When to capitalize "Earth"?
I know when referring to earth as "dirt" then it shouldn't be capitalized. Similarly, when Earth is referred to as a "planet" then E should be capitalized.
I am still not sure. Sometimes earth is written with a capital 'E' and sometimes with small 'e' even when it is referred to as a planet. Which is the correct usage?

well ahmed acording to my little knowledge, earth is always written in capital letters,like EARTH to specify it,but you know when we have to mention it in a long sentence then we can write it in small letter for our own convience,bcoz it would look odd to write one capital word in a whole long sentence.

e-g The period of rotation of earth is 23 hours,56min and 4sec.

The shape of the EARTH is oblate sphere.

i hope it`ll clear your confusion.

__________________
GEO to aise k sab tumhara ha,MARO to aisa jaise tmhara kuch bhi nhi.

Maza deti han zindagi ki thokerin unko,jinhen NAAM-E-KHUDA le kar sanbhal janey ki adat ho.

No i'm still not sure... could our marks be deducted on such mistakes?

__________________
A clean dwelling place is the sign of a disturbed mind.

yes ofcourse dear,i think if i am telling you somthing then i am on my experience because i am a geographer dear, ok if u r still not sure then give me your own example which is confusing for you nay one from your book then i hope i can clear your confusion.

__________________
GEO to aise k sab tumhara ha,MARO to aisa jaise tmhara kuch bhi nhi.

Maza deti han zindagi ki thokerin unko,jinhen NAAM-E-KHUDA le kar sanbhal janey ki adat ho.

If you are talking about the Earth as a proper noun, as a planet or celestial body, then you can capitalize Earth and use no article (the): How far is Earth from the Sun?

But it is also fine to leave it as lowercase and use the with earth if you are talking about it as the planet we live on: The earth rotates on its axis. It is ok to do the same with the sun and the moon.

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. The word earth is derived from Greek era. Here is something interesting - we never hear people say the Mercury, the Pluto - but we do say the Earth, the Moon. Using the definite article "the" in front of an uncapitalized "earth" has its roots in the worldview that we are separate from and fundamentally different. (P.S. There is a town called Earth, Texas.)

Dictionary.com

Earth: Generally put in lowercase, but capitalize when used as a proper name.

ex.: In his garden, he enjoyed the feeling of earth between his fingers.

"You Earth creatures make me VERY angry, " the Martian shouted as he pulled out his disintegrator.

Writer's Web: Capitalization