7.2 The Moon reflects the Sun’s light

7.2

The Moon reflects the Sun’s light

Learning intentions

By the end of this topic, you will be able to: • identify and describe the phases of the

Moon and lunar eclipse • contrast a solar and a lunar eclipse.

Interactive 7.2 The path of the Sun

Key ideas

• The Moon rotates as it orbits the Earth. • The same face of the Moon always faces towards the Earth. • The phases of the Moon are caused by the light from the Sun shining on different parts of the Moon.

phases of the Moon changes in the shape of the Moon as seen from Earth

Full moon

Many scientists believe that a giant collision ‘seas’) because they look like dark oceans. We between two planetary bodies resulted in now know these plains to be solidified lava. the formation of Earth and the Moon. Early In 2020, NASA’s special SOFIA telescope astronauts collected samples from the surface mounted in an aeroplane (the Stratospheric of the Moon and compared them with the Observatory for Infrared Astronomy) identified surface of Earth: they are almost identical. small molecules of water on the surface of the The first scientific description of the Moon Moon for the first time. The amount of water was made in 1609 by Italian astronomer and found was very small (100 times less than the physicist Galileo Galilei (1564–1642), based Sahara Desert). This will not be enough to on his observations through a telescope. At the supply all the needs of the NASA astronauts time it was believed that the Moon had a smooth that will be landing on the Moon by 2024. surface, which explained its ability to reflect light from the Sun. Galileo knew differently. He saw Moonlight the rough, mountainous terrain and vast craters that we know cover the surface of the Moon. Unlike the Sun, the Moon does not create He even described large flat plains that we call its own light. Instead, it reflects sunlight. ‘maria’ (pronounced ‘MAHR-ee-ah’; Latin for The amount of light reflected varies with the different phases of the Moon, but even the full Moon only provides a faint light that appears bluish to the human eye. We always see the same side of the Moon from Earth because the Moon rotates at the same speed as it orbits. This is just like walking around a person, making sure you always face toward them. The Moon takes 27.3 days to completely orbit the Earth. Sometimes, only a part of the Moon is visible. You might see half a Moon, a crescent or a fully round Moon. Sometimes the Moon cannot be seen at all, even though it is in the sky. These changes in the shape of the Moon are called the phases of the Moon (Figure 1). Of course, the Moon does not change shape – it is always round. What changes is the amount of the Moon that is lit by the Sun, which makes it possible for us to see the Moon from Earth. We are really looking at the ‘day’ and ‘night’ parts of the Moon. The Moon rises and sets, just like the Sun. The Moon rises approximately 50 minutes later from one day to the next. The Moon is always in the sky; Figure 1 The phases of the Moon as they appear from Australia however, during the day, the sky is usually so (the southern hemisphere) light that the Moon is hard to see.

First quarter

Waxing half moon gibbous moon Third quarter half moon Waxing crescent moon New moon

Waning gibbous moon

DRAFT

Waning crescent moon

Exploring the Moon

The Moon is the only body in space on which humans have actually stepped. It has a weak gravitational pull and very little atmosphere; therefore, there is not enough oxygen to breathe. Astronauts must wear space suits fitted with breathing apparatus.

Neil Armstrong and Edwin ‘Buzz’ Aldrin were the first humans to walk on the Moon in July 1969 as part of the Apollo 11 mission (Figure 2). They found ‘kangaroo hopping’ easier than walking on the Moon. The astronauts could jump higher and further because the pull of gravity on them was only about one-sixth of Earth’s gravity. Figure 2 The first Moon landing was televised around the world and was front-page news on 21 July 1969.

The surface of the Moon is made of fine grains of dust that stick together like damp sand. The footprints made by the Apollo 11 astronauts should still be visible in a million years because there is no erosion to destroy them. However, the footprints may be covered with dust from meteor impacts. It was possible to beam images of the Moon landing around the world because of the satellite dishes located at Honeysuckle Creek in Canberra and Parkes in New South Wales (Figure 3). Lunar eclipse In eighteenth-century China, a young scientist called Wang Zhenyi developed a way to model a lunar eclipse. This eclipse occurs when the Earth moves between the Moon and the Sun. The Moon passes into the Earth’s shadow and appears dark (Figure 4). Wang modelled this by hanging a lamp from the roof as a Sun, above a circular table that acted like the Earth. She then used a circular mirror that acted like the Moon. By moving the Moon mirror under the ‘Earth’ table, she was able to model the lunar eclipse. 7.2 Check your learningDRAFT

Figure 3 Australian scientists at the Parkes Observatory played a critical role in the Moon landing.

Figure 4 A time-lapse photograph of a lunar eclipse

Retrieve Analyse

1 Identify if these statements are true or false. a The Moon creates light. b The Moon does not supply light to the Earth. c The Moon changes shape during different phases. d The Moon is the closest body in space to the Earth. e Craters are large indentations on the Moon’s surface. f Astronomers are pseudoscientists. g We can see both sides of the Moon from the Earth. 2 Recall why astronauts can jump higher on the Moon than on Earth.

Comprehend

3 Use Figure 1 to describe the waxing and waning of the Moon. 4 Greek philosopher and scientist Aristotle noticed that, during lunar eclipses, the Earth’s shadow was always round. Consider how this led him to suggest the Earth was spherical in shape. Apply

5 Investigate an alternative explanation for the phases of the Moon as told by early First Nations Australians.

Explain how they saw the variation in the appearance of the Moon.

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