Why is HDTV's aspect ratio 16:9?

One of the defining features of the HDTV format is an aspect ratio of 16:9 (expressed alternatively as 1.78:1). And yet, when I watch movies from Blu-Ray discs, I’m still seeing letterboxing. In other words, the movies were made in an even higher aspect ratio than HDTV. Supposedly the two most common aspect ratios for cinema are 1.85:1 and 2.39:1.

So why did they make the HDTV format “boxier” than the most common aspect ratios used in cinema? Instead of letterboxing cinematic movies to present them on boxy HDTV screens, why didn’t they define the HDTV format match one of those aspect ratios (1.85 or 2.39 to 1), and then have all HDTV television programming produced to match that aspect ratio?

Some photographer bloke named Ken Rockwell sez:

Earlier threads:

From MrFloppy’s link, in the first thread Earl Snake-Hips Tucker linked to (I’ll repeat the part he quoted):

These inner and outer rectangles are determined by the largest and smallest ratios, 1.33 and 2.35, the intermediate ones don’t matter. But they only turned out equal because 1.33^3 = 2.35, so you get 1.33^2 = 2.35/1.33 = 1.77 = 16/9.

(Equations accurate to about 1% or better)

You know, cinematographers could also start making things to fit the TV screens. Masking on a projected screen is much less noticeable.

As it is now, a lot of things are released in with some weird stretching that I always unstretch for screenshots.