- A handful of earth.
- The earth under this house.
- The earth beneath my feet.
- The heavens above and the earth below.
- The earth moved.
In the examples above, earth should be lowercased. In all the examples cited, earth is synonymous with the following common nouns: soil, land, dirt, terrain, ground, and world. In addition, the expression in number 6. is idiomatic but it doesn't normally refer to earthquakes, it means that something, often a sexual experience, was highly pleasurable.
- A handful of soil/dirt
- The ground under this house
- The land beneath my feet.
- The heavens above and the world (or ground) below.
- The terrain moved OR The world moved
I feel example 4. could go either way, Googling the phrase does not really help to clarify.
- What on earth?
What on Earth?
On the first page of Google, earth is capitalised 14 times out of 20 but many of the instances cite the name of an American TV series, the titles of podcasts; events and shows; science exhibitions; they also form the name of several websites, e.g. What on Earth is Happening?; a BBC science page, and titles of books.
Cambridge Dictionary says of the phrase
used for showing surprise:
What on earth is going on in there?
Lexico, formerly Oxford Dictionaries, has an entry for on earth, lowercased
Used for emphasis, especially in questions and negative statements.
‘who on earth would venture out in weather like this?’
‘So my question is, how on earth do you keep tabs on all of the available programming?’
‘So why on earth does he want to make yet another death-defying walk in the Grand Canyon?’
Dictionary.com uses lowercase for the following idioms
- move heaven and earth.
on earth, in the world:
Where on earth have you been?
Merriam-Webster includes another idiom, earth is still spelled with a lowercase "e"
(a) heaven on earth
and when earth used to be flat,
from the four corners of the earth
People came from the four corners of the earth to see the sight
The following definition by Macmillan Dictionary
used for emphasizing that someone or something is the best, worst, biggest etc in the world
The Great Wall is the largest man-made structure on earth.
nothing/nowhere etc on earth used for adding emphasis to negative statements
Nothing on earth could get me to speak to her.
But in one dictionary, TFD, the word earth is capitalised
go to the ends of the Earth
To do all that one can in an attempt to achieve something.
I would go to the ends of the Earth to help my children.
The one sentence where I would capitalise earth is the last one
- We returned quickly to Earth.
The MLA Style Center says
We usually lowercase sun, moon, and earth, but, following The Chicago Manual of Style, when the does not precede the name of the planet, when earth is not part of an idiomatic expression, or when other planets are mentioned, we capitalize earth:
The earth revolves around the sun.
The astronauts landed on the moon.
The space shuttle will return to Earth next year.
The four planets closest to the sun—Mercury, Venus, Earth, and Mars—compose the inner solar system.
I'm surprised by their first example, it's clear that the term earth refers to the planet on which we live, and it cannot be replaced by soil, land, ground, dirt etc. Even though it is preceded by the definite article the, it does not dissuade me from wanting to capitalise that all-important "e"
Lexico provides the following example sentences
- ‘The discovery suggests that life could exist on planets very different from Earth.’
- ‘The course of life on planet Earth might even turn out to be described by such a picture.’
- ‘Mercury is also the only planet other than Earth that has a global magnetic field.’
- ‘Imagine a perturbation of the Earth's orbit big enough to change the size of the sun in the sky.’
In none of the above examples could the word earth be substituted with any of counterparts without affecting or changing its meaning substantially.
Interestingly, in the article Scott Kelly’s Year in Space, published in February, 2016. The New York Times journalist cleverly avoids using the article with Earth.
The International Space Station zips around Earth at more than 17,000 miles per hour, or once every 90 minutes.
Of course, on the space station, Mr. Kelly was never more than about 250 miles from Earth.
Water is heavy and expensive to transport from Earth, so for efficiency, water is continually recycled.
Whatever Earthly things Mr. Kelly may have been missing during the mission, the Internet was not one of them.