Bernard O’Kane, a professor of Islamic art and architecture at American University in Cairo, has a message and a nudge for travelers standing outside a mosque, wondering whether they should enter: Go inside. An object of architectural beauty and cultural fascination awaits. His new book, “Mosques: The 100 Most Iconic Islamic Houses of Worship,” isn’t a guidebook per se; the coffee table tome weighs (20 pounds) and costs ($895) more than many pieces of furniture. But his selections are inspiring and deserve a time slot in any travel itinerary. To better understand mosque construction and design, as well as visitor etiquette, we reached out to O’Kane at his home in Cairo. The interview has been edited for style and space.
What is a mosque?
A mosque is created for anyone who makes a space set aside to pray. There’s no architectural definition of a mosque, which means there’s an enormous variety of possible spaces.
Is a mosque always related to Islam?
Nobody else specifically uses the word “mosque” as a place of prayer. It comes from the Arabic word meaning a “place of prostration,” where you bow down while praying to God.
Do mosques share similar characteristics?
You can make your own personal mosque by marking off a space to pray, but the mosques people visit communally will have some distinguishing features, such as a mihrab, a niche in the center of the wall facing Mecca. The mihrab is a commemoration of the place where the prophet Muhammad led the first prayer.
Why did you choose these mosques?
I was asked to pick 100, basically what I thought were the most representative and exciting buildings. I also wanted to give some idea of the geographical and chronological range of the buildings.
Did you visit all 100?
I visited 80 to 85 percent of them.
How many continents are represented in the book?
We have examples from North America, Europe, Africa and Asia.
Do the mosques demonstrate regional differences?
There are certainly stylistic differences that are both regional and chronological. There’s a big difference between mosques in the west of Africa, mosques in Egypt, and mosques in Iran and India. The building materials can be very different, the types of decoration can be different, and sometimes the forms differ substantially.
Can you provide some examples?
Let’s pick one in my hometown, Cairo. The Mosque of Ibn Tulun was built in the 9th century but on a plan that resembles those from Samarra, the former capital of the Abbasid Caliphate in Iraq, where the patron Ibn Tulun came from. It’s built on a plan that uses a hypostyle form, meaning having many piers supporting lots of arcades, and has decorative stucco work, which is very similar to what was being produced in Iraq at the same time. That’s totally different from a 15th-century mosque in Xi’an, China, where you find the terra-cotta warriors. It’s built in the form of a Chinese temple but with some Arabic calligraphy that distinguishes it from the other temples.
What is inside mosques, and do the interiors vary?
There aren’t really any surviving old mosques that have the exact furnishings from when they were built. The carpets, if they were old, would have been transferred to museums. In Turkey, for instance, in the middle of the 20th century, they found some extraordinarily old carpets in some of the medieval mosques, but they are now in the carpet and rug museums in Istanbul. Most mosques have pulpits called minbars, which are frequently made of carved wood, tile or stone. Some have wooden stands for Koran readers and smaller stands on which Korans are placed. Of course, the decorative materials can differ substantially. Places with good stone for quarrying used the material for building and decoration. When stone wasn’t available, they used brick. Initially they laid it in different ways to create decorative patterns, but later on, from the 14th century, colored tile revetment became very popular. Iran and northwest India favored tiles.
What do modern mosques look like? Are they sleek and minimalist like a Frank Gehry design?
You have a huge variety in modern mosques. You have some like the Sheikh Zayed mosque in Abu Dhabi, which is an Arabian Nights fantasy and an eclectic building that mixes decoration and forms from all different periods and parts of the Islamic world. And you have some extremely minimalist buildings. There are a couple in Bangladesh that have minimal decoration and just concentrate on basic forms, such as the dome and the lighting.
How many of the 100 mosques are open to the non-Muslim public?
Almost all of them are. Just a few countries like Algeria and Tunisia restrict entrance to Muslims only, but in other parts of the Islamic world, it’s not a problem.
When I was in Morocco, I remember the guide telling us that we could not visit the Hassan II mosque in Casablanca. I thought it was because we were not Muslims. My mother thinks it was because we were short on time. Is this mosque open to everyone?
That one has become a tourist attraction, and they do allow non-Muslims to visit. But it isn’t one of my favorites. It’s based too much on earlier styles, with absolutely nothing new in it. You may have visited the madrassas in Fez, which are theological schools that have the same kind of decorations as the mosques. But for some reason the authorities don’t have any problem with letting tourists in to see those. They don’t really function as madrassas anymore. They’re really just places tourists visit.
How do you know if you are allowed to enter?
Just walk in and take your shoes off. If someone says you can’t come in, you can proceed from there. But assume that you can visit unless there is a notice to the contrary.
What is the proper etiquette of visiting a mosque?
You shouldn’t go during communal Friday prayer at noon. In fact, it’s best to miss communal prayer if an imam is leading prayers inside. Wait the 20 minutes for it to be over. You should dress conservatively. Try not to go in shorts or tank tops. You should maintain decorum inside, so don’t speak or laugh loudly. Other than that, there are very few restrictions.
What about taking photos?
Normally no problem, although it is usually discouraged during prayer time.
Should women cover their heads?
It depends on the country. In many countries it’s not necessary at all.
Which mosques would you recommend visiting?
I lived in Iran for a few years, and some of my favorite mosques are in Isfahan, which is one of the most beautiful cities in the world. But there are also other cities like Istanbul, Cairo and Delhi that have an amazing concentration of buildings from a wide range of periods. This is an excellent way to get the feel of an Islamic city.
Non-Muslims are not permitted to go to Mecca, correct?
That’s correct. While Mecca might be an amazing experience, the Saudis have unfortunately not been keen on preserving any of the old buildings and have knocked most of them down to build new structures.
Any mosques you can recommend in the States?
The Islamic Center in New York is one of the finest buildings in the world. It’s by the American architecture firm Skidmore, Owings & Merrill. It has a beautiful dome chamber with a great sense of space inside.
Any encouraging words for people who might feel intimidated entering a mosque?
There’s no reason to be. The vast majority of Muslims are very welcoming of visitors, even non-Muslims.
Can you suggest a portable book on this subject?
I can recommend one of my own that’s smaller and lighter. It’s called “Treasures of Islam: Artistic Glories of the Muslim World.” It includes art as well as architecture. But a pocketbook for travel? I don’t really think there is one that covers the whole Islamic world.
I think I know what your next project is.
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