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How Accurate Is a Sundial?

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Sundials are mostly obsolete nowadays. While prehistoric, ancient, medieval, and even renaissance men relied on sundials in the past, modern clocks, watches, and phones help us accurately tell time now. Just how accurate is a sundial, and can you still use one to check the time? How were the sundials replaced by the modern clocks we know now?

How Accurate Is a Sundial?

How Accurate Is a Sundial?

Though nowhere near as accurate as many modern clocks, sundials will forever have a place in history as some of the most ingenious ways mankind has harnessed the power of nature in order to make sense of the things around him.

Even with all the adjustments made like time zones, location, season, Daylight Saving Time, and other corrections, you may not be able to get a truly accurate sundial reading. You’ll still be able to make use of a sundial to get accurate readings of up to a 30-second to 2-minute difference.

There are many factors that can contribute to the accuracy (or inaccuracy) of a sundial, but one contributing factor is the shadow casted by the sun and the sundial’s gnomon. From earth, the sun is about ½° across, so the shadows can be fuzzy near the edges.

Corrections for Sundial Reading Accuracy

Sundial readings (and sometimes even the sundials themselves in the case of movable-gnomon sundials) must be adjusted to get accurate time readings. Here are some adjustments to make readings more accurate:

Daylight Saving Time

If DST is observed in the country you’re in, you can adjust the sundial reading correspondingly. Adjustments regarding DST readings also depend on the sun, so as to maximize sunlight and get more things done before the sun sets.

Time-zone Correction

Longitudes invisibly divide the earth into time zones, and the accuracy of a sundial depends on where it lies on the time zone too. You should make adjustments according to the difference from the center of the time zone if the sundial doesn’t sit on an exact reference longitude (usually one that is a multiple of 15°).

For every degree off of the reference longitude, 4 minutes must be added or subtracted from the reading. For example, if the sundial sits 5° west of the reference longitude, subtract 20 minutes from the reading. If it sits east of the reference longitude, add 4 minutes for every degree off.

Equation of Time Correction

Just like the earth isn’t perfectly round, its orbit around the sun isn’t perfectly circular. And while the tilt of the Earth’s axis also known as its obliquity doesn’t change too much over tens of thousands of years, it isn’t really fixed as well.

So while the sun appears to rotate around the earth in a perfectly circular motion, there are only about 4 days in every year wherein sundial readings don’t need to be corrected. Every day otherwise, sundials can be up to 15 minutes early or up to 15 minutes late. Unlike time-zone corrections though, equation of time corrections are uniform worldwide and is not dependent on the location of the sundial.

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