I do not quite get the need for the articles in this sentence. IMHO, the correct translation is Tero moviĝas ĉirkaŭ Suno.
According to the examples provided by PIV-o regarding the sun, considered as an astronomical object, there should not be any article, and it should be written with capitalization: Suno. The same goes for Earth.
Duolingo's tips notes section recommendations (only for Earth) are:
Tero is the word for the planet Earth. We omit the article la when we speak of it in that context:
Tero moviĝas ĉirkaŭ la suno.
(The) Earth moves around the sun.
La Tero is the term for the Earth we live on:
Kie ni vivos, kiam la Tero estos tro varma?
Where will we live, when the Earth is too hot?
Those are (more or less, including capitalization) the same rules that Collins suggests for the English language. I suppose that Duolingo is influenced by English grammar and rules, thus the capital for the Earth, regarded as the home of humanity, and the lower case for the sun. What I find hard to understand is why Duolingo does not follow its own rules regarding the articles.