Compass in the antipodes

[Connemara]

I can't see an obvious place to put this question, but perhaps navigation is the best.

I bought a pair of binoculars with a built-in compass from a store in Canada planning to use them as binoculars (duh) and a bearing compass on the boat in New Zealand.

The compass part worked fine when I got them, but did not function in New Zealand. The display was incorrect and would not move smoothly when I moved the glasses.

Got back. Now the display appears to work fine, but there is something rolling around (looks like a washer or some other circular flattish object).

The question is: shouldn't the compass work south of the equator? If I sailed from here to there, I'd expect my boat compass would still function.

If so, it might be that something came loose (the washer-like object) and is now so loose that it no longer interferes with function, but does get in the way of seeing the display.

Connemara

[barnakiel]

You buy a different compass for different parts of the world. I think Plastimo makes theirs for 3 zones.

Or else hold the binnocs at an odd angle.

https://en.wikipedia.org/wiki/Magnetic_dip

Cheers,
b.

[Roy M]

I just spoke with the friendly, and knowledgeable, folks in San Diego, Baker Marine Compass Adjustors about this question. They confirm your issue is as you surmised. You may be able, depending on the manufacturer, to get a corresponding compass unit for your binos. You will have to switch them out when "crossing the line", but it isn't rocket science. Or, as an alternative, buy a backup pair when you get back south.

[Connemara]

Well, who'd a thunk it?

Thanks for the quick replies and explanation. I learn something new every day.

Connemara

[barnakiel]

Plastimo, happy boating to you

Plastimo's take on the subject.

b.

[CaptTom]

Nah, you just have to turn them upside-down when you're in New Zealand.

[Adelie]

Ok it sounds like the opposite hemisphere issue has been resolved. Except for explaining why ship's compass worked and binos didn't. The bino compass card is flat inside a flat case so when reverse dip tried to tilt the card it dragged on the case. Ship's compass almost certainly was gimballed in a spherical case. You may have notes that when the boat was upright at rest the compass card was probably still tilted 10-15-20 degrees.

The washer like object sounds like a bubble in the compass fluid.

[JPA Cate]

It depends on what kind of compass you have. We had our ship's compass re-balanced in NZ. The guys with Ritchies didn't have to. None of our northern hemisphere handbearing compasses work down here, all hang up, except the flux gate one.

If you're planning a circumnavigation, you can buy a equatorial balance compass which will work okay at somewhat higher and lower latitudes.

Good luck with the binoculars, it's all so small in them, little clearance for dip.

Ann

[Hydra]

Minox makes binoculars with a digital (fluxgate) compass: MINOX: Overview Nautic Binoculars

Beautiful but heavy and not cheap...

Alain

[captmikem]

It is called Dip which is the tendency of the compass needles to point down as well as to the magnetic pole. Dip is greatest near the poles and least near the Magnetic Equator. The compass card is designed to operate in the horizontal, therefore, any movement from the horizontal plane introduces dip error.

Michael

[S/V Alchemy]

Originally Posted by Ann T. Cate

It depends on what kind of compass you have. We had our ship's compass re-balanced in NZ. The guys with Ritchies didn't have to. None of our northern hemisphere handbearing compasses work down here, all hang up, except the flux gate one.

If you're planning a circumnavigation, you can buy a equatorial balance compass which will work okay at somewhat higher and lower latitudes.

Good luck with the binoculars, it's all so small in them, little clearance for dip.

Ann

This reflects what I anticipate. I have a Ritchie at the helm, but I'll have to trade the hand-bearings for someone heading north, I figure.

[svHyLyte]

Originally Posted by Connemara

I can't see an obvious place to put this question, but perhaps navigation is the best.

I bought a pair of binoculars with a built-in compass from a store in Canada planning to use them as binoculars (duh) and a bearing compass on the boat in New Zealand.

The compass part worked fine when I got them, but did not function in New Zealand. The display was incorrect and would not move smoothly when I moved the glasses.

Got back. Now the display appears to work fine, but there is something rolling around (looks like a washer or some other circular flattish object).

The question is: shouldn't the compass work south of the equator? If I sailed from here to there, I'd expect my boat compass would still function.

If so, it might be that something came loose (the washer-like object) and is now so loose that it no longer interferes with function, but does get in the way of seeing the display.

Connemara

The magnet attached to the underside of a compass card aligns with the earth's magnetic field. At the magnetic (verses rotational) equator, the earth's magnetic field is essentially tangent to the surface of the earth hence the compass card remains "flat". As one progresses north or south from the equator, the earth's magnetic field curves more and more radically into the earth until it is essentially vertical near the poles. E.g.






This field curvature causes the card to "dip" out of the horizontal plane, eventually enough to come into contact with the edges of the compass bowl and become "stuck". As noted by others, compass cards are commonly weighted or "corrected" with either a small weight or a small vertical magnet beneath the card that can be adjusted outward from the center of the card toward the edges to compensate for the dip of the card. Inexpensive compasses, however, are normally weighted or compensated with fixed compensators for the locale/latitudes where they are expected to be used hence can become dysfunctional at other higher or lower latitudes. It may be possible to obtain an alternate compass adjusted for your new locale to be substituted into you binoculars. Alternately, buy yourself a hand held bearing compass. Those with a round "ball" type compass generally remain effective at very high latitudes.

FWIW...

[foufou]

Had the same surprise when I shipped my international canoe to Australia for a championship regatta years ago. A mind boggling event for the unaware. But once explained it all makes sense. Nice drawing of the magnetic field

[El Pinguino]

Diameter of the card determines how much effect 'dip' has on a particular compass.
Big ship compasses ( typically a 12 inch diameter card ) aren't affected at all.... tiny ones are affected quite a bit.
When compasses are sold for different regions they have ballast in them. Equatorial compass = no ballast. One for use in the northern hemisphere... the north seeking end of the needle is trying to point down so a bit of ballast is fitted by the south seeking end of the needle. V/V in a 'southern hemisphere' compass.
If a compass is ballasted in this manner and it is taken into the opposite hemisphere then the ballasted acts with the dip and compounds the effect and everything turns to custard.

Before someone picks me up on it... big ship - and indeed all- compasses are affected by dip in that the closer you get to the magnetic poles and the greater the dip, the greater the vertical component and the smaller horizontal component of the earth's magnetic field and the less enthusiastic the compass needles ( big ship compasses have multiple needles on the same card ) are to seek north.

[knockabout]

round ball aviation and 4x4 compass on ebay for a couple of dollars works upside down / some point south some point north / both work in either hemisphere / keep them away from magnets in speakers / have given many of them away to travellers having compass difficulty / no one has ever complained about the cheapness as they are surprisingly accurate / also good to suction cup on to the dinghy to hold a heading to a point onshore and reverse / there is also one in the grab kit / one suctioned upside down onto the AIS box for quick reference in the night for swing on anchor