The Earth and the Solar System

The Earth is a planet that orbits a star, the Sun. One full orbit takes approximately 365.25 days, which we define as a year. The Earth also spins on its own axis, an imaginary line running through the North and South poles. One full rotation takes 24 hours, causing day and night. As the Earth rotates, different parts of its surface face the Sun (daytime) or away from it (night-time). The Earth's axis is tilted at an angle of 23.5 degrees relative to its orbit. This tilt is the reason for the seasons. When the Northern Hemisphere is tilted towards the Sun, it receives more direct sunlight for longer periods, resulting in summer. Six months later, it is tilted away from the Sun, causing winter. The Moon is a natural satellite that orbits the Earth, taking about 27 days. We see the Moon because it reflects sunlight. The 'phases of the Moon' are the different shapes we see, which depend on how much of the Moon's sunlit side is visible from Earth as it orbits us.

The Solar System consists of the Sun and all the objects that orbit it due to its gravity. The Sun itself contains over 99.8% of the total mass of the Solar System. There are eight planets, which can be divided into two groups. The four inner planets are Mercury, Venus, Earth, and Mars. They are known as terrestrial planets because they are relatively small, dense, and have rocky surfaces. The four outer planets are Jupiter, Saturn, Uranus, and Neptune. They are known as gas giants because they are very large, have low densities, and are composed mainly of gas (like hydrogen and helium) with no solid surface. Between Mars and Jupiter lies the asteroid belt, a region containing millions of rocky objects called asteroids. Other objects in the Solar System include dwarf planets (like Pluto), which are spherical bodies that orbit the Sun but haven't cleared their orbital path, and comets, which are lumps of ice, dust, and rock with highly elliptical orbits.

An orbit is the curved path of a celestial object around a star, planet, or moon. Orbits are caused by gravitational force. The Sun's immense gravitational pull is what keeps the planets in their orbits. This force acts as a centripetal force, constantly pulling the planets towards the centre (the Sun) and changing their direction of motion. Without this force, a planet would travel in a straight line off into space. The strength of gravity depends on the mass of the objects and the distance between them. It gets weaker as the distance increases. This is why planets further from the Sun orbit more slowly and have longer years. For an object to maintain a stable circular orbit, its speed must be just right. The speed(v) for a circular orbit can be calculated if you know the orbital radius(r) and the time it takes for one orbit, known as the orbital period (T).

Our Sun is a star. A star is a massive, luminous sphere of plasma held together by its own gravity, which generates energy through nuclear fusion in its core. The Sun is just one of billions of stars in our galaxy, the Milky Way. A galaxy is a huge collection of stars, gas, and dust held together by gravity. The Universe is everything that exists, and it is made up of billions of galaxies. The distances between stars and galaxies are so vast that metres or kilometres are impractical. Instead, astronomers use the light-year. A light-year is the distance light travels in one year. Since light is the fastest thing in the universe, this distance is enormous (about 9.46 trillion km). The distances between galaxies are typically millions of times greater than the distances between stars within a galaxy.

The Solar System formed about 4.6 billion years ago from a nebula - a vast, cold cloud of gas and dust. A nearby event, like a supernova explosion, may have sent a shockwave through the cloud, causing it to start collapsing under its own gravity. As the cloud collapsed, it began to spin faster and flatten into a rotating disk, known as an accretion disc. Most of the mass was pulled to the centre, becoming hotter and denser, forming a protostar. When the core of the protostar became hot and dense enough, nuclear fusion began, and it became a true star: our Sun. In the surrounding accretion disc, tiny particles of dust and ice began to stick together. Through a process called accretion, these clumps grew into larger bodies called planetesimals. Over millions of years, these planetesimals collided and merged to form the planets, moons, asteroids, and comets we see today.

The Big Bang theory is the leading scientific model for how the Universe began. It states that the Universe started from an extremely hot, dense single point around 13.8 billion years ago and has been expanding and cooling ever since. There are two key pieces of evidence for this. The first is the redshift of distant galaxies. When we observe light from distant galaxies, the wavelengths are longer than they should be, meaning the light is shifted towards the red end of the spectrum. This is a Doppler effect for light, and it tells us that these galaxies are moving away from us. Crucially, the further away a galaxy is, the greater its redshift, meaning it's moving away faster. This observation of universal expansion implies everything started from a single point. The second piece of evidence is the Cosmic Microwave Background (CMB) radiation. This is faint microwave radiation detected from all directions in space. Scientists interpret it as the 'afterglow' or leftover heat from the Big Bang, which has cooled down over billions of years as the Universe expanded.