Don’t you also need a fairly precise compass? I know there’s an app for that, but the iPhone also comes with an app for telling time.
For a typical sundial, yes. A typical sundial uses the sun’s direction, this one uses the sun’s elevation. On a particular day, the sun will only be at that elevation twice, once in the morning, once after noon. But the higher your elevation, the lower the sun will get, so it’s set to latitude.
How accurate is this?
Latitude of New York = 40.7° N
Latitude of Madrid = 40.4° N
According to http://www.esrl.noaa.gov/gmd/grad/solcalc/azel.html for today, Dec 4, 2014:
New York local apparent noon (the sun’s highest elevation) is at 11:46:22 and will be 27.03 degrees.
Madrid local apparent noon is at 11:05:00 and the sun will be at 27.35 degrees.
It will take 27:20 for the sun’s elevation to drop to 27.03 in Madrid. So your New York ring would be off almost half an hour. Apparently it’s very accurate.
Of course, looking at the picture, I can’t tell if that’s at 10, 10:30 or nearly 11. My inner navigation geek is still craving it.
Hmm.
I guess that longitude still matters, because time zones?
If I was in Indiana, at the same latitude as NYC (I dunno, N of Lafayette), the apparent noon wouldn’t be until almost an hour after NYC?
So, at least, you’d need to correct for daylight savings and the effect of the time zone to get the ‘real’ time?)
The ring gives you local solar time.
Local apparent solar time differs from local time-zone clock time for two reasons:
First, it varies throughout the year according to the Equation of Time which corrects for the eccentricity of Earth’s orbit and the obliquity of the ecliptic.
Second, every location has a fixed offset plus or minus according to how far east or west you are of your time zone’s meridian.
So it’s not so much absolute longitude that matters so much as the longitude delta from the local time zone’s centerline, combined with the current Equation of Time offset.
And, yeah, any DST correction, too.
There’s a cheaper one that’s been on the market for years. Often sold at Renn Fairs. Costs about $40. Probably pewter.
Not that accurate. City tuning is mostly a gimmick, totally swamped by other issues.
Indeed. I bought one of those thirty years ago.
This one adds the dubious value of needing to be insured.
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