Why is ‘Earth’ often spelt with a lowercase e, even when referring to the planet?

Question

The word earth has several meanings; the most central one is ‘soil, dirt’, that thing we walk on when we’re outside. It’s also used as a name for the planet we live on.

The Lexico definition for this sense has:

(also Earth)
The planet on which we live; the world

Note the word also: the entry is lowercase, and this sense, in which it functions as a proper noun, also appears capitalised.

But proper nouns are as a rule always capitalised. Mars, Pluto, Venus, and all the other planet names in our solar system are always capitalised.

The first two of the following examples of lowercase earth are from the King James Bible and show that this isn’t a new thing; it’s been like this for a long time:

Job 26:5–13, Authorized (King James) Version (AKJV)

5. Dead things are formed from under the waters,
   and the inhabitants thereof.
6. Hell is naked before him,
   and destruction hath no covering.
7. He stretcheth out the north over the empty place,
   and hangeth the earth upon nothing.
8. He bindeth up the waters in his thick clouds;
   and the cloud is not rent under them.
9. He holdeth back the face of his throne,
   and spreadeth his cloud upon it.
10. He hath compassed the waters with bounds,
    until the day and night come to an end.
11. The pillars of heaven tremble
    and are astonished at his reproof.
12. He divideth the sea with his power,
    and by his understanding he smiteth through the proud.
13. By his spirit he hath garnished the heavens;
    his hand hath formed the crooked serpent.

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Psalm 24:1–3, Authorized (King James) Version (AKJV)

1. The earth is the Lᴏʀᴅ’s, and the fulness thereof;
   the world, and they that dwell therein.
2. For he hath founded it upon the seas,
   and established it upon the floods.
3. Who shall ascend into the hill of the Lᴏʀᴅ?
   or who shall stand in his holy place?

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William Shakespeare, A Midsummer Night’s Dream, Folio 1, 1623

The Lunaticke, the Louer, and the Poet,
Are of imagination all compact.
One sees more diuels then vaste hell can hold;
That is the mad man. The Louer, all as franticke,
Sees Helens beauty in a brow of Egipt.
The Poets eye in a fine frenzy rolling, doth glance
From heauen to earth, from earth to heauen.

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Chuck Palahniuk, Damned, 2011

What makes earth feel like hell is our expectation that it should feel like heaven.

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But why is the Earth’s name so different from the other planets’ names? Why is this proper noun so inconsistently capitalised in English?

Answer 1

I have this impression that British 17th century authors avoided capitalising Earth because it felt disrespectful to their Lord.

The Old Testament is mostly focused on the character of God, there are many stories that illustrate how incredibly powerful, all-knowing, vindictive and almighty God is, so these authors must have asked themselves: “How do we show mankind the power, the awesomeness of God, our Lord and Saviour?” and “How do we show our love, our fear and our worship of Him?” And someone replied: “I know, let's not just limit to capitalising the first letter of “Lord”, let's place the entire word LORD in all capital letters and anything else that is not a name of a person, a country, town, village etc. we'll just leave in lowercase.”

Which meant the first letter of places that had proper names such as heaven, hell and earth were in lowercase because they had to pale into insignificance compared to the name of LORD; however, the authors didn't stop there, they capitalized the names that God and later, Adam, gave to things: Day, Night, Heaven, Earth, Seas, Woman, and Man, thereby creating this wonderful contrast, this sense of wide-eyed awe mixed with submission and veneration.

Genesis 1

¹ In the beginning God created the heaven and the earth. ² And the earth was without form, and void; and darkness was upon the face of the deep. And the Spirit of God moved upon the face of the waters. ³And God said, Let there be light: and there was light. ⁴ And God saw the light, that it was good: and God divided the light from the darkness. ⁵ And God called the light Day, and the darkness he called Night. And the evening and the morning were the first day. […] ⁸ And God called the firmament Heaven.[…] ¹⁰ And God called the dry land Earth; and the gathering together of the waters called he Seas:

Genesis 2

⁸ And the LORD God planted a garden eastward in Eden; and there he put the man whom he had formed. ⁹ And out of the ground made the LORD God to grow every tree that is pleasant to the sight, and good for food; the tree of life also in the midst of the garden, and the tree of knowledge of good and evil. ¹⁰ And a river went out of Eden to water the garden; and from thence it was parted, and became into four heads. ¹¹ The name of the first is Pison: that is it which compasseth the whole land of Havilah, where there is gold; ¹² and the gold of that land is good: […]
²¹ And the LORD God caused a deep sleep to fall upon Adam, and he slept: and he took one of his ribs, and closed up the flesh instead thereof; ²² And the rib, which the LORD God had taken from man, made he a woman, and brought her unto the man. ²³ And Adam said, This is now bone of my bones, and flesh of my flesh: she shall be called Woman, because she was taken out of Man.

