Aspect ratio refers to the ratio of width to height of an image (or, in the case of a motion picture, each individual frame).

Some common aspect ratios are:

• 1.33:1 (i.e. 4:3) early films and standard-definition TV
• 1.78:1 (i.e. 16:9) high-definition TV
• 1.85:1 standard for theatrical films (sometimes called “Flat”)
• 2.39:1 wide-screen theatrical films (sometimes called Panavision, CinemaScope or “Scope”)

When a movie is shown on a screen with a different aspect ratio, the image can be cropped but, more commonly, black space is added. If the screen is taller than the picture, the black space is on the top and bottom (called “letterboxing”). If the screen is wider than the picture, the black space is on the sides (called “pillarboxing”).

The movie version of Project Hail Mary uses two different aspect ratios. The scenes in Space are shot in 1.43:1 and those on Earth in 2.39:1.

Why the 1.43:1 ratio? This is characteristic of IMAX. So if you’re watching the film on an IMAX screen, the Space scenes will take up the full screen and the Earth scenes will be letterboxed. If you're watching the movie on wide, 2.39:1 screen, the Earth scenes will take up the full screen and the Space scenes will be pillarboxed.

Clearly the optimal viewing experience (especially for the Space scenes) will therefore be on an IMAX screen.

The movie is not shot on film with IMAX cameras, however. It was shot on Arri Alexa 65 large-format digital cameras but is being distributed in a variety of formats including one that can be viewed with IMAX film projectors (which have tall 1.43:1 screens).

You’ll see movies like Project Hail Mary described as “Filmed For IMAX®” in contrast to “Shot in IMAX®”. Only the latter means an IMAX film camera was used. The former means the camera used was certified by IMAX for distribution in an IMAX format.

This IMAX distribution format is sometimes called 15/70 because it uses 70mm film but rotated horizontally so the width of the film is used for the height of the frame and the width (1.43 times wider) spans 15 perforations on the film.

Typical 70mm film (not rotated) uses the width of the film for the width of the frame and so the height is 5 perforations.

This can be seen in the image below (cropped from a Wikipedia image)