What's the difference between 'the Earth' and 'Earth'? Why is there an article before Earth on the first sentence, and none on the second one?
1) Sputnik, Russian for fellow traveller around the Earth marked the beginning of Space Age.
2) It took about 98 minutes for Sputnik to make one complete orbit around Earth.
There's really two questions here: when to use the article "the", and when to capitalize the word ("Earth" vs "earth").
The first example ("around the Earth") technically should be either "around Earth" or "around the earth". When used as a common noun, it's frequently lowercase, and used with the article ("the"). When it's a proper noun -- like a person's name, "Bob", or even "God" vs "the god" -- then it's capitalized ("Earth"), and often without the article ("the"). And sometimes the word "earth" is simply unnecessarily capitalized, and no one notices, because no one agrees on a single standard usage.
In general, when referring to the uniquely named Planet Earth, especially in the context of other (also capitalized) celestial bodies, then we're usually capitalizing, with or without "the".
Unofficially, if the sentence makes sense by substituting "Pluto" for "Earth", then use that as a guide whether to use "the". For example: "It orbited the Pluto" vs "It orbited Pluto" -- the second is correct, so prefer "It orbited Earth". But either option would actually "work" for "Earth".
But, in any case, if you're shoveling dirt, you're definitely shoveling "earth" (common noun); not "Earth", nor "the Earth". If you're using colloquial phrases like "what on earth are you doing?", then it's also lowercase.
As another complicated example: style guides used to capitalize "the Internet" when referring to that thing on which the World Wide Web is built; except when used as a common noun, for the generic "internet" technology. But more recently, it's been demoted to just "the internet". So sometimes, English (or is it "english"?) is simply a work in progress.
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My understanding of it is thus: "The Earth" is the planet - a physical object. "Earth" is a more abstract concept which encompasses the planet, the life on it, its culture etc.
Think of "He's the greatest trumpeter on Earth" - this really just means he's better than all the other trumpeters. Then if you say "He's the greatest trumpeter on the Earth", this conjures up more of a physical image, perhaps of him standing on the Earth, and most people would prefer the former because you're really talking about humans, rather than the physical planet.
As another example, if you were talking about geology, you would tend to say "the Earth" (or perhaps "the earth", I think the rules for capitalisation may be a bit flexible with the definite-article version). Eg "The Earth's core is approximately 5,000 miles wide" (I just made that number up).
Having said that, I think they can be used fairly interchangeably without too much complaint.
Related article: About definite article before "Earth", "Moon" and "Sun"
Earth is an entity.
The Earth is a planet.
Although there is no universally accepted grammar rule regarding the usage of The before Earth but in most scenarios when pointing to Earth as a proper noun we ideally use The before it.
