What is a dome?
Imagine a large ball, cut in half. Each half represents a dome.
On the face of it, this is an extremely efficient way of covering a large area. Unlike a conventional roof it doesn't need any central supports under it; just strong walls to sit on around the edges. it is a robuste structure; stress forces are equally distributed across the surface. A dome can carry a load that many a flat or even pitched roof would collapse under.
It gives an architect a versatile canvas to work on; windows can be incorporated to produce unique lighting effects, a building interior can appear to soar towards the heavens (very suitable for a building with religious significance), decorative elements can be designed into it.
Building one however is not so easy! Thanks to their use of concrete the Romans were able to construct the Pantheon which has a span of about 43 metres but it was a huge 1300 years before another dome came anywhere near it in size, the dome of Florence Cathedral, The Cathedral of Santa Maria del Fiore with a slightly smaller span of around 42 metres, was built in brick in 1436. it was yet another 400 years or so before this was surpassed in size; the crown went to the Royal Albert Hall in London, with an eliptical dome of glazed wrought iron.
New materials have brought ever increasing spans and allowed for lower curvature; it could be argued that the roof to the Astrodome sports stadium in Houston, Texas, with a span of nearly 200 metres or the even larger Caesars Superdome in New Orleans are true domes at all. What is very likely, though, is that these buildings will never exceed the lifespan of the Pantheon, which is now approaching it's second millennium and still remains the largest unreinforced concrete dome in the world.
The single unsupported dome is the most striking component of many of the most famous buildings in the world, including the Dome of the Rock in Jerusalem, the Blue Mosque in Istanbul, the Taj Mughal in India and St Basil's Cathedral in Moscow. All of these have presented unique challenges to their builders, but the technology has been well known for a long time; in fact the first known large dome goes right back to more than 3000 years ago!