A practical way to determine geographical meridian

[vcsharp2003]

Homework Statement

How to determine practically the geographical and magnetic meridians at a place P on the surface of the Earth?

Relevant Equations

None

A geographical meridian at a place P is a vertical plane that passes through the geographical north-south axis of the Earth.
A magnetic meridian at a place P is a vertical plane passing through the magnetic north-south axis of the Earth.

For the magnetic meridian, I would place a magnetic compass on a horizontal table. Then, the vertical plane passing through the north-south axis (after its needle has aligned) of the compass will be the magnetic meridian at place P.

I cannot find a practical way to determine the geographical meridian at a place P. I know that there are infinite vertical planes at any given point on the Earth. Perhaps one practical way could be to stand facing the sun at sunrise, then a vertical plane perpendicular to the direction you are facing would be the geographical meridian. I am not sure if this is accurate or approximate.

 

[.Scott]

I cannot find a practical way to determine the geographical meridian at a place P.

Certainly the most common (old-style) way is to locate either Polaris (Northern Hemisphere) or The Southern Cross.
A crescent moon provides a far less precise celestial indicator. Draw a line through the crescent moon that cuts the "C" vertically and extend it to the horizon. That point will be approximately south in the northern hemisphere and approximately north in the southern hemisphere. If you are near the equator, that line will run pretty close to North/South.

It daylight, you are probably best using a clock and and a sundial with compass points. Adjust the sundial so that it shows the correct time.

If you have ephemeris data and the time of day, you can use those directly against the sun or any other visible celestial body. Of course, if you have a phone app that uses GPS, just turn it on and find North.

Perhaps one practical way could be to stand facing the sun at sunrise, then a vertical plane perpendicular to the direction you are facing would be the geographical meridian. I am not sure if this is accurate or approximate.

That would be most accurate near the equator and/or during either equinox.
Far better would be to track the sun with a sundial over a full day marking its shadow every 30 minutes or so. Whenever the shadow was shortest, the sun would have been directly north or south.

 

[vcsharp2003]

Certainly the most common (old-style) way is to locate either Polaris (Northern Hemisphere) or The Southern Cross.
A crescent moon provides a far less precise celestial indicator. Draw a line through the crescent moon that cuts the "C" vertically and extend it to the horizon. That point will be approximately south in the northern hemisphere and approximately north in the southern hemisphere. If you are near the equator, that line will run pretty close to North/South.

It daylight, you are probably best using a clock and and a sundial with compass points. Adjust the sundial so that it shows the correct time.

If you have ephemeris data and the time of day, you can use those directly against the sun or any other visible celestial body. Of course, if you have a phone app that uses GPS, just turn it on and find North.

That would be most accurate near the equator and/or during either equinox.
Far better would be to track the sun with a sundial over a full day marking its shadow every 30 minutes or so. Whenever the shadow was shortest, the sun would have been directly north or south.

The problem with sundial method is that its not a standard device that is easily available and its time-consuming to find the shortest shadow time. Isn't there a way that uses an easily available option/method to get the geographical meridian? Using the compass on my mobile phone does not give the geographical north-south direction, so that's not an option.

 

[.Scott]

The problem with sundial method is that its not a standard device that is easily available and its time-consuming to find the shortest shadow time. Isn't there a way that uses an easily available option/method to get the geographical meridian?

Your homework statement is: "How to determine practically the geographical and magnetic meridians at a place P on the surface of the Earth?".

Clearly, using a phone app is the most practical thing in most cases.

But I guess "practically" depends on what your situation is. The sundial shown below is available from Home Depot for $48:

If you were travelling at sea before the 20th Century, you would determine you longitude by sighting on stars and using ephemeris data (but not from NASA).

Until 2000 or so, whenever I was "bushwhacking" (hiking off trail), I would use either a topo map; the combination of a magnetic compass and the current local magnetic declination (which is normally provided in the margins of the USGS topo maps); or (on sunny days), the position of the sun combined with the current time and the sunrise and sunset times. That last method can be a bit of a problem if you are in the tropics.

Using the compass on my mobile phone does not give the geographical north-south direction, so that's not an option.

Use the map feature. It will put north to the top. then just orient yourself to the map.

 

[haruspex]

The problem with sundial method is that its not a standard device that is easily available

A vertical stick in the ground would have been good enough for Eratosthenes. Need a plumb line, though.

and its time-consuming to find the shortest shadow time.

The context is unclear. Are we to assume the tools available to an architect of Stonehenge or to a modern visitor?