by CKR
A few days ago, I walked out to pick up the mail at a little before noon. I saw this remarkable sight.
The shadow of the wall continues in a straight line with the wall. That must mean that the wall is exactly north-south, I thought, because it is lined up with the sun at noon.
That is more likely to be true at this time of year, when the shadows are longer, so that their direction is more obvious. It wouldn’t have been that way at noon during daylight saving time.
Then I thought some more, and I realized that it also wouldn’t be that way just anywhere in a time zone.
For most of the nineteenth century, each town had its own time. When the sun was overhead, it was noon. Then came the railroads, which required that the time in one town have something to do with when the train would arrive in the next town. That was in 1883. It was not until 1918 that a federal law went into effect in the US defining standard time, time zones, and daylight saving time. 
Indiana and Arizona, at the edges of their time zones, have a different view of time, and other states have made changes over the years.
If time zones were determined rationally, they’d be defined by longitude: 360 degrees of longitude divided by 24 hours means that time would change by an hour every 15 degrees of longitude. Here’s a world map with time zones and longitude indicated. The United States isn’t particularly bound by longitude in its time zones, but Asia is where things really get exciting. China, spanning a little over sixty degrees of longitude (four rational hours), has defined itself into one time zone. 
If you live on the eastern edge of a time zone, you get the sun before everyone else. So if noon is defined as noon in the center of the time zone (rationality again, silly me!), the sun will be at its highest in the sky for you before noon. If you live on the western edge, the sun will be at its highest after noon.
Congress has recently decreed that we in the US will live under daylight savings time for an additional four weeks, from the second Sunday of March to the first Sunday of November, instead of this year’s first Sunday of April to the last Sunday of October. This will put the sun overhead an hour later in the day for everyone, except Arizona and Indiana, which don’t observe daylight saving time.
But how much daylight will this measure save? The second Sunday of March occurs earliest on March 8 and latest on the 14th, and the range for the first Sunday of November is 1-7. The March dates are one to two weeks before the spring equinox, and the November dates six to seven weeks after the fall equinox. Day length is twelve hours at the equinox, less in the period after the fall equinox and before the spring equnox. Exactly how much less depends on your latitude.
To take the US extremes, Presque Isle, Maine, is at 46.7 degrees north latitude, and there’s a bit of Maine further north than that. Brownsville, Texas, is at about 26 degrees north latitude. On March 1, Brownsville will have a little over eleven and a half hours of sunlight, and Presque Isle will have a little over eleven, not too different. On November 7, Brownsville will have a little over eleven hours and Presque Isle will have about nine and three quarters. Here's the calculator.
When you’ve got nine and three quarters hours of sunlight, it’s worth saving every minute. But it doesn’t much matter when you get up or go to bed; it’s going to be dark a good part of the time. Even at eleven hours, it’s dark a lot.
And it will throw off my lovely linear wall shadow by an hour. From what I know about my house, that wall is pretty close to north-south. But I don’t have a good way to prove it from that noon shadow.