Astronomy

Introduction

This topic explores the key concepts of astronomy as they relate to:

• the celestial coordinate system
• the appearance of the sky
• the calendar and time
• the solar system and beyond
• space exploration
• gravity and flight.

Key concepts of astronomy

The activities in this topic are designed to explore the following key concepts:

Earth

• Earth is spherical.
• ‘Down’ refers to the centre of Earth (in relation to gravity).

Day and night

• Light comes from the Sun.
• Day and night are caused by Earth turning on its axis. (Note that ‘day’ can refer to a 24-hour time period or the period of daylight; the reference being used should be made explicit to students.)
• At any one time half of Earth’s shape is in sunlight (day) and half is in darkness (night).

The changing year

• Earth revolves around the Sun every year.
• Earth’s axis is tilted 23.5° from the perpendicular to the plane of the orbit of Earth around the Sun; Earth’s tilt is always in the same direction.
• As Earth revolves around the Sun, its orientation in relation to the Sun changes because of its tilt.
• The seasons are caused by the changing angle of the Sun’s rays on Earth’s surface at different times during the year (due to Earth revolving around the Sun).

Earth, the Moon and the Sun

• Earth, the Moon and the Sun are part of the solar system, with the Sun at the centre.
• Earth orbits the Sun once every year.
• The Moon orbits Earth in one lunar month (about twenty-eight days). The Moon is Earth’s only natural satellite.
• The Moon turns on its axis at a rate that means we always see the same face.
• The Moon orbits Earth at an angle to the plane in which Earth and the Sun are located.

The phases of the Moon and eclipses

• The Moon is visible because it reflects light from the Sun.
• The Sun always illuminates half of the Moon’s sphere.
• The Moon appears to change shape each month (its phases) because we see different amounts of the illuminated surfaces of the Moon at different times each month due to the relationship between the positions of Earth, the Sun and the Moon at a particular time.
• The phases of the Moon occur in a regular pattern.
• Eclipses occur in two ways: when Earth lies between the Sun and the Moon, causing a shadow—full or partial—over the Moon (that is, a full or partial eclipse), or when the Moon lies between Earth and the Sun and casts a shadow—full or partial—over part of Earth (that is, a full or partial solar eclipse). These occur regularly.

The solar system and stars

• Stars emit light. The Sun is a star. The Sun emits light.
• The Sun is the centre of the solar system and is the only body in the solar system that emits light.
• The Sun is the solar system’s main source of energy.
• The planets orbit the Sun. Some planets, other than Earth, have their own moons (natural satellites).
• The planets are great distances from Earth, but relatively much closer than the stars, apart from the Sun.

The universe

• The solar system is only a small component of one particular galaxy, the Milky Way, which is made up of millions of stars.
• Even the nearest stars (apart from the Sun) are gigantic distances away compared to the planets.
• The universe (which is everything that exists) comprises countless galaxies. Our galaxy, the Milky Way, is not the centre of the universe.

Students’ alternative conceptions of astronomy Research into students’ ideas about this topic has identified the following non-scientific conceptions: The seasons

• The seasons are caused by the elliptical orbit of Earth. When Earth is closest to the Sun it is summer.
• Summer occurs when Earth is tilted towards the Sun and is therefore closer to it.

The Moon

• The Moon is not in free fall.
• The Moon gives off its own light.
• Earth blocks the Sun’s light, casting a shadow on the Moon.
• The different shadow effects are due to Earth’s tilt, its rotation or its revolution around the Sun.
• The amount of light reflected off Earth onto the Moon causes the changed shapes.
• The side of the Moon reflecting the Sun’s light affects the shapes.

Gravity

• The force that acts on an apple is not the same as that acting on the Moon.
• The gravitational force is the same on all falling objects.
• There are no gravitational forces in space.
• The gravitational force on a space shuttle is nearly zero.
• The gravitational force acts on one mass at a time.
• The Moon stays in orbit because the gravitational force on it is balanced by the centrifugal force.
• There is no gravity in a vacuum.
• Earth’s spinning motion causes gravity.
• Gravity only acts on things that are falling.
• Free-falling objects can only move downwards.

Comets

• Comets’ tails are created as comets burn up passing through Earth’s atmosphere.
• Comets only appear to have long, fiery tails; this is due to their speed and/or that they are really ball-shaped.
• Comets are made up of gases and/or dust (these gases are stated or implied to be burning).

Space travel

• Spacecraft travel in straight lines from one planet to another.
• Spacecraft can be launched any time to travel from one planet to another.
• Spacecraft are not affected by the Sun.
• Jets can fly in space.
• Weightlessness means there is no gravity.
• Rockets need something (like air) to push against.

Stars and outer space

• Stars reflect light from the Sun.
• Stars are planets.
• Stars seem to have points because they are a long distance from us.
• The composition of stars explains the points;, for example, burning balls of hydrogen whose flames appear as points.
• Space is not something.
• Black holes are big.
• Light always travels in straight lines.
• Things in space make sounds.
• If the Sun were to become a black hole, Earth would get sucked into it.

Tides

• Tides are caused by the Moon orbiting Earth every twenty-four hours.
• There is only one high and one low tide each day.
• The elliptical orbit of the Moon around Earth causes the tides; when the Moon is closer to Earth, it is high tide.
• High tides occur when the Moon is visible (maybe only at night).
• High tide occurs on the opposite side of Earth to the low tide.
• The Sun has no effect on the tides.

The planets

• The morning and evening star is not equated with the planet Venus, but is believed to be a star.
• Planets give out their own light.
• The planets contain dust and rocks and a gaseous atmosphere and water.
• The planets are all similar in structure to Earth.
• Planets’ orbits are circular.
• All the planets revolve about the Sun, taking the same period of time.
• Revolution is the same as rotation.

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