The word “sun” is not capitalized except when it is used in the astronomical context. Like every proper noun, the name of the sun is written in a capital letter. However, some confusion arises because our solar system sun is referred to as The Sun.
Therefore, the word to denote The Sun can either be a common or proper noun depending on the context.
When Is “Sun” Capitalized?
Take for example this sentence: “are all suns hot similar to The Sun?” The word “suns” in this sentence is a common noun, while “The Sun” (our sun’s name) is the proper noun which is why it starts with a capital letter. According to NASA’s style guidelines, when you refer to the sun at the center of our Solar System, you should capitalize it.
To capitalize or not to capitalize the word “sun” is not a question of grammar, but more of emphasis and usage. If you are advising people to watch an eclipse or warning them not to look at the sun eclipse directly, do not capitalize the word “sun.”
However, if it is a poetic metaphor whereby the Sun stands for something which is adding to the astronomical gadgets, for instance, something powerful which should be treated with extra care against something interesting yet not dangerous, the capitalizing sun is appropriate. But if you see it as the generic astronomical body, you don’t capitalize it. For example, “in an alien sky, we watched the blue sun rising above the horizon.”
Every time the word “sun” is applied apart from when referencing the star that our planet revolves around, capitalization follows the same rule as any other common noun. Don’t capitalize unless grammatical placement demands it. This may be in direct quotation first word, a new sentence first word, or a table cell or list entry. A common noun usage would be “we played in the sun the whole day.”
To sum up, there are not many fast and hard rules when it comes to the word “sun,” but generally, it is capitalized when applied like a proper noun, but it is not capitalized when used as a common noun.