So is used as an adverb and doesn't refer to the subject of the sentence. It usually refers back to something mentioned before. It is not a placeholder as it has the following meanings defined in Oxford Online Dictionary.

(1) That is the case: ‘Has somebody called an ambulance?’ ‘I believe so’

(2) The truth: I hear that you’re a writer—is that so?

(3) Similarly; and also: times have changed and so have I

(4) Expressing agreement: ‘There’s another one.’ ‘So there is.’

Example sentences above means:

(1) I believe somebody called an ambulance.

(2) Is that true that you're a writer?

(3) I have also changed as times have changed.

(4) Yes, there is another one.

As you can see, the adverb so functions mainly to avoid repetition of a word or words that were previously mentioned.

Your example sentence will be

The moon rotates itself but it (the moon) rotates itself very slowly.

So is used as an adverb and doesn't refer to the subject of the sentence. It usually refers back to something mentioned before. It is not a placeholder as it has the following meanings defined in Oxford Online Dictionary.

(1) That is the case: ‘Has somebody called an ambulance?’ ‘I believe so’

(2) The truth: I hear that you’re a writer—is that so?

(3) Similarly; and also: times have changed and so have I

(4) Expressing agreement: ‘There’s another one.’ ‘So there is.’

Example sentences above mean:

(1) I believe somebody called an ambulance.

(2) Is that true that you're a writer?

(3) I have also changed as times have changed.

(4) Yes, there is another one.

As you can see, the adverb so functions mainly to avoid repetition of a word or words that were previously mentioned.

Your example sentence will be

The moon rotates itself but it (the moon) rotates itself very slowly.

So is used as an adverb and doesn't refer to the subject of the sentence. It usually refers back to something mentioned before. It is not a placeholder as it has the following meanings defined in Oxford Online Dictionary.

(1) That is the case: ‘Has somebody called an ambulance?’ ‘I believe so’

(2) The truth: I hear that you’re a writer—is that so?

(3) Similarly; and also: times have changed and so have I

(4) Expressing agreement: ‘There’s another one.’ ‘So there is.’

Example sentences above means:

(1) I believe somebody called an ambulance.

(2) Is that true that you're a writer?

(3) I have also changed as times have changed.

(4) Yes, there is another one.

As you can see, the adverb so functions mainly to avoid repetition of a word or words that were previously mentioned.

So is used as an adverb and doesn't refer to the subject of the sentence. It usually refers back to something mentioned before. It is not a placeholder as it has the following meanings defined in Oxford Online Dictionary.

(1) That is the case: ‘Has somebody called an ambulance?’ ‘I believe so’

(2) The truth: I hear that you’re a writer—is that so?

(3) Similarly; and also: times have changed and so have I

(4) Expressing agreement: ‘There’s another one.’ ‘So there is.’

Example sentences above means:

(1) I believe somebody called an ambulance.

(2) Is that true that you're a writer?

(3) I have also changed as times have changed.

(4) Yes, there is another one.

As you can see, the adverb so functions mainly to avoid repetition of a word or words that were previously mentioned.

Your example sentence will be

The moon rotates itself but it (the moon) rotates itself very slowly.