# Midday Sundial curiosity

34 views

### Andrew

May 25, 2022, 6:20:11 AMMay 25
to
Back in January when we had a bright sunny day, I marked the
position of the Midday shadow from my south-facing patio door jamb
on the kitchen marley floor with some 2 inch wide blue masking tape.

Yesterday I drew another line at 13:00 BST expecting it to be
in exactly the same position as the january line but it is
rotated around slightly, which is not what I was expecting.

Why would the increase in the angle which the 'midday' suns
rays are hitting Europe alter the angle of the shadow ?.

Is there some extra maths that I have overlooked ?

Also, my Olympus Tough TG6 camera indicates a compass
when the info button is pressed and according to that
North is way off my floor markings. I only expected it to
be indicative, but not that far off.

Andrew

May 25, 2022, 6:29:18 AMMay 25
to
Andrew,

Equation of time <https://en.wikipedia.org/wiki/Equation_of_time> and the
analemma <https://en.wikipedia.org/wiki/Analemma> all come into this -
it's not as simple as one might imagine - as for the compass there is the
difference between magnetic north & true north (that varies depending on
where you are and, in the long term, when)

HTH BIBID

Avpx

--
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### Another Dave

May 25, 2022, 7:32:46 AMMay 25
to
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
--
Change nospam to techie

### Max Demian

May 25, 2022, 7:50:37 AMMay 25
to
On 25/05/2022 11:29, The Nomad wrote:
> On Wed, 25 May 2022 11:20:06 +0100, Andrew
> <Andrew9...@mybtinternet.com> wrote:
>
>> Back in January when we had a bright sunny day, I marked the position of
>> the Midday shadow from my south-facing patio door jamb on the kitchen
>> marley floor with some 2 inch wide blue masking tape.
>>
>> Yesterday I drew another line at 13:00 BST expecting it to be in exactly
>> the same position as the january line but it is rotated around slightly,
>> which is not what I was expecting.
>>
>> Why would the increase in the angle which the 'midday' suns rays are
>> hitting Europe alter the angle of the shadow ?.
>>
>> Is there some extra maths that I have overlooked ?

> Equation of time <https://en.wikipedia.org/wiki/Equation_of_time> and the
> analemma <https://en.wikipedia.org/wiki/Analemma> all come into this -
> it's not as simple as one might imagine

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

--
Max Demian

### NY

May 25, 2022, 7:55:35 AMMay 25
to
"The Nomad" <no...@the.desert.invalid> wrote in message
news:_xnjK.2955810\$391.2...@usenetxs.com...
> Equation of time <https://en.wikipedia.org/wiki/Equation_of_time> and the
> analemma <https://en.wikipedia.org/wiki/Analemma> all come into this -
> it's not as simple as one might imagine - as for the compass there is the
> difference between magnetic north & true north (that varies depending on
> where you are and, in the long term, when)

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.

### Brian

May 26, 2022, 1:53:17 AMMay 26
to
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.

### alan_m

May 26, 2022, 2:23:27 AMMay 26
to
On 25/05/2022 11:20, Andrew wrote:

>
> Also, my Olympus Tough TG6 camera indicates a compass
> when the info button is pressed and according to that
> North is way off my floor markings. I only expected it to
> be indicative, but not that far off.
>
> Andrew

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.

--
mailto : news {at} admac {dot} myzen {dot} co {dot} uk

### alan_m

May 26, 2022, 2:32:43 AMMay 26
to
On 26/05/2022 07:23, alan_m wrote:
> On 25/05/2022 11:20, Andrew wrote:
>
>>
>> Also, my Olympus Tough TG6 camera indicates a compass
>> when the info button is pressed and according to that
>> North is way off my floor markings. I only expected it to
>> be indicative, but not that far off.
>>
>> Andrew
>
>
> 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.

### Vir Campestris

May 27, 2022, 4:52:46 PMMay 27
to
On 25/05/2022 11:20, Andrew wrote:
> Back in January when we had a bright sunny day, I marked the
> position of the Midday shadow

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

### Andrew

May 28, 2022, 3:44:30 PMMay 28
to
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 ...

### Robin

May 28, 2022, 5:04:47 PMMay 28
to
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 :)

--
Robin

### Brian D

May 28, 2022, 6:37:09 PMMay 28
to
Braind

### williamwright

May 28, 2022, 9:32:34 PMMay 28
to
On 28/05/2022 20:44, Andrew wrote:
> 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 ...

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

### Nick Odell

May 29, 2022, 9:03:06 AMMay 29
to
On Wed, 25 May 2022 12:32:42 +0100, Another Dave <dmar...@nospam.com>
wrote:
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

Nick

### Robin

May 29, 2022, 9:17:24 AMMay 29
to
Would it be wearable?

### Andrew

May 29, 2022, 9:32:10 AMMay 29
to
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.

### Nick Odell

May 29, 2022, 10:00:06 AMMay 29
to
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

### Max Demian

May 29, 2022, 1:24:02 PMMay 29
to
On 29/05/2022 14:03, Nick Odell wrote:
You just need a sundial with a figure-of-eight shaped hole as the gnomon
to correct for the equation of time.

--
Max Demian

### SteveW

May 29, 2022, 5:24:58 PMMay 29
to
On 29/05/2022 14:03, Nick Odell wrote:
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.

### Thomas Prufer

Jun 3, 2022, 1:13:44 AMJun 3
to
Actually...

http://www.praezisions-sonnenuhr.de/

I'd say accurate to five minutes or so, with a bit of generosity. Mostly because
the width of the sun smudges the shadow, and it's not a sharp transition.

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

Thomas Prufer

### Thomas Prufer

Jun 3, 2022, 1:19:11 AMJun 3
to
On Sun, 29 May 2022 15:00:02 +0100, Nick Odell <ni...@themusicworkshop.plus.com>
wrote:
There is an analog digital sundial.

https://www.deutsches-museum.de/en/museum-island/exhibitions/sundial-garden

"Digital Sundial

"This sundial, which displays the true solar time digitally, was only developed
and patented in 1994 by Werner Krotz-Vogel and Daniel Scharstein. No other
sundial can be read as easily and clearly.

"The sunlight falls at an angle that depends on the time, first through a
periodic array of slits and then through a second array at a suitable distance.
There, it only lights up the slits that belong to a single number at any given
moment."

And there's 3d printed models, where holes in the Gnomon spell out the digits.

https://all3dp.com/2/3d-printed-sundial-3d-printed-digital-sundial-3d-models/

Thomas Prufer