The Earth and the Solar System

Our planet Earth is in constant motion. It spins on its axis, an imaginary line running through the North and South poles. This rotation takes approximately 24 hours and is what causes day and night. As the Earth spins, different parts of its surface are illuminated by the Sun (daytime) while others face away into darkness (night-time). Simultaneously, the Earth revolves, or orbits, around the Sun. This journey takes about 365.25 days, which we define as one year. The Earth's axis is not straight up and down; it is tilted by about 23.5 degrees. This axial tilt is the reason we have seasons. When a hemisphere is tilted towards the Sun, it receives more direct sunlight for longer periods, resulting in summer. When it's tilted away, it receives less direct sunlight, causing winter. Spring and autumn occur in between. The Moon is Earth's natural satellite, orbiting our planet approximately every 27-28 days. The Moon does not produce its own light; we see it because it reflects sunlight. As the Moon orbits the Earth, the amount of the sunlit side we can see from Earth changes. This causes the different phases of the Moon, from New Moon (invisible) to Full Moon (fully illuminated) and back again.

The Solar System consists of our star, the Sun, and everything gravitationally bound to it. This includes eight planets, their moons, dwarf planets, asteroids, and comets. The Sun is at the centre and contains over 99.8% of the total mass of the Solar System, which is why its immense gravity holds everything in orbit. The planets orbit the Sun in the same direction and on roughly the same plane. The order of the planets from the Sun is: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. A useful mnemonic is 'My Very Educated Mother Just Served Us Noodles'. The planets are split into two groups. The four inner planets (Mercury, Venus, Earth, Mars) are known as terrestrial planets; they are relatively small, rocky, and dense. The four outer planets (Jupiter, Saturn, Uranus, Neptune) are known as gas giants; they are much larger, have low densities, and are composed mainly of hydrogen, helium, and other gases with no solid surface. Between Mars and Jupiter lies the asteroid belt, a region containing millions of rocky objects called asteroids. Comets are icy bodies that have highly elliptical orbits, developing a visible 'tail' as they get closer to the Sun. Dwarf planets, like Pluto, are celestial bodies that orbit the Sun and are nearly round, but have not cleared their orbital path of other debris.

An orbit is the curved path an object takes around a more massive object due to gravity. The Sun's immense gravitational force pulls on all the planets, constantly changing their direction to keep them moving in a near-circular path. This gravitational pull provides the necessary centripetal force. An orbit is a stable balance: the planet has a forward velocity (its tendency to travel in a straight line), but gravity continuously pulls it inwards. If the planet's speed were too low, it would fall into the Sun; if it were too high, it would escape the Sun's gravity and fly off into space. The strength of gravity depends on mass and distance. A more massive central object exerts a stronger pull. The force of gravity also gets weaker as the distance from the object increases. This means planets further from the Sun experience a weaker gravitational pull. Consequently, they orbit at a slower speed and take a much longer time to complete one orbit. The time taken for one full orbit is called the orbital period. We can calculate the average orbital speed(v) if we know the orbital radius(r) and the orbital period (T) using the formula for the speed of an object in a circle: v = (2 × π ×r) / T.

Our Sun is a star - a massive ball of hot gas that produces light and heat through nuclear fusion in its core. It is just one of billions of stars in our galaxy, the Milky Way. A galaxy is a vast collection of stars, gas, dust, and dark matter held together by gravity. When you look up at the night sky and see a faint, hazy band of light, you are looking at the combined light from distant stars in the plane of the Milky Way. The Universe is everything that exists: all of space, time, matter, and energy. It contains billions of galaxies. The distances in space are so enormous that using kilometres or miles is 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 (travelling at 300,000 km per second), this distance is huge: about 9.46 trillion kilometres. The scale of the Universe is mind-boggling. The distance to the nearest star other than our Sun is over 4 light-years. The distance to the next nearest major galaxy (Andromeda) is about 2.5 million light-years. This shows that the average distance between galaxies is vastly greater than the average distance between stars within a galaxy.

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, and small point approximately 13.8 billion years ago and has been expanding and cooling ever since. It is important to understand this was not an explosion in space, but rather the expansion of space itself. There are two key pieces of evidence for the Big Bang theory. The first is the redshift of distant galaxies. When we observe light from galaxies far away, we find that the light is shifted towards the red end of the spectrum. This is called redshift. It is an example of the 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 is moving away faster. This observation implies that the entire Universe is expanding. The second piece of evidence is Cosmic Microwave Background (CMB) radiation. This is faint microwave radiation that is detected coming from all directions in space, with a very uniform temperature of about 2.7 Kelvin (-270 °C). Scientists interpret the CMB as the leftover heat, or 'afterglow', from the Big Bang, which has cooled down as the Universe expanded.