# Pinning a gyroscope will resist change in orientation

## Main Question or Discussion Point

I know when spinning a gyroscope will resist change in orientation so that the rotor is still pointing in the same direction.

But, is that direction with respect to earth's center of gravity or is it an "absolute direction".

So, if I point a gyroscope upwards and spin the rotor and I go directly to the otherside of the earth would the rotor be pointing downwards or will it be pointing upwards still?

Also, if I spun a gyroscope for a day, would I see it change direction (with respect to my reference frame) with Earth's rotation?

And if I spun it for a year would I see it change direction (again with respect to my reference frame) along with Earth's rotation and orbit?

Considering these questions my guess is it stays in the same direction with respect to Earth's Center of Gravity.

## Answers and Replies

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It's an "absolute direction" kind of thing.

So assuming the rotor is spinning forever regardless of where I take it in the universe it will always point in the exact direction that I started spinning it at?

If that is the case then how do things which use gyroscopes to determine direction compensate for the earth's spin and orbit?

russ_watters
Mentor
That is how they compensate. That's the whole point of the gyroscope - that it isn't affected by the earth's spin and motion. For example, a gyrocompass on a ship has its axis aligned to the earth's axis and it stays that way, regardless of which way the ship is pointed or the earth moves (the earth's axis doesn't move either, for the same reason), enabling the ship to know its direction.

Ah, I see how that works.

Though if it isn't aligned with the Earth's axis it would not work well as a compass?

You normally use gyroscopes for short time scale things, like acrobatic maneuvers. Over longer time-scales, most gyroscopes tend to slowly "drift" and need correcting anyway (say using accelerometers, or compasses perhaps.. in which case the slow daily rotation may be removed then).

Is that simply because real world is not a "perfect world" situation?

vanesch
Staff Emeritus
Gold Member
Is that simply because real world is not a "perfect world" situation?
It is because the "gyroscope pointing in the same direction" is only correct if no torque is applied to the gyroscope (that is, a perfect gyroscope is an axially symmetric rigid body that has a fixed point, lying on its axis of symmetry, and is totally free to rotate about that axis, without undergoing any external torque wrt to that fixed point). The suspension of a gyroscope always includes some form of friction or so, which induces a very small torque on the gyroscope, and can slowly alter the direction in which it points.

Thanks for the info guys. :-)

russ_watters
Mentor
Ah, I see how that works.

Though if it isn't aligned with the Earth's axis it would not work well as a compass?
The compass is free to float (IIRC, it is suspended magnetically), so it still works if it isn't aligned quite perfectly. Ships keep logs of their compass error and check it daily against the sun.

Danger
Gold Member
'How It Is Made' on Discovery had a segment on the construction of a marine gyroscope a while back. It's quite amazing how complex it is.