Question

why The locar time is

decided by the noon time

Answer 1

there are three events in a day that are easily observable: sunrise, sunset and noon. In most parts of the world, sunrise and sunset are long-drawn-out things. For example, take Madrid, roughly in the middle of the longitude range for the Western world and look at Sunrise and sunset times in Madrid. Today (22 July) the sunrise has three different times (astronomical twilight, nautical twilight and civil twiight), at 5:07, 5:52 and 6:31. That’s a difference of nearly 1½ hours, and it happens again at sunset (22:10 to 23:33). So sunrise and sunset aren’t useful. That leaves noon, which can be measured relatively accurately.