Noon should be noon

The sun crosses the sky at fifteen degrees of longitude per hour. That's all a time zone is: a band about fifteen degrees wide where everyone agrees to set their clocks the same, with the middle of the band at the spot where the sun is highest at 12:00.

In the United States the middles are 75°, 90°, 105° and 120° west. The honest edges sit halfway between, at 82.5°, 97.5° and 112.5°. On a map drawn that way, nobody's clock is ever more than about half an hour off the sun.

That isn't the map we have.

The day with two noons

Before 1883 every town kept its own time, set by the sun. Chicago's noon was a few minutes behind Detroit's and ahead of St. Louis's, and there were more than a hundred local times in use around the country. That was fine for farmers. It was a mess for railroads, which had to publish schedules in dozens of times at once.

So the railroads fixed it themselves. Their General Time Convention, meeting at the Grand Pacific Hotel in Chicago in October 1883, adopted a plan for standard zones. On Sunday, November 18, 1883, the Allegheny Observatory sent a telegraph signal at noon on the 90th meridian, and railroad clocks across the country were reset. Towns east of their zone's middle saw the clock hit noon twice that day.

The federal government made the zones law in 1918, in the Standard Time Act. Indiana, for the record, was put entirely on Central time.

How the lines drifted

The 1918 lines were reasonably close to the sun. Then came a century of petitions. A city near a zone line wants to keep the same clock as a bigger city to its east, for business or just out of habit. The Department of Transportation, which handles the boundaries now, has granted a lot of those requests. One county at a time, Eastern time crept west across Indiana and Michigan, and Central time crept west across the Dakotas and Kansas.

The result is that Detroit, at 83° west, is on Eastern time, and in summer the sun is highest there at 1:32 in the afternoon. Williston, North Dakota, sits on Central time at nearly 104° west. Indiana spent decades fighting about which zone it belonged in, and today 80 of its 92 counties are on Eastern time, a full zone east of where the sun puts them.

Why the western edge matters

When the clock runs ahead of the sun, the sun sets late on the clock, and people stay up later. They still have to get up for work and school at the same clock time as everyone else.

Economists Osea Giuntella and Fabrizio Mazzonna compared people living on either side of US time zone borders, where the clock jumps an hour but the sun doesn't. They found that an extra hour of evening light cut sleep by about 19 minutes a night on average. The western-side group also showed higher rates of conditions linked to disrupted body clocks, including obesity, diabetes and cardiovascular disease.

A University of Colorado study of fatal car crashes after the spring clock change found the biggest spikes on the western edges of time zones, in places like Amarillo, Texas, and St. George, Utah, where the morning was already darkest.

Not every claim about time zones holds up. A 2017 study reported higher cancer rates on the western sides of zones, and a larger 2023 follow-up using nearly every county in the country found no overall relationship. We'd rather tell you that than leave it out. The sleep findings are the solid part.

What this map does

Every zone line goes back to the meridian the sun says it belongs on, and bends only to keep a metro area together. A few places move.

These are mean solar noon. The real moment drifts by up to about a quarter of an hour over the year, which nobody can fix with a clock.

The lower 48 still only needs four zones. What changes is where the lines sit.