Should I use True North or Magnetic North to align my sundial?

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In summary, the sun light travels a perfect path across Earth, 50% before solar 12 noon and 50% after solar 12 noon. However, online information says the sun is at solar 12 noon at 11:55 am, but this month of April the sun is at 11:45 am. This discrepancy may be an error.
  • #1
gary350
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I'm not sure I asked the correct question. I am trying to learn when our sun is at solar 12 noon on my yard, does the sun light travel a perfect path across earth, 50% before solar 12 noon and 50% after solar 12 noon.

I built a large sun dial in my yard 3 ft tall 6 ft long. I used a compass to line it up magnetic north south. Should I have used True north to line up sun dial instead of magnetic north?

The numbers that I have do not match up with online information of where solar 12 noon is at my house???

Sun Dial Reading Online information

Dec 21 30.5° 29.15°

Jan 21 38.4 33.32

Feb 21 43.4 42.63

March 21 54.0 52.73

April 21 65.4 62.83

Sun dial says solar 12 noon is 11:55 am CT at my house. Daylight saving time 12.55pm

Online info was solar 12 noon at 11:55 am but this month of April solar 12 noon is 11:45 am. How is that possible or is that an error.

NEXT: Online research shows if a farmer plants a 1000 acre field with north south rows and another identical 1000 acre field with east west rows, the north south crop will be a $30,000. more profit than the east west row crop. Research says east west rows never has full direct sun on the north side of the plants.

I have a home garden. I am trying to be accurate as possible. Can I rely 100% on my sun dial to tell me sun really is solar 12 noon at 11:55 am? Is it correct for crops to line up with magnetic north south OR should crops be lined up with True north.???

Should I use magnetic north of True north for my sun dial?
 
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  • #2
For solar related purposes you should use true north, not magnetic north.
 
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  • #3
gary350 said:
I am trying to learn when our sun is at solar 12 noon on my yard, does the sun light travel a perfect path across earth, 50% before solar 12 noon and 50% after solar 12 noon.
Yes it is symmetrical, but standard clock time and solar time do not track exactly.

gary350 said:
I used a compass to line it up magnetic north south. Should I have used True north to line up sun dial instead of magnetic north?
Use true north, based on the stars, not on the Sun, or your clock.

Do NOT use the position of the Sun to find true north, because it wanders seasonally, by about ±15 minutes. https://en.wikipedia.org/wiki/Equation_of_time
Solar time is also offset from your clock, by your longitude within your standard time zone.

Do NOT use Grid north from a 2D map, as that is not true north everywhere on a 3D spherical Earth.

You could find true north on a clear night by using a star sight.
There are several ways. For example, if you are in the Northern Hemisphere, use the North Star, Polaris. https://en.wikipedia.org/wiki/Polaris
Today, hang a weight on a string to establish vertical above a reference point on the ground, maybe the centre of your sundial. Then later, at any time of the night, mark a point on the ground that is in line with the pole star and the vertical string. Next day, the line on the ground between the marked point and the string, is close enough to true north. The elevation of the pole star is equal to your latitude, so you may need a long string, or a different technique.
 
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I was shocked to discover that over my lifetime (3 score and 11 yrs) the position of the magnetic north pole has changed enough to greatly diminish the incidence of big aurora borealis displays in the lovely state of Maine. C'est dommage
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012-017_Notas_277-info.jpg
 
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  • #5
I suppose I should make the finer delineation that the "geomagnetic pole" (which is where your compass points) has shifted less than this. The "(not geo) magnetic pole" pictured above is where the field lines are vertical at the surface and thereby guide the solar ejecta. The last really ripping good Maine aurora I saw was on my birthday in1982...the whole damned sky was on fire
 
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  • #6
gary350 said:
Online info was solar 12 noon at 11:55 am but this month of April solar 12 noon is 11:45 am. How is that possible or is that an error.

Should I use magnetic north of True north for my sun dial?
You need to use True North for a sun dial. It should be sufficient to use a magnetic compass and apply Magnetic Deviation (called Magnetic Variation in UK) for you location, which should be on maps.
If setting up a sun dial using the Sun, its motion is not uniform over the year. Clocks are based on the average Solar position (Greenwich Mean Time or Universal Time) but the actual position wanders ahead and behind with the seasons. This fluctuation is called the Equation of Time.
The time of solar noon at Greenwich for each day is given in almanacs. If we know your longitude (or the city you are in) we can find your clock time when the Sun is at local solar noon on a particular day. So at that time you just mark the position of the shadow and that is True north.
 
