Edmund Halley is the appositive in the sentence.
An appositive is a noun or noun phrase that renames another noun right beside it. The appositive may be a short or long combination of words.
In this case, the apossitive "Edmud Halley" renames "the British astronomer" previously mentioned in the story.
In addition to this, nonessential appositive is always placed in the sentence between commas.
