# Suppose you are navigating in Egypt deserts by trusting the compass

#### salsero

Suppose you are navigating in Egypt deserts by trusting the compass to determine the South direction. If your destination is 15 km to the North of you, approximately by how much will you miss it?

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#### LURCH

I geuss it depends on what part of the Egyptian desert you walk through. The thing that's going to take you off-course is called the "anagonic angle"; the difference between magnetic North and celestial North. Here where I live, the North Pole described by the Earth's axis of rotation and the Magnetic North Pole are very close together. If I take the ferry across Lake Michigan to go to Minisota, I will cross the line along which the two are perfectly lined up (the Agonic Line). So magnetic deviance in Michigan is almost zero (we preferr to be deviant in other ways ).

Now I just heard a few days ago that the anagonic angle in the UK is about 5o. That is, the magnetic pole is 5o West of the rotational pole. If you look at a globe, you can get a pretty good guestimate of the anagonic angle for Egypt.

Inetresting note:
The Agonic line runs through both the Bermuda Triangle and its far eastern twin, the Dragon's Triangle. It also runs through the less well-known, but somewhat more dangerous "Great Lakes Triangle", right near the Western shore of Michigan (the place where I take a lot of my vacations) .

#### chroot

Staff Emeritus
Gold Member
Most people, including pilots, just refer to the difference between geographic and magnetic north as "magnetic declination."

- Warren

#### salsero

Thank you very much!

I just wanted the order of magnitude such declination can be. If you say 5 degrees in UK, it is of the same order of magnitude as I got. I was surprised that it was so large. Before, I thought that I can really trust the compass. I knew that the magnetic and the real poles do not coincide, but I didn't know that the effect is so big.

#### birdus

Here in Arizona, USA, there is a place called Sedona. The rocks there are so laden with iron that it causes huge bends in the magnetic field. I can't remember exactly how much the magnetic north is but I believe it is more than 10 degrees. It also changes from day to day. Compasses there are perfectly useless. The magnetic phenomenon that occurs there as attracted a cult following. Many of the people that live there think it is caused by UFOs and such.

#### salsero

Is there a physical explanation for the daily changes there in Arizona?

Has anyone seen UFOs there?

#### birdus

Everybody there claims to have seen UFOs, some claim to have been abducted, and others claim to be aliens from another dimension-- but none of them, I think, can be considered the brightest bulb in the string.

The earths magnetic field is constantly in flux due to motion of magma under the crust. To get an accurate reading of north from a compass you have to look up correction figures in an almanac.

#### chroot

Staff Emeritus
Gold Member
Originally posted by salsero
I was surprised that it was so large.
Think about people who live on the line between the geographic and magnetic north poles -- for them, geographic north and magnetic north are directly opposite!

- Warren

#### Integral

Staff Emeritus
Gold Member
5deg is actually pretty small, where I live in the Pacific Northwest it is about 20deg.

#### russ_watters

Mentor
Originally posted by chroot
Most people, including pilots, just refer to the difference between geographic and magnetic north as "magnetic declination."

- Warren
Actually, in the Navy (on nautical charts) it is referred to as "variation."

Figuring out what direction you are going on a ship can be a laborious exercise. Lots of things to consider/relate to each other:
True north
Magnetic north
(magnetic) Compass error
Gyro (compass) error
variation
deviation

Variation is the difference between magnetic and true north. Deviation is the difference betwen your actual magnetic heading and what your magnetic compass reads (since you are on a hunk of metal, you need to adjust for this). Compass error is the total difference between your magnetic compass and true heading. Gyro error is the difference between your ship's gyrocompass (which reads in degrees true) and actual true north.

#### chroot

Staff Emeritus
Gold Member
Originally posted by russ_watters
Actually, in the Navy (on nautical charts) it is referred to as "variation."

Figuring out what direction you are going on a ship can be a laborious exercise.
Many of the same things apply to flight, as well. I have to deal with quite a lot of that (gyro drift, declination, etc.) for every flight. Not to mention the wind...
Lots of things to consider/relate to each other:
True north
Magnetic north
(magnetic) Compass error
Gyro (compass) error
variation
deviation
What's the difference between variation and deviation? In some lattitudes you also have to deal with "dip," which is the result of the fact that the field lines are not parallel to the earth's surface -- they become vertical at the poles.

- Warre

#### LURCH

Originally posted by chroot
Most people, including pilots, just refer to the difference between geographic and magnetic north as "magnetic declination."

- Warren
Now that's interesting, since the place where I first learned the term "anagonic angle" was from my flight instructor! I waonder if that's a regional thing here in Mi? Doesn't seem likely; terminology among pilots is not very location-specific because universal standards of comunication are so important to safety in air travel.

#### chroot

Staff Emeritus
Gold Member
Originally posted by LURCH
Now that's interesting, since the place where I first learned the term "anagonic angle" was from my flight instructor! I waonder if that's a regional thing here in Mi? Doesn't seem likely; terminology among pilots is not very location-specific because universal standards of comunication are so important to safety in air travel.
I'll look in my ground school book when I get home to check, but I'm pretty sure it uses the term "declination." I used the King Schools CD-ROM-based ground school program.

- Warren

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