Midday Sundial curiosity

Andrew,

Equation of time

HTH BIBID

Avpx

Like the ancients, you're expecting the position of the sun's shadow at midday to follow a straight line as the year passes. The fact that it describes a flattened vertical figure of eight shows that the orbit of the sun round the earth (or the earth round the sun, it comes to the same thing) is not a circle. It took 2,000 years for Kepler to figure out that the orbit is an ellipse.

Meanwhile, all sorts of epicycles, offsets etc were used by Ptolemy and others used to "save the appearances".

Another Dave

For a start, the thing that casts the shadow (the gnomon of a sundial) has to be parallel to the Earth's axis, which your door frame won't be.

And the varying orbital speed of the Earth in its elliptical orbit around the Sun has a cumulative effect leading to a disparity of up to about 15 minutes either way between solar and "clock" time. This is what the "equation of time" linked above is about.

The true north / magnetic north / grid north difference can be a real hassle.

We use an Android app on our phones/tablets which displays the GPS position and track on various scales of OS maps (you have to pay for the OS map data). The old package (Viewranger) has been bought out and replaced by the new owner's Outdoor Active app. We noticed that all the grid lines on new maps were a few degrees off vertical, which looked really naff. When we reported it to Outdoor Active, we got a garbled explanation which effectively said that they aligned the maps to true north rather than grid north (though they didn't use anything as technical as that). I can see that maps that use latitude and longitude lines should be oriented to true north, but maps which have grid lines drawn on them, as part of the map, need to be oriented so those lines are perfectly vertical and horizontal, even if this is mathematically wrong to a lat/long purist.

Despite the explanation of "that's how we do it" (with the implication "we're not going to fix it"), I see that a few months later an updated version of the app now orientates OS maps properly.

There are two ‘contributors’ to the error which the correction * the equation of time yields. One is, as you say, the elliptical shape of the orbit, the other is the tilt of the Earth’s axis to the orbital plane. Each give rise to a periodic error ( different periods) which sum to give the collective error we see.

• it is used to get corrections, perhaps less so now. When sun compasses were used for navigation the EoT was used to produce tables used with the sun compass to apply corrections when taking fixes etc. Sun compasses can be used on vehicles where magnetic ones can’t.

On a smartphone the "electronc" compass has to be calibrated before use often by moving the phone through a figure of 8 sequence. I expect the same type of function in your camera requires the same calibration sequence.

From the manual..... Electronic compass errors may be caused by the effects of strong magnetic or electrical fields (such as televisions, microwaves, large motors, radio towers and high-voltage lines). To restore the electronic compass function, hold the camera firmly and move in a figure of 8 while turning your wrist over.

Just to add to the other comments - was that mid day GMT, or mid day on your local time? (unless you happen to be on he meridian!)

Andy

Midday GMT (Sussex), and 13:00 BST recently.

I was simply trying to identify actual South so that I might put up a freesat dish without a signal tester. Bill will probably pour scorn on this, but nothing ventured ...

Takes me back to elderly spinster showing us how to use a sextant.

a. find time of local apparent noon. Can get that from the nautical almanac (or now from sites that cater for the sundial-ists)

b. set watch and wait for local noon to arrive, with sharp sticks to mark your line to due South

c. repeat until the British weather stops pulling your chain :)

The easiest way for would be to open Multimap and click on the Ordnance Survey option. Find your house and the area generally to the south. Use the largest scale possible (either 1:50,000 or 1:25,000). Locate one or more landmarks as close to due south of you as possible. Use these to determine due south. OS grid south is near enough to true south to make no difference. If there are no landmarks visible to the south use another direction and measure the angle, just the same. I sometimes find a landmark exactly opposite the wanted direction, which makes things easier.

Bill

Do you suppose that the world might be ready for a solar-powered computer controlled sundial with a bezel that rotates so that the correct time is always under the shadow (or in the UK, where the shadow ought to have been)?

Nick

Would it be wearable?

Thanks. I'll try that but I had already compared paper OS maps with online maps that allow zooming in, so I am fairly sure how my house is orientated, i.e. the rear wall faces south but is slightly to the east of due south (which might explain why I get regular phone calls from 'green' energy companies trying to flog me solar panels).

To the south is the South Downs and because I am down in a hollow any landmarks are hidden. Aligning maps with the intersecting A roads and the railway line in the village is all I have to go on.

What a good idea! People could wear them like rucsacks so that everybody could look at everybody-else's sundial to find out the time.

Nick

You just need a sundial with a figure-of-eight shaped hole as the gnomon to correct for the equation of time.

Although not solar powered, back in the '80s IIRC, one of the electronics magazines published a project for an indoor sundial, with a fixed lamp and a moving gnomon.

Actually...

Made to match a location, and one swaps out the gnomon on the solstices.

Thomas Prufer