Study guide

1. The Earth:

• Rotation and Orbit: "The Earth is a planet that rotates on its axis... This rotation is responsible for the cycle of day and night." The Earth completes one rotation in approximately 24 hours. The Earth also orbits the Sun in approximately 365 days (one year).

• Seasons: "The Earth's orbit, combined with the tilt of its axis, is responsible for the changing seasons." The axial tilt (23.5 degrees) causes different hemispheres to receive varying amounts of sunlight throughout the year.

• Moon's Orbit & Phases: "The Moon orbits the Earth once every approximately 29.5 days, which is roughly one month. This movement of the Moon around the Earth explains the phases of the Moon." The phases are determined by the relative positions of the Earth, Moon, and Sun.

2. The Sun:

• Composition & Energy: "The Sun consists mostly of hydrogen (about 75%) and helium (about 24%)." The Sun radiates energy across the electromagnetic spectrum, primarily in the form of infrared radiation (heat), visible light, and ultraviolet radiation.

• The Sun as a Star: "The Sun is a star of medium size compared to other stars in the universe." It's a main sequence star, indicating it is in a stable phase of its lifecycle.

3. The Solar System:

• Structure: The Solar System consists of the Sun, eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune), minor/dwarf planets (e.g., Pluto), asteroids, moons, comets, and other smaller bodies.

• Planetary Arrangement: Planets are arranged in order of increasing distance from the Sun. The inner planets (Mercury to Mars) are "terrestrial planets," while the outer planets (Jupiter to Neptune) are "gas giants or ice giants." The Asteroid belt sits between Mars and Jupiter

• Formation (Accretion Model): The Solar System formed approximately 4.6 billion years ago from a cloud of gas and dust. "The force of gravity caused the interstellar cloud to collapse into a rotating disc." Lighter gases accumulated in the outer regions, forming gas giants, while heavier elements formed rocky terrestrial planets closer to the Sun.

• Gravity: "The Sun contains most of the mass of the Solar System—over 99%." This gives it an immense gravitational pull. The planets are kept in orbit by the balance between the Sun's gravitational pull and their orbital speed. The gravitational field strength of a planet depends on its mass. Gravity decreases with distance.

• Distances: Light travels at 300,000 km/s. Light from the Sun takes approximately 8 minutes to reach the Earth.

4. Galaxies and the Universe:

• Galaxies: "Galaxies are massive systems that contain billions of stars along with gas, dust, and dark matter, all held together by gravity." The Milky Way is our home galaxy.

• Milky Way: The Milky Way is a spiral galaxy that contains our Solar System and hundreds of billions of stars. "The diameter of the Milky Way is about 100,000 light-years."

• Measuring Astronomical Distances: Astronomical distances are measured in light-years. "A light-year is the distance that light travels in a vacuum in one year." Since light travels at about 300,000 km/s, a light-year represents an enormous distance.

• Redshift: "Redshift is the increase in the wavelength of light from a star or galaxy, making it appear shifted toward the red end of the electromagnetic spectrum." It occurs when an object is moving away from the observer due to the Doppler effect. "The greater the redshift, the faster a galaxy is moving away."

• Expanding Universe: "The redshift of distant galaxies shows that space itself is expanding, carrying galaxies away from each other." This supports Hubble’s Law, which states that the speed at which a galaxy moves away is proportional to its distance from Earth.

• Big Bang Theory: "If the Universe is expanding, it must have been smaller in the past. This suggests the Universe originated from a single, extremely dense and hot point about 13.8 billion years ago—this is the Big Bang Theory." The discovery of cosmic microwave background radiation (CMBR) further supports this theory.

• Accretion: The process by which particles in a cloud of gas and dust clump together to form larger bodies, such as planets.

• Asteroid Belt: A region in the Solar System between the orbits of Mars and Jupiter, where many asteroids are found.

• Big Bang Theory: The prevailing cosmological model for the universe, which suggests that it originated from a single, extremely dense and hot point about 13.8 billion years ago.

• Cosmic Microwave Background Radiation (CMBR): The leftover heat from the Big Bang, which provides evidence supporting the theory.

• Doppler Effect: The change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source.

• Galaxy: A massive system containing billions of stars, gas, dust, and dark matter, all held together by gravity.

• Gas Giant: A large planet composed mainly of gases, such as hydrogen and helium (e.g., Jupiter, Saturn, Uranus, and Neptune).

• Gravitational Field: The region around a massive object where other objects experience a force of attraction due to gravity.

• Light-Year: The distance that light travels in a vacuum in one year, approximately 9.461 x 10^12 kilometres.

• Milky Way: The galaxy that contains our Solar System.

• Orbit: The curved path of an object around a star, planet, or moon, due to gravity.

• Redshift: The increase in the wavelength of light from a star or galaxy, making it appear shifted towards the red end of the electromagnetic spectrum. It indicates that the object is moving away from the observer.

• Solar System: The system of planets, moons, asteroids, comets, and other celestial bodies held together by the gravitational pull of the Sun.

• Sun: The star at the centre of our Solar System, providing light and heat to Earth and other planets.

• Terrestrial Planet: A planet composed mainly of silicate rocks or metals (e.g., Mercury, Venus, Earth, and Mars).

• Universe: All of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy.