Above is a bust of the Emporer Constantine, who ruled the Roman Empire from 306-337 AD. During his reign, the persecution of Christians was stopped by the Edict of Milan in 313. He also convened the Council of Nicea in 325 and Christianity became the established religion of the Empire,declaring that God is the Father and Jesus is his only begotten son.
Constantine also established an Eastern Capitol of the Empire in Byzantium, renaming it Constantinople.
The Hagia Sophia (Holy Wisdom) in Constantinople (now Istanbul) was built as a palace church later by the Emperor Justinian, who ruled from 527-565 AD.
Two of the pendentives are shown at the back on either side of the main arch in the picture above. Here is a simplified version:
It is tempting to go into the history, but I am sticking with the art, which is still pretty complicated. Sophia Hagia became a Muslim Mosque and then a museum, which it is now. Below is a contemporary picture of the Hagia Sophia. During that time, the dome fell in 558, when there was an earthquake, but it was rebuilt in 563.
The interior of Hagia Sophia is decorated with mosaics, including one depicting Mary holding the baby Jesus, with Justinian presenting him with a model of the Hagia Sophia and Constantine presenting him with a model of the city of Constantinople.
Another interesting mosaic in the Hagia Sophia is the Virgin Theotokos, from 867 AD, which is actually a post iconoclast work.
A little history is necessary now. Works depicting religious figures, or icons, were made unlawful by the Iconoclast Council of Hiera in 754, based on biblical teachings that there should be no making and worshiping "graven images" or idols. So, new images were prohibited and some of the existing images were destroyed. This is known as the Iconoclast Era. The second council of Nicaea reinstated the use of icons in 787, stating that holy images should be seen frequently so that those who see them are drawn to remember and pay tribute to the images. The work above was executed in that spirit. This is an opportunity to point out how many of the images of Mary and the baby Jesus depict them seated on a throne, whether in an earthly realm or in heaven. The image above can be considered them in heaven, primarily because of the metallic gold background.
Another important art center was Ravenna, Italy. Ravenna was Western Capitol of the Roman Empire from 402 to 476. There are numerous mosaics there which are very well preserved. Ravenna had been a port city and was important in transportation between Rome and Constantinople. However, the port silted over and so it was no longer usable. In present day jargon--no-one went there anymore, especially during the Iconclastic era and subsequent invasions of the Roman Empire. The result is that the outstanding mosaics are preserved. In Ravenna's Basilica of San Vitale, there is an important mosaic depicting the Emporer Justinian. In it, he is shown with his military officials on one side and the ecclesiastical officials on the other, showing that he had both military and religious authority.
There is also a mosaic of Justinian's wife, Theodora, that shows her with men and women of her court. I have actually seen these mosaics and they are stunning.