When does 'the Earth' become 'Earth'?

[H.Ibraheem]

We notice that the word ' Earth ' is sometimes preceded by the definite article 'the' and sometoimes used without ' the ' , is there a rule for this?

[H.Ibraheem]

My lesson tomorrow is Theme 4 -Grammar ( Grade 11 ). In exercise F, table 5, there is the following sentence:
The Sahara is the biggest desert on Earth.
And there are three rules below the table, the last one is:
No article is needed to talk about certain special times or places, such as Friday, Ramadan, home, Earth
Whereas in the Teacher's Book, page 327, we read the following:
We use the definite article the for something which is unique in the world, e.g.:the Sun, the Earth
The problem here is with the word Earth, so what's the rule?

[H.Ibraheem]

Studying a number of example sentences, I noticed the following:

- When the word 'Earth' is the subject, it is always preceded by the.
- When talking about specific information about the planet Earth comparing it with the other planets, it is often preceded by the.
- After prepositions, it can be preceded by the and it is also correct not to use the.
- After preposition on, it is rarely preceded by the.

Can these be rules for using the definite article before the word 'Earth'?

[Terry Phillips]

This is a strange one. I think it is quite unusual for native speakers to use an article with the expression 'on Earth' e.g. 'There has been life on Earth for millions of years.' On the other hand, it is rare to begin a sentence with 'Earth'. e.g. The Earth has existed for millions of years.
But in many other cases, the choice between Earth and the Earth is a personal one. Native speakers might say e.g. 'The first astronauts took pictures of Earth / the Earth from space.'

[PhilB]

As a native speaker, I too have been confused when I've stopped to think about this question. I agree with Terry's observations, but I just want to add that it depends – of course – on the context, as always. If 'Earth' is being thought of as the unique lump of matter on which humans reside – together with a unique sun ("the sun") and moon – then it is "the Earth", or even "the earth" (see below). But when it is seen as one member of a set of planets – Mars, Jupiter, etc – it becomes "Earth". "Earth has existed for millions of years" seems acceptable to me if if the context is cosmological. Otherwise, "The earth has existed ..." would be correct (see below).
The Oxford Writers and Editors Dictionary says that the capital letter should only be used in a planetary context. Hart's Rules (OUP) also says that sun, moon and earth are capitalised only in astronomical contexts. Perhaps space travel and science fiction have increased incidences of this astronomical usage? Wordsworth, of course, wrote: "Earth has not anything to show more fair:". How on earth can we improve on that!

[Terry Phillips]

Excellent observations and great ending. Course, he couldn't use an article, could he, in this case, or it would have messed up the scansion!