The definite article is generally used before a noun when it is clear who or what we are talking about. It happens in several cases, mostly because ‘the’ represents a unique thing or person in the context. We can use ‘the’ when there is only one of something in the room, in the building or in the country we are in, and the listener knows which one we mean.
For example, we say ‘Give me the cup, please’ – and there’s only one cup in the room; or we say ‘the president had a speech yesterday’ – we don’t need to say which one, he is the only one in our country.
Sometimes, there is only one of something on Earth, or even in the universe. For example, we use ‘the’ with the word ‘sun’ – ‘Everything rotates the sun in our solar system’ as there’s only one sun close to us (anyway!) and everybody knows which one we mean.
Let’s look at even more examples when it’s the only one, unique thing:
The earth is growing warmer and warmer.
In this case we mean the place we live on, and there is only one planet Earth, so we use a definite article ‘the’.
The Queen is Head of State of the UK and 15 other Commonwealth realms.
There’s only one person who is the queen of the UK, and everybody knows about it, so we use ‘the’. Even if you may not know about that fact, it is still well known by many or most people, so it is true.
The noun has been mentioned before, so it becomes the only one:
I observed a star last night. The star was really beautiful.
Which star was beautiful? The one, I observed last night. So, we are talking about already known thing.
And the third example shows us how a phrase or a clause after the noun makes it clear:
The captain checked the position of the Northern Star to determine the location of his ship.
We can see the person, so we put ‘the’ before ‘captain’, ‘Northern Star’ is a unique object, so, we put ‘the’ before it – ‘the Northern Star’. And there are also two more articles ‘the’ – before ‘position’ and ‘location’. We put them because we know that it is a position of a definite object – the Northern Star, and ‘location’ has phrase ‘of the ship’ after it, so it is not just ‘a location’, but ‘the location’ of a definite object – ship.
I suppose everything is absolutely clear with the second and the third groups, but the first one… what things are so special?
Let’s look at what objects are considered to be unique.
Here is the list of some of them:
the solar system
Other examples are:
the Northern Star
the North Pole
Let’s look at some sentences:
The earth is round.
The moon is already out.
The sun is shining.
The sky is full of stars.
I love to jog most than anything else l in the world.
Don’t worry about the past.
I want to be a pilot in the future.
Our policy is to build for the future not the past.
There is an urgent need for people to help clean up the environment.
But we also should be careful using the definite article with these words, because if their meanings change, the article changes, too.
We should bear in mind that we use ‘the’ with sun, moon and galaxy when we mean our local sun, situated in our solar system, or our moon, as Earth satellite, or our Milky way galaxy, where our solar system is.
But when we refer to any abstract group of stars, we should say ‘a galaxy’;
What is a galaxy?
A galaxy is a system of millions or billions of stars, together with gas and dust, held together by gravitational attraction. (Oxford Dictionary)
When we talk about any other solar system in the universe and its sun, we’ll say ‘a sun’ or we talk about various types of suns in the universe, we’ll also add indefinite article and say ‘a sun’;
What is a sun?
A sun is any star around which a planetary system revolves. (Collins Dictionary)
When we describe, for example, any of numerous Saturn’s satellites, we’ll say ‘a moon’.
What is a moon? It is a natural satellite of any planet. (Oxford Dictionary)
Some words about capitalisation. How should we write the first letters in these words?
We should capitalise Earth, Moon, Sun when we use them in astronomical context, in scientific articles, or as proper names.
I saw the Moon above the horizon.
But: Phobos is one of the two moons of Mars.
And we shouldn’t capitalise words ‘earth’, ‘moon’, ‘sun’ when they are used in non-scientific texts, and we use them just as examples of celestial bodies. When you're referring to the Sun's light or to the Sun’s heat output, you don't need to capitalize the first letter, either.
The Sun sets in the west.
But: Tom could feel the sun on his face.
And also the article before the noun ‘Earth’ is absent from time to time when it is used as the name of our planet, as a proper name, just like Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, other words, as our own names, such as Marcy and Carlton.
Look: "Of the eight planets orbiting our Sun, Earth is the third inner planet"; here ‘Earth’ is the proper name of our planet. So, in contexts where you might use the name of any other planet as a proper name, you would generally not use the article. One more example:
"Aliens visiting Earth may fail to find life intelligent enough to be worth conversing with".
We also omit definite article ‘the’ when we use this word in idioms, such as:
What on earth do you think you’re doing?
What in the world do you think you’re doing?
The word ‘world’ has absolutely the same meaning as ‘earth’ in these phrases but it is always used with definite article – the world. These expressions are used for emphasis, to express astonishment or disbelief, especially in questions and negative statements.
Let’s listen to some more examples to compare:
I am the happiest man on earth. He travelled all over the world.
The word ‘universe’ also always requires the definite article – we will always use ‘the universe’. However, ‘outer space’ or just ‘space’, the synonym of the cosmos, is used without any article.
Edwin Hubble was the first person to use the Hale Telescope. Hubble's work led to new research on the birth of the universe. The universe had not been quiet and unchanging since the beginning of time, as many people had thought.
Sergei Pavlovich Korolev was the lead Soviet rocket engineer and spacecraft designer. He developed the first intercontinental missile and then launched the world's first satellite, Sputnik 1. He also put into space the first dog, the first two-man crew, the first woman, the first three-man crew; directed the first walk in space; created the first Soviet spy satellite and communication satellite.