What time is it?

 ¡SkyCaramba! Weekly astronomy blog for the week ending November 10, 2012 

This is the time of year when people in many countries set their clocks back an hour. This comes about six months after setting them ahead an hour. People and their governments just aren’t satisfied with when darkness sets in.

Millennia ago, people told time by day parts. It was dawn, sunrise, morning, noon, etc. Eventually, people developed somewhat reliable ways of dividing the day into smaller parts. As long as the sun is out, a sundial can show you how much of the day has passed. And as long as you can see the stars, you can estimate how much of the night has gone by.

A few centuries ago, when mechanical clocks provided much more reliable ways of keeping track of time’s passage, more specific appointment times could be set. At first, people still relied mostly on getting things done according to what part of the day it was. You could tell your friend you’d meet him in the morning without having to say exactly when. If you didn’t see him right away, you just waited. But as people’s lives got busier, specific times became important so people could fit events in between other events.

In the early 1800s, each town had its own clock by which everyone else told time. If you were to appear at some place at 1:30 in the afternoon, and the town clock said it’s 2 o’clock, you should have been there already. The clock in the next town a few miles away might say it’s 1:30, because whoever set it made a different judgment about how much of the day had passed.

Train companies couldn’t take every town having its own clock and each one having a different time. It’s bad for business if people think it’s 10:30 and they miss the train that wasn’t supposed to leave until 10:45 because the engineer’s watch says it’s 11:00 already. So the train companies decided that their trains would run by their clocks and their clocks in big regions would be synchronized. This caused regular people to become more aware of something they usually didn’t have to think about.

It’s hard for the regular person to just look up at where the sun is and know exactly what time to set a watch by. You might think it’s high noon while your friend standing next to you thinks it was high noon a few minutes ago. If your friend were several miles to the east or west and communicating with you by telegraph, your disagreement would become bigger. Even if you both had the best scientific equipment by which to measure the sun’s position, the person to the west would find that high noon doesn’t happen for a few seconds or minutes yet.

If you stood in Philadelphia while your friend stood in Pittsburgh, he’d report high noon 16 minutes later. Add another friend in Chicago to your network of timekeepers and you’re waiting another 28 minutes. Train company officials knew nobody in Chicago would accept that it’s almost 12:45 when the sun is just now reaching the high noon spot. So they divided the United States into time zones. Train companies on other continents did similar things.

But there have been times when someone decided it was important to pretend it’s noon when it’s not. If we adjust our clocks ahead in the summertime, we can schedule events at 7pm without having to turn on so many lights. To nations at war, saving energy and therefore saving resources needed to produce energy is a big deal. Even nations at peace are worried about energy use. So quite a lot of countries tell their citizens to adjust their clocks forward an hour for the summer months and reset them to normal time for the winter.