Assume for simplicity that the Earth’s magnetic north pole is at the same location as its geographic north pole. If you are in an airplane flying due west along the equator, as you cross the prime meridian (0° longitude) facing west and look down at a compass you are carrying, you see that the compass needle is perpendicular to your direction of motion, and the north pole of the needle dipole points to your right. As you continue flying due west, describe how and why the orientation of the needle will (or will not) change.
Question by OpenStax is licensed under CC BY 4.0
Final Answer

The needle direction will not change. Please see the video for an explanation.

Solution video

OpenStax College Physics for AP® Courses, Chapter 22, Problem 2 (Test Prep for AP® Courses)

OpenStax College Physics, Chapter 22, Problem 2 (AP) video thumbnail

In order to watch this solution you need to have a subscription.

Start free trial Log in
vote with a rating of votes with an average rating of .
Video Transcript
This is College Physics Answers with Shaun Dychko. We imagine that you are in an airplane that's flying along the equator so let's draw an airplane here and it's flying towards the west. So this is the geographic north pole of the Earth and here's the geographic south pole and when I say geographic, I mean the north pole and south pole terminology that you are used to so the Arctic is up here and Antarctica is in the south pole. Okay! And you look at a compass that's in the airplane and I'll draw the compass really big here and the compass has a needle pointing towards the north pole and the other end of the needle is pointing towards the south pole and one thing that we are meant to do to simplify things is we are meant to imagine that the magnetic pole up here is in the same position as the axis of rotation so the magnetic pole is in the same position where the north geographic pole is. So normally the magnetic pole is some distance away from the axis of rotation... I think it's somewhere in Baffin Island in the north of Canada and this angle between where the magnetic pole is and where the geographic pole is this angle is called declination but we are told that we don't have to worry about that so good... let's get rid of that! So the magnetic pole is in the same position as the axis of rotation in order to simplify the question. Now where this question makes things confusing though is in the incorrect wording that it has about well, I don't know if it's incorrect or not but anyway you have to be really clear about what this means; it says the Earth's magnetic north pole is at the same location as its geographic north pole but if we imagine that the Earth is like a bar magnet, which it is, and we were to draw that bar magnet here so I am drawing the bar magnet that we imagine is inside the Earth the end that's at the geographic north pole— the end that's in the Arctic in other words— has southern polarity that's why the north end of the compass needle points there because this has southern magnetic polarity and so the north end of the compass magnet is attracted to the opposite pole being the south magnetic pole and down here in black, we have the north magnetic polarity at the geographic south pole and this sort of opposite business is confusing and the way to make sense of it is to say well... this pole was labeled on a map with the word north because the end of the magnet in the compass— that is labeled north—points there. This is the north end of the magnet here that's pointing towards well, the north pole of course because the north end of the magnet in the compass is pointing to what we'll call the north pole since the north end of the magnet is pointing there. Okay! But the reason that it's pointing there is because the geographic north pole has southern magnetic polarity and opposites attract here. Okay! So hopefully that made some sense and so I disagree with the way it's worded here they are saying magnetic north pole... I guess what they mean is well, I think it's wrong... I think it should say magnetic south pole. Okay! So there! Fine! Now with all that said, let's answer this question: question is what's going to happen to this needle when it currently has its north pole pointing to the right and so if you are on the plane here and you are sitting in the cockpit say your right hand would be on the north side so this would be the right hand and then your left hand would be facing towards the south and as you go along this equator whether you are passing the prime meridian of zero degrees or not it's nothing special about the prime meridian in this case any longitude... they are all the same in this respect in the sense that the needle will not change, the direction that it's pointing; the north end of the compass needle will always point to the geographic north pole as you go along this equator.