This is a panorama film, which was a very common camera effect in the early cinema of single shot-scene actualities: the camera pans over a view, which in this case is at the Pan-American Exposition. Nothing special there. The notable trick here is in the edit, or the stopping of the camera and the resuming of the pan, so that half the film is a pan in the daylight and the latter half continues the panorama into a night scene. As an Edison Company catalogue description put it:
"The emotional and sensational effects were also secured by starting the panoramic view by daylight and revolving the camera until the Electric Tower forms the center of the field of the lens. Our camera was then stopped and the position held until night, when we photographed the coming up of the lights, an event which was deemed by all to be a great emotional climax at the Pan-American Exposition."
This effect is well done in that it is as near to seamless as can be expected, with a fade in between the time-lapse edit and excellent work at continuing the pan at the same steadiness in both shots (although, as I said, panoramas were common, so the filmmakers had plenty of practice). In addition to an early instance of time-lapse photography, although not as impressive in that respect as "Demolishing and Building Up the Star Theatre" (also made in 1901), "Pan-American Exposition by Night" is one of the first films where lighting is used as an effect. Historian Charles Musser ("The Emergence of Cinema") called it "a technical tour de force".
(Note: One of the filmmakers of this film was Edwin S. Porter. In early cinema, film-making was generally a collaborative process between two filmmakers—sometimes they exchanged camera duties, and, often, neither one could be distinguished as a film's sole director or producer. There seems to be some uncertainty as to whom the other filmmaker here was; James Blair Smith and James H. White have been suggested.)