the nabla symbol was first used by Hamilton.

∇ is a Δ (greek capital delta) turned upside down. Therefore the symbol is sometimes refered to as atled, but aparently I have never heard anyone call the Delta alban. The name nabla for the symbol is the greek word for a hebrew harp which has a similar shape.
In math the symbol usually denotes a differential operator or a connection. the latter are used to model the concept of parallel transport on a curved surface: Assume stand at the north pole of our world. Choose a direction. and walk in that direction until you are at the equator. Now walk along the equator, say for a quater of its length and allways keep track of your original direction (still pointing south of course). In the end walk back noth to the pole. If you allways kept track of your original direciton, it will now point backwards but you will find, that it does not coincide with the initial direction you headed when leaving the north pole in the beginning (in fact it will be 90 degrees off). What you have encountered here is the same phenomenon, that makes a Foucault pendulum
(here is one at the south pole) preceed.

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