Our Sun Odyssey: Light Years Travelled

The Earth moves around the Sun at about 30 kilometres per second, which equates to roughly 62 billion kilometres in 66 years. However, the Earth's orbit around the Sun is not its only movement through space. The Earth is also travelling with the Sun as it orbits the centre of the Milky Way galaxy, at a speed of about 230 kilometres per second. This means that in 66 years, the Earth has travelled about 479 billion kilometres. The Earth is also moving through the universe as part of the Milky Way, which is expanding since the Big Bang.

Earth's movement through space

Earth's Movement Around the Sun

Earth orbits the Sun once every 365.3 days, travelling at an average speed of about 30 kilometres per second. This speed translates to approximately 62 billion kilometres in 66 Earth years. The distance between the Earth and the Sun is approximately 150 million kilometres, or one astronomical unit (AU). This distance varies slightly as Earth's orbit is not perfectly circular, with the closest point to the Sun being the perihelion and the farthest point being the aphelion.

Earth's Movement Through the Milky Way

While the Earth's movement around the Sun is a well-known fact, it is even more intriguing to realise that we are also journeying through the Milky Way galaxy. The Sun, along with the entire solar system, is orbiting the supermassive black hole at the centre of our galaxy, known as Sagittarius A*. This journey through the Milky Way takes our solar system about 230 million years to complete. The speed of this galactic orbit is approximately 448,000 miles per hour (720,000 km/h), which equates to about 230 kilometres per second.

The Expanding Universe

On an even grander scale, the universe itself is expanding. Since the Big Bang approximately 13.8 billion years ago, all galaxies have been moving away from each other. This expansion is not relative to a central point, as the universe has no centre or edge. However, our Milky Way galaxy is being pulled towards the Leo and Virgo constellations by the gravitational influence of a group of galaxies known as the Great Attractor. This movement towards the Great Attractor occurs at a speed of over 600 kilometres per second, nearly three times faster than our orbit around the Milky Way's centre.

The Sun's Migration

Adding another layer of complexity, the Sun's position within the Milky Way has not always been the same. The Sun is currently located around 26,000 light-years from the galactic centre, but it is believed to have formed much closer to the core, possibly around 16,300 light-years away. This outward movement, known as radial migration, occurs along the spiral arms of the galaxy due to the momentum of the spinning limbs. As a result, the Sun's orbital period has increased over time, and it has likely completed more trips around the Milky Way than initially estimated.

The speed of light

All forms of electromagnetic radiation, including visible light, travel at the speed of light. Light and other electromagnetic waves will appear to propagate instantaneously for many practical purposes, but for long distances and very sensitive measurements, their finite speed has noticeable effects. For example, much of the starlight we see on Earth is from the distant past, allowing humans to study the history of the universe.

The speed at which light propagates through transparent materials, such as glass or air, is less than its speed in a vacuum. The speed of light in a vacuum is usually denoted by a lowercase 'c', which stands for "constant" or the Latin "celeritas", meaning swiftness or celerity.

The finite speed of light has practical effects in telecommunications and computing. For example, in computers, the speed of light imposes a limit on how quickly data can be sent between processors.

Light can be slowed down when it passes through an absorbing medium, such as water or glass. Light moves more slowly when travelling through diamond than when moving through air, and it moves through air slightly slower than it can in a vacuum.

While faster-than-light travel is not guaranteed to be impossible, it would require the harnessing of exotic physics. For example, a spaceship could fold a space-time bubble around itself.

Earth's orbit around the sun

The Earth's orbit around the Sun is a well-known concept, but what exactly is it, and how far has the Earth travelled in this orbit?

The Earth orbits the Sun once every 365.3 days, travelling at about 30 kilometres per second. In light-years, the Sun is 0.00001581 light-years away, or 8.20 light minutes away, which equates to 500 light-seconds. In terms of distance, the Sun is one astronomical unit (AU) away from Earth, which is equivalent to 150 million km or 93 million miles.

