After he gained control of the roman empire, constantine moved the capital to
byzantium and renamed it constantinople.
rome and renamed it constantinople.
created a second capital in byzantium.
created a second capital in constantinople.

When Constantine gained control of the Roman Empire, he moved the capital to Byzantium and renamed it Constantinople promptly. This was the new capital of the Eastern Roman Empire and is today known as Istanbul, the biggest city of Turkey. 

You should question a source’s credibility prior to using it.

One should question: if the source is current (what is the date of the research), who is the author of the source (what authority/knowledge/experience do they have), what depth does the source reach, who is the audience, what is the purpose of the source, etc.