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Time Zones and the Spherical Earth

Week of April 5, 1999

A few months ago, I wrote about how we know the Earth is round. The idea was that as you move north or south (that is, change your latitude), different stars appear in the sky, and familiar ones change their apparent height over the horizon. That's only possible if we live on a sphere.

Or is it? Actually, if we lived on the edge of a disk we'd see the same thing! All that matters is that the Earth is curved along a north-south path. You can imagine the Earth is a disk with some finite but large thickness, like a hundred kilometers. If you live on that curved part (and not the face of the disk), and the disk is oriented such that the curved part moves north-south, then it all hangs together. As you move along the perimeter of the disk, you'd see new stars, and old ones would change position.

What we need is to go that extra dimension. How do we show that the Earth is round both north-south and east-west? In other words, how can we show that the Earth is a sphere?

Actually, there's a funny way, and it takes time. Literally. Imagine you are on the Earth (this should not be very hard to do!). Now also imagine the Sun is straight up. If the Earth were indeed a big ball, then, for someone all the way around the Earth, the Sun would be straight down, beneath their feet. It would be nighttime.

This is of course the way things really are. When it's noon for me in Washington, DC, it's roughly midnight in parts of China. As the Earth rotates, different parts of it face the Sun. Most animals, and that includes us, use the cycle of day and night as a rough clock. We talk about morning, afternoon, evening, night. It's how we divide our day. It's natural, literally, to use that cycle as a clock. When we became intelligent enough to spread civilization across the globe, it quickly became apparent that people's clocks weren't synchronized very well. Noon for me wasn't noon for anyone else on a different line of longitude! So we invented the idea of time zones.

Time zones are lines that run roughly north-south, and we define them to divide the Earth up into zones of time (hence the name, of course!). Everyone who lives in one time zone sets their clocks to the same time. The lines don't run due north-south because sometimes cities didn't want to get divided into two zones, or the lines zig-zag a bit to follow state or country borders.

This idea solved a lot of problems (like getting train schedules worked out correctly in the U.S.), but there is still a subtle problem. The east-west extent of a single time zone can cover hundreds of kilometers. A funny thing happens if you move west across a single time zone: the Sun will set at a different time at different locations! For example, someone in St. Louis, Missouri (a city in the U.S.) may see the Sun set at 7:00 local time, but my nephew Derek in Cunningham, Kansas, may see it set more than a half hour later! Why is this?

Aha! It's because the Earth is a sphere! The Earth is a rotating ball, and people that live on different longitudes see the sun set at different times. To someone in Missouri, the Sun may be setting, but to Derek, who is much farther west, the Sun is still above the horizon. Even though their clocks read the same time, their natural clock, the Sun, indicates differently.

I have noticed this myself when visiting my in-laws in Kansas, or my brother in Atlanta (who is in my time zone, but a long way west). The next time you travel, try it for yourself! Time the sunset at home, then again at your destination. Very few of us may ever get into space to see the Earth in all its spherical glory, but us flatlanders can get a taste of it if we just keep ourselves aware of it, and we're clever enough to figure out how.

©2008 Phil Plait. All Rights Reserved.

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