The distance between the Earth and the Sun varies as the Earth moves in its orbit. At its closest point, the Earth's perihelion, it reaches 147.5 million km or 91.3 million miles. At its farthest point, the aphelion, it is approximately 152 million km or 94.5 million miles, a little over one AU.

Over its 4.6 billion-year lifetime, the Earth has travelled incredibly far around the Sun. To put this into context, in 66 years, an individual on Earth would have travelled about 62 billion kilometres as the Earth orbits the Sun. This is a tiny distance in stellar terms, but it is still a significant distance for us.

The Earth's orbit around the Sun is a stable and predictable path, but it is fascinating to consider the immense distances covered in our journey through space.

The sun's orbit around the Milky Way

The Sun, along with the entire solar system, orbits the centre of the Milky Way galaxy. This means that the Earth is simultaneously making two journeys: one around the Sun, and one around the Milky Way.

The Milky Way is a spiral galaxy, consisting of a central bulge, four major arms, and several shorter arm segments. The Sun is located near the Orion arm, between the two major arms of Perseus and Sagittarius. The diameter of the Milky Way is approximately 100,000 light-years, and the Sun is about 26,000-28,000 light-years from the galactic centre.

The Sun orbits the centre of the Milky Way at an angle of 60 degrees, and bobs up and down above and below the galactic plane. The Sun's orbit around the Milky Way is much less stable than the orbits of the planets around it, making it difficult to calculate exactly how many times it has circled the galaxy. However, given the Sun's current speed of around 448,000-514,000 mph (720,000-828,000 km/h), it takes approximately 230 million years to complete one orbit.

The Sun is estimated to be around 4.6 billion years old, and the Earth was formed about 100 million years after that. This means that if the Sun's orbital path had remained constant, it would have completed about 20 orbits of the Milky Way during its lifetime, with the Earth in tow for around 98% of those orbits. However, the Sun's orbit has not been constant, and it has likely moved significantly since its formation. The Sun was probably born much closer to the galactic centre, around 16,300 light-years away, and has since moved outward through a process known as radial migration. As a result, the Sun's orbital period was initially much shorter, taking around 125 million years to complete one orbit.

While the Sun's orbit is now relatively stable, it is possible that it will continue to migrate outward, though the extent of this movement is unpredictable.

The expansion of the universe

The Earth's orbit around the Sun is not the only movement our planet makes. The Sun itself is moving through space, orbiting the centre of the Milky Way galaxy. The Sun's orbit around the Milky Way is estimated to take around 230 million years, meaning it has travelled around 20 orbits in its lifespan of 4.6 billion years. However, the Sun's orbit is not stable and has likely changed over time, so it is hard to be certain of the exact number of orbits.

Now, to address your question about the expansion of the universe:

The universe is expanding. This means that the distance between gravitationally unbound parts of the observable universe is increasing over time. This expansion is a fundamental aspect of Big Bang cosmology and can be modelled mathematically. The concept of an expanding universe was first proposed in the context of general relativity by Albert Einstein, who initially stated that the universe was static. However, in 1922, Russian scientist and mathematician Alexander Friedmann published a paper detailing multiple possibilities for the history of the universe, including the possibility that it is expanding. This was followed by a paper from Belgian astronomer Georges Lemaître in 1927, who also incorporated Einstein's theory of general relativity and demonstrated that the equations supported the idea of an expanding universe.

The Big Bang

Dark Energy and Cosmic Acceleration

While the expansion of the universe was expected to slow down over time due to the gravitational pull of matter, observations have revealed that this is not the case. Instead, the expansion began to speed up again around 5 or 6 billion years after the Big Bang due to a mysterious force known as dark energy. Dark energy is thought to make up approximately 68.3% to 70% of the universe, and it is causing the universe to expand at an accelerating rate. The nature of dark energy is still not fully understood, but it has been described as having the effect of negative pressure that pushes space outward.

Implications and Ongoing Research

Frequently asked questions