Sentences start with a capital letter.
When the word is a name, place, title or initial, it is capitalised. [capitalised is English spelling and capitalized is American spelling]
Not doing so can change the meaning
the poet wishes to communicate.
Some examples are-
earth is dirt/ Earth is a planet
moon is vacant expression/ Moon is a planet
heaven is state of being, ideal/ Heaven is God's abode
hell is a bad situation/ Hell is place of damnation
Place names are capitalised/ eg Australia, India, Africa,
as are geographical features like rivers. Murray, Ganges, Nile,
cities Sydney, Calcutta, Lagos.
regions Riverina, Riviera.
Public holidays/ Easter, Christmas.
People's names/ Jane, Robert.
Titles/ Mr, Mrs, Sir, Aunt Jane or Uncle Bob.
Official titles/ Sergeant Bob, Colonel Jane, Senator Hicks, King Id, Queen Erg, President Ego.
Initials and abbreviations/ D. H. Lawrence, F.B.I., B.B.C.
Titles of literature/ War and Peace, not War and peace.
Institutions/ School of Arts, Yale University, Institute of Poetry.
Awards/ Nobel Peace Prize, Pulitzer, Academy Awards.
Street names/ Beasley St, Abbey Road, Clover Lane.
Movements/ Cubism, Tea Party, Marxism.
Historical times/ Middle Ages, Victorian, Renaissance.
When referring to where something originates/ English or American spelling often depends on whether you had an Australian, Asian or African education
Seasons of year- bit of controversy about correct way, but spring is curved metal or action of jumping/ Spring is the season. In this context it operates as the name of a season. Likewise poets often personify the season, eg Spring spread her arms of fresh shoots, which once again has Spring as a name needing a capital. However spring shoots, in the spring can operate without capitals. Use of a capital to describe spring will avoid confusion in for example 'the spring of hares in spring..
What other examples can you think of?
Adding this thought- someone, some time, decided that each line of a poem should start with a capital letter. I personally find it makes poems hard to read as my brain is trained to see a capital letter as the start of a sentence. Means any poetic thought that goes more than a line is cut in half.
Starting each line of a poem with a capital letter is a trend; not a rule of writing?