Distances and Space
The distances across the Earth range up to ~1x104 m, which can be easily understood using our own understanding of the world around us. When the distances are to objects across our Solar System, galaxy or the observable Universe, the numbers quickly become too big to understand.
Distance to edge of observable Universe = 9.46x1024 m
In order to make sense of the distances, different distance units must be used:-
1 Astronomical Unit (A.U.) - The distance from the Earth to the Sun - used within the Solar System.
1 Light-year (ly) - The distance Light travels in one year - used within the Milky Way.
1 Parsec (pc) - Distance equal to 3.26 Light-years - Used for distances to other galaxies, usually in the Mpc range.
The Astronomical Unit
The Astronomical Unit is used to measure distances within the Solar System. It is equal to the average distance between the Earth and the Sun:-
1 A.U. = 150x109 m
The Astronomical Unit is only useful within the Solar System. Even the nearest star, Proxima Centauri, is 15x103 A.U. away.
In order to easily describe the distances within the Milky Way, the Light-year must be used. One Light-year is the distance that light travels in one year and can be calculated by multiplying the speed of light by the number of seconds in 1 year:-
1 ly = (3x108 x 365.25 x 24 x 60 x 60) m
1 ly = 9.46x1015 m
Some example distances :-
Proxima Centauri is ~ 3.4 Light-Years away.
The Centre of the Galaxy is ~ 50,000 Light-Years away.
Nearest Galaxy ( Andromeda ) ~ 2.54x106 Light-Years away.
Tour of the Universe
The link below will open a virtual tour of the scale of the Universe, both to the smallest and largest of scales:-
The Earth (Latin : Terra)
Our Planet is (as far as we know) unique in the Universe, as it is the only place to support Life. The Earth is approximately 4.5 billion years old, but has only supported life for approximately 3.5 billion years. In a later Section (Exoplanets) we will look in greater depth at the formation of the Earth.
The Moon (Latin : Luna)
The Moon is the only natural satellite of the Earth, and is one of the largest natural satellites in the Solar System when compared with its parent planet. The Moon is thought to have formed nearly 4.5 billion years ago, when the early Earth collided with another protoplanet called Theia (comparable in size to Mars). This collision caused millions of tonnes of liquid rock to be thrown into space, which coalesced together to form the Moon.
The video below gives a short summary of the formation of the Moon:-
The Earth orbits around a medium-sized yellow star which is commonly called the Sun. This star is approximately 5 billion years old and is about half-way through its life cycle (see Stellar Evolution). The Sun is the largest object in our Solar System (the Earth could fit within it, several thousand times over), but there are much larger stars within our own galaxy. The Image below shows a size comparison between the Sun and one of the largest known stars, VY Canis Majoris:-
The Solar System
The Earth is joined by 7 other Planets, several Dwarf Planets, Hundreds of Moons and Millions of Asteroids and Comets in its orbit around the Sun. As can be seen from the image below, the Solar System is a very busy place:-
Note - Not to scale
The Solar Systems main Planetary Bodies can be split into two main groups :-
1. Small rocky planets (Mercury, Venus, Earth, Mars).
2. Large gas giants (Jupiter, Saturn, Uranus, Neptune).
Note - Pluto is not included in this list because in 2006 the International Astronomical Union downgraded Pluto to the Status of dwarf planet.
In order to be classed as a planet, the object must:-
1. Orbit a central star.
2. Have sufficient Gravity to form a nearly round shape.
3. Clear its orbit of other debris (pluto fails this test).
The video below gives a brief guided tour of the Solar System :-
The Milky Way Galaxy (Latin : Via Lactea)
The Solar System is only a tiny part of the extended area around Earth, as the Sun is only one of millions of Stars that make up our Galaxy.
A galaxy is a large collection of stars, gas and dust, all rotating around a dense core, (normally containing a Supermassive Black Hole). The image above shows our view of the Milky Way galaxy from within it. By studying how the stars appear in the sky, and by observing distant galaxies, scientists have been able to develop a clear understanding of what the Milky Way looks like.
Our home galaxy, the Milky Way is a barred spiral galaxy approximately 100,000 Light-Years across. The below diagram shows the approximate position (bottom of image, about 2/3 away from centre) of our Solar System within the Milky Way:-
Our home galaxy, the Milky Way, is only one of hundreds of millions of galaxies that are visible in the night sky. All matter, all galaxies, dust, gas, stars, planets and so on, combine together to make up the Universe. The Universe is thought to be ~13.8 billion years old, and started from an infinitely dense and hot singularity in The Big Bang.
There are two descriptions regarding the Universe that must be understood:-
The Universe - This is the whole of Space-Time, everything that has or will ever exist. (Infinite in every direction.)
The observable Universe - The section of the Universe that is observable from Earth (a sphere of radius 13.8 billion light-years with Earth at the centre).
Note - The Earth is not at the centre of the Universe, the observable Universe has Earth at the centre as that is where we happen to be observing from, no other special reason.
The video below shows a brief tour of the Universe in 5 minutes:-