Local Solar Time Clock

Sundial?

Mark L. Fergerson

Why is a sundial unsuitable?

If you have a reliable UT1 source (or UTC/GPS time with lower

Greenwich to your longitude (4 minute time correction for each degree of longitude).

minutes) depending of the day of the year. For calculating the solar time for a few centuries before or after todays date, you need to equation-of-time equation for that year (basically two sinusoids).

I can't think of anywhere that would bother to make or sell one!

You can buy sidereal clocks that show star time with 24 sidereal hours between transits of the distant fixed stars, but there is literally no demand at all for a clock with the same defects as a sundial!

I have a sidereal clock design somewhere direct drive LCD from a 16F877 that could I suppose be modified to do this by the addition of a daily calendar and lookup table but I fail to see why it is worth the effort.

If you want to do it in software I can recommend the book "Astronomical Algorithms" by Jean Meeus. It has algorithms that you can use to calculate sun time.

Bill

If you have a solar tracking solar panels, this might be of _minor_ interest. With longitude correction (mean solar time) but without equation of time correction, the time could have an error about 20 minutes, thus the sun would be off by 5 degrees from optimal orientation, causing a 0.4 % (worst case) loss of power compared to ideal orientation. Is it worth the effort, is an other question.

Depending on the electric price in your location and the total area of your panels, this minor 0.4 % peak, perhaps 0.1 % awg. gain might justify some 100 USD/Euro increase in controller cost.

A sidereal clock does not solve anything compared to a mean solar time clock, the only difference is the 23:56 day compared to the average

I also recommend this book. You can get the equation of time correction for a few thousand years in the past as well as in the future.

On a sunny day (Sat, 18 Jan 2014 08:32:24 -0600) it happened Bill Gill wrote in :

I have the sunwait program on my PC (Linux), i tis open source:

# sunwait -p 23.2514N 15.5995W Using location: 23.251400N, 15.599500W Date: 18 Jan 2014 Local time: 16:02 Day length: 10:57 hours With civil twilight 11:42 hours With nautical twilight 12:37 hours With astronomical twilight 13:32 hours Length of twilight: civil 0:22 hours nautical 0:50 hours astronomical 1:17 hours Current specified time zone: CET (1 from UTC) Sun transits meridian 1413 CET

...Jim Thompson

If you need realtime output, can't help. But this site will give you the data

gnomon, didn't know I wanted to know that. Mikek

Though I've been there many times... it's the site of art shows and wine tasting's, I didn't know that either. ...Jim Thompson

Digital sundial:

Wristwatch electronic sundial:

What you want is to convert UTC to solar time: using the "equation of time": Note that the various time references will not produce exactly the same time. For example, GPS time is currently about 16 seconds ahead of UTC time:

Years ago, I threw together a sun tracker using 4 solar cells, two diff amps, two motors (X-Y), and a gimbaled mount. At the junction of the 4 solar cells, there was a rod, which cast a shadow on one of the cells depending on where it was pointed. When the shadow disappeared, the light level to all 4 cells was identical and the rod would be pointed at the sun. Scaled up to running a solar power panel array, it produced some interesting exercises in damping control, automobile headlight avoidance, and flying insect interference. It made a nice demonstration piece until someone stole it.

If all you want is when the sun is at its zenith, then you don't need a full sundial. Instead, you need a gnomon. Basically, it's just a stick in the ground that casts a shadow. Mark the end of the shadow with small pebbles. When the shadow is longest, it's high noon.

A noon sight using a sextant is more accurate but not as convenient.

Of course, you could cheat, and simply calculate when the sun is at its zenith and just check a lookup table: etc...

On Sat, 18 Jan 2014 08:46:45 -0800 in sci.electronics.design, Jeff Liebermann wrote,

Shortest?

Well if you set your clock to Az time , Im sure it's pretty close to solar time.

Cheers

Sorry... shortest shadow at high noon. See graphs of shadow lengths:

It's a good thing they built that thing in AZ. Otherwise it would be a royal bitch having to move those number plates twice a year for DST. Art

We don't need no daylight savings time... we have enough daylight already >:-} ...Jim Thompson

This sounds like the best lead so far.

Will subscribe now. Thanks.

Glen Lewis