Do I have anything else that supports my initial impression? Well, yes I do. Before risking my hard-earned Internet points on the line, I searched in Wikipedia and found the following article

Reverential capitalization

Reverential capitalization is the practice of capitalizing words, particularly pronouns, that refer to a deity or divine being, in cases where the words would not otherwise have been capitalized:

    and God calleth to the light 'Day,' and to the darkness He hath called 'Night;' and there is an evening, and there is a morning — day one.
— Genesis 1:5, Young's Literal Translation (1862)

In this example, "God" is in capitals because it is, like "Day" or "Night", a noun which is here a proper name, whereas "He" is an example of reverential capitalization, since while proper names are capitalized universally, reverence for any particular divinity—belief therein implied on the part of the author who capitalizes pronouns in reference to such being—is not universal. In short, when pronouns are capitalized which usually are lowercase, this usually implies that the writer personally reveres and regards as a deity the antecedent of that pronoun.

Nouns, which are not proper names, can also be capitalized out of reverence of the entity they refer to. Such examples include "the Lord", "the Father", "the Creator".

As to when the word earth should be capitalized, the following guideline says

4. Proper nouns
   • Names of celestial bodies: Mars, Saturn, the Milky Way. Do not, however, capitalize earth, moon, sun, except when those names appear in a context in which other (capitalized) celestial bodies are mentioned. "I like it here on earth," but "It is further from Earth to Mars than it is from Mercury to the Sun.
   • Names of courses: Economics, Biology 101. (However, we would write: "I'm taking courses in biology and earth science this summer.")

But according to the MLA Style Center

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.

Answer 2

It seems to depend on whether it's used as a proper noun or not. From Dictionary.com's page When To Capitalize “Earth”:

Following this rule, when Earth is discussed as a specific planet or celestial body, it is capitalized: It takes six to eight months to travel from Earth to Mars. When Earth is a proper noun, the is usually omitted.

When you are talking about the ground or soil as a surface or stratum, then you must lowercase the word: The archaeologists excavated the earth at the site. It is acceptable to leave earth lowercase and use the with earth if you are talking about it as the planet we live on: The earth rotates on its axis.

Source: https://www.dictionary.com/e/earth/

Answer 3

According to etymonline.com, 'earth' has this etymology:

Old English eorþe "ground, soil, dirt, dry land; country, district," also used (along with middangeard) for "the (material) world, the abode of man" (as opposed to the heavens or the underworld), from Proto-Germanic *ertho (source also of Old Frisian erthe "earth," Old Saxon ertha, Old Norse jörð, Middle Dutch eerde, Dutch aarde, Old High German erda, German Erde, Gothic airþa), perhaps from an extended form of PIE root *er- (2) "earth, ground." The earth considered as a planet was so called from c. 1400. Use in old chemistry is from 1728. Earth-mover "large digging machine" is from 1940.

The word 'earth' was used only as a common noun until around 1400, when it began to be also used as the planet we live on. This use of the word as a planet I think is a metonym, because the planet we live on is mostly the ground, soil, dirt, etc that we live on.

Therefore, 'earth' as a planet is not a prototypical proper noun but is only derived from the prototypical common noun 'earth', unlike other planet names such as Mars, Pluto, Venus, etc, which are all prototypical proper nouns. So it's only natural that capitalization is not consistent for 'earth'.

Moreover, the original meaning of 'earth' as ground, soil, dirt, dry land, etc. is still alive and kicking. So you should normally use 'the' to indicate that 'earth' refers to the planet we live on (because in spoken English you can't figure out the capitalization) unless there's no such confusion in context, in which case you can do without 'the'.

For example, in the dictionary you cited, only these three sentences do without 'the', because it's clear that 'earth' refers to the planet:

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.

Note also that you could add 'the' in the first and third sentences, but never in the second sentence where you use the expression planet Earth.