Say this about the Oculus: It is breathtaking from the inside — luminous, intricate, uplifting and tranquil. Photos of it resemble idealized architectural renderings.

There is nothing like it back home, unless home is Milwaukee or Liège, Belgium, where some of Mr. Calatrava’s masterworks have been constructed.

Elsewhere at the trade center, a family of four (two parents with a 10- and a 6-year-old) can expect to spend $179 just to get into the National September 11 Memorial Museum and the One World Observatory.

The Oculus, however, will be free. It will be free because it is a public space.

It is bound to be one of the most popular destinations in Lower Manhattan. Its balconies, cantilevered like diving boards at either end of the elliptical hall, will become Selfie Central. A comparison with the balconies at Grand Central Terminal is almost inevitable. Not surprisingly, Mr. Calatrava cites the terminal as inspiration.