On the east and west coasts the network affiliate newscasts are at 11:00 PM. In the midwest it’s 10:00 PM. Where exactly are the dividing lines and when/why did thus come to be?
The dividing lines are the time zones. Network broadcasts in the central time zone are simultaneous with those in the eastern time zone, so all the local time slots (like news) are shifted by that hour as well.
Edited to add: this wouldn’t work all the way to the west coast, since no one wants to watch Letterman at 8 PM, so west coast TV schedules are not tied to the east coast (which is why, for instance, you hear things like “Immediately following the game, we’ll show 60 Minutes, except of the west coast,” (…where it is still only 4 in the afternoon.)
Out in a village in Western Alaska we would get Denver channels for the networks. In summer we might joke about Mid-afternoon with David Letterman.
Pasta gave the answer that I was going to give, but the truth is that it is an incomplete answer, because in the Mountain Time Zone the schedule is the same as in Central, but the broadcasts are an hour later, and in the Pacific the schedule is the same as in the Eastern.
So my guess is that Central and Mountain zones are dominated by country folk who go to bed earlier, and the Eastern and Pacific zones are dominated by city folk who go to bed later.
The winners are broadcasters near the Eastern/Central border, who don’t have to worry about viewers on the other side. I hope that the Central/Mountain and Mountain/Pacific borders areas are more sparsely populated and pose less of a problem.
Back in the days when there were only 3 channels available in the U.K. the BBC used to have their news broadcast at 21:00, so ITV put theirs on at 22:00 so it wouldn’t compete. Similarly for 18:00 and 18:30 (and later Channel 4 at 19:00)
It, of course, dates from radio. And at the time, the Mountain Time Zone was sparsely populated, so the networks didn’t consider them important.
A radio show would be broadcast at 8 pm Eastern and 7 pm Central. It would either be redone at 11 pm Eastern to go out to the west coast (or recorded, once that technology was developed). Mountain time stations with network feeds could choose either the Eastern time zone feed or the Pacific.
You didn’t do anything for the time zone shift between Eastern and Central because the demarcation lines were generally in more populated areas within reach of stations from both time zones. You wanted to have a show running simultaneously in both zones for people on the border. The borders between the Mountain time zone and the Central and Pacific are in areas with low population, so this wasn’t an issue.
And in the early days of network TV, the stations in the Mountain Time Zone were on the Eastern schedule, while the Pacific stations were on the Central schedule, so you had program times like “Tonight at 9:00…8:00 Central and Pacific time.”
Regarding the Mountain Time Zone, there actually weren’t any MTZ specific feeds until the 1980s. Before then local TV stations had to tape the East Coast feed and play it back later. Only the largest stations had enough videotape machines to delay the schedule a full hour all the time, so it was common to broadcast a mix of live and tape delayed feeds. The real reason the MTZ matched the Central Time Zone was that delaying stuff two hours would mix up an already confused schedule and make time sensitive shows like the national news vulnerable to changing events.