the beam is not bright enough, move the stack of books closer. Turn off or dim any other sources of light to get thebest results.

Results

Question 1

Create: Once your model is set up, create the following three effects and take photos of the results as viewed from the direction ofthe Earth. Upload your photos into the project space along with any other observations you find interesting.

1. Phases of the Moon: Slowly rotate the wire so that the Moon orbits around the Earth. By viewing the Moon from the oppositeside of the Earth as it revolves, you can observe the phases of the Moon. To capture the full moon phase without a lunareclipse you might need to temporarily raise either the Moon or the Sun.

2. Lunar eclipse: Shift the Moon into full moon position and see what sort of lunar eclipse results. If the eclipse is only partial, seewhat you need to do to your model to create a total eclipse.

3. Solar eclipse: Shift the Moon into new moon position and create a solar eclipse. Taking a photo from the direction of the Earthmight be difficult but you can take a photo of the shadow cast on the Earth by the Moon.

You can also upload your best photos into the discussion board at the bottom of the page and start a conversation with yourclassmates. 

Hint: You might need to ask your teacher to open the discussion board first.

MethodTo create the model:

1. Using the scissors, make at least eight vertical cuts around each end of the cardboard tube. Each cut should be about 2 cmlong.

Question 2

Compare: The model you've created is not a scale model – theballs representing the Earth and Moon are much too bigcompared to the size of the torch or "Sun" and the distancesbetween the balls and the torch are much too small.

Compare the size of the real Moon's shadow on the Earthduring a total solar eclipse with the size of the shadow createdin your model. How could your model be modified so that itrepresents the real situation better?

Satellite image of the shadow cast by the Moon on theSahara Desert during the total solar eclipse of 29 March,

2006.

Discussion

Career: Eclipses

Alcoholic space lasers, cosmic water fountains, and the world’s largest radio telescope: it all sounds like something out of anew sci-fi film. But for Lisa Harvey-Smith, it’s just part of her job as an astronomer.

Lisa’s love for astronomy began when she was a little girl in asmall village in southeast England. After reading an article aboutthe planet Mars in the local newspaper she decided to tryidentifying it from her bedroom window. The first thing she everlearnt about astronomy was that you can’t see the stars fromindoors with the lights on!

She’s come a long way since then. Lisa is now head of theAustralian Square Kilometer Array Pathfinder telescope project– a project that will lead to what will be the world’s largest radiotelescope. She works at CSIRO’s observatory in Sydney whereshe tries to figure out how the universe works – she’sparticularly interested in studying the birth and death of stars.But Lisa has also studied other fascinating features of thecosmos, like bright laser-like signals from alcohol-containingclouds and fountains of water that shoot out of stars at over 400km per second.

When she isn’t busy with her own research Lisa spends her timesharing her love for astronomy with other people. She regularlyappears on television to talk about the latest developments. Lisathinks astronomy is important because it appeals to ourfundamental need to understand our place in the universe. Andadvances in astronomy have inspired new technologies inmedical imaging, wireless communications and materialsscience – so it’s a win-win situation, she says.

As a long-distance runner Lisa gets to some interesting parts ofthe planet – when she isn't investigating interesting parts of thesky. She's taken part in races in South Africa and Australia’sSimpson Desert, and is currently training for a 435 kmmarathon.

Question 1

Investigate: Lisa Harvey-Smith points out that scientific discoveries often inspire new technologies in apparently unrelated areas.Do a quick internet search for "astronomical medicine" and then write a short summary of the surprising connection betweenastronomy and medical imaging.

Cosmos Lessons team

Lesson author: James DriscollIntroduction author: Yi-Di NgProfile author: Yi-Di NgEditors: Jim Rountree and Campbell EdgarArt director: Wendy JohnsEducation director: Daniel Pikler

Image credits: iStock, Alberto Levy/Sky & Telescope,NASA, Astronomy Magazine, Maximilian ReuterVideo credits: BBC, Mr. Rasmussen's YouTube Page, Zoomin.TVScience, MonkeySee, Physics Girl, YouTube