It's kind of like a collocation, some sentences don't work if you use the article. If in doubt, leave out the article. Earth is a proper noun, which should be capitalized.
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Thinking SpainAnother question, should it be capitalised or not?It depends. The planet is capitalized: Venus and Earth are rocky planets. Other than that, do not capitalize, even when you think you have to: He moved heaven and earth to find her. From angels bending near the earth, to touch their harps of gold. The greatest city on earth.
Thinking SpainHi ennon,Thank you very much for your commentaries. One more question please:If I have a picture of our planet, I have to say, 'This is the earth', with the 'definite article', haven't I?TSI used to have a NASA photo of the planet on my wall. I would write that I had a picture of planet Earth. "This is Earth" works, but if you use "the", you hav