  • #7
hutchphd said:
I was shocked to discover that over my lifetime (3 score and 11 yrs) the position of the magnetic north pole has changed enough to greatly diminish the incidence of big aurora borealis displays in the lovely state of Maine. C'est dommage
/

View attachment 325952
Look at it go! It isn't all that important though. First a little background. The magnetic field is generated by huge vortexes of liquid iron rising inside the Earth. It's like your bathtub drain vortex but rising instead of going downward. The liquid iron in the outer core is carrying heat away from the solid inner core as the Earth very slowly cools. We don't know much detail about these vortexes but we can observe three main concentrations of the magnetism they generate. These are underneath Canada, Siberia, and in between Australia and Antarctica. What we are seeing is the Canadian vortices growing weaker than the Siberian ones. Today they are roughly equal. I expect this trend to continue so that in thirty to seventy years the magnetic pole will be in Siberia.
 
  • #8
hutchphd said:
I suppose I should make the finer delineation that the "geomagnetic pole" (which is where your compass points) has shifted less than this. The "(not geo) magnetic pole" pictured above is where the field lines are vertical at the surface and thereby guide the solar ejecta. The last really ripping good Maine aurora I saw was on my birthday in1982...the whole damned sky was on fire
Actually the solar ejecta are maximal where the field lines are maybe 85 degrees relative to Earth's surface. Auroras don't occur much at the magnetic poles. This is easily seen in photos of auroras from other planets.

The trapping of the ions is complicated and I never bothered to try to understand it.
 
  • #9
Hornbein said:
The trapping of the ions is complicated and I never bothered to try to understand it.
The ions move in helical paths about magnetic field lines. Where the lines converge, the ions are reflected. The lines near 85° form resonant cavities, with the aurora visible at both ends of the field line, in both hemispheres. You may notice patches of regularly pulsating light where huge bunches of particles are being bounced back and forth between the hemispheres.

Lines close to vertical also converge, and they do reflect the particles, but the particles are then lost, far out into space, so do not resonate between hemispheres.

There is insufficient field convergence to form resonant paths near the equator, where the field is close to horizontal in the ionosphere.
 
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  • #10
Maybe then you also know the origin of the Van Allen radiation belts. My guess is that the ions get trapped in there and radiate with they collide.
 
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Baluncore said:
The ions move in helical paths about magnetic field lines. Where the lines converge, the ions are reflected. The lines near 85° form resonant cavities, with the aurora visible at both ends of the field line, in both hemispheres. You may notice patches of regularly pulsating light where huge bunches of particles are being bounced back and forth between the hemispheres.
You mean they bounce between the northern and southern hemispheres?
 
  • #12
Hornbein said:
You mean they bounce between the northern and southern hemispheres?
Yes. The Earth often plays a gentle game of ping-pong with clouds of ions, using the magnetic equator as the net.
There is often symmetry between aurora borealis and aurora australis. It is complicated by the daylight difference between summer and winter, and the fact that it is difficult to observe the two displays at the one time.
 
  • #13
Baluncore said:
Yes. The Earth often plays a gentle game of ping-pong with clouds of ions, using the magnetic equator as the net.
There is often symmetry between aurora borealis and aurora australis. It is complicated by the daylight difference between summer and winter, and the fact that it is difficult to observe the two displays at the one time.
Well whaddya know.
 
  • #14
tech99 said:
You need to use True North for a sun dial. It should be sufficient to use a magnetic compass and apply Magnetic Deviation (called Magnetic Variation in UK)
I think you mean Magnetic Declination (which is called Magnetic Variation in the UK and elsewhere): Magnetic Deviation is something else.

tech99 said:
The time of solar noon at Greenwich for each day is given in almanacs. If we know your longitude (or the city you are in) we can find your clock time when the Sun is at local solar noon on a particular day. So at that time you just mark the position of the shadow and that is True north.
Now Google Maps (and other mapping services) as well as GPS devices in mobile phones have made determination of longitude simple for everyone, this is an easy and more precise way to find True North.
 
  • #15
More precise than what? How do I find the North if I just know my longitude?
 
  • #16
nasu said:
More precise than what?
Anything based on a compass.

nasu said:
How do I find the North if I just know my longitude?
By using this method:
tech99 said:
The time of solar noon at Greenwich for each day is given in almanacs. If we know your longitude (or the city you are in) we can find your clock time when the Sun is at local solar noon on a particular day. So at that time you just mark the position of the shadow and that is True north.
Or use the calculator here: https://gml.noaa.gov/grad/solcalc/ to find the local time corresponding to solar noon at your position.
 
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