>how do they decide that the earth spun faster on it's axis rather than the earth taking longer to orbit the sun?
It's both effects really. Celestial bodies' orbits do undergo decay and also their rotation undergoes decay. The question is which happened to which degree, and I think the rotational slow down is the dominant effect in Earth's case.
Orbits decay due to various forms of drag. The long-term rate of decay of orbits in the Solar system is relatively well established.
Rotation also is slowed down due to drag, but in our case there's another major force: the tidal influence from the Moon. Earth's Moon is a relatively large companion (at 1.23%[1] by mass). Both bodies influence each other tidally, and that influence saps away rotational energy and also Moon's orbital energy; the Moon already got tidally locked to Earth. Aside of that there's a (smaller) tidal influence from the Sun, which again saps Earth's rotational energy.
Since the magnetic field is tied to rotation of the planet’s core, I would expect it to slowly weaken as it caught up with the lack of rotation of the surface.
It's both effects really. Celestial bodies' orbits do undergo decay and also their rotation undergoes decay. The question is which happened to which degree, and I think the rotational slow down is the dominant effect in Earth's case.
Orbits decay due to various forms of drag. The long-term rate of decay of orbits in the Solar system is relatively well established.
Rotation also is slowed down due to drag, but in our case there's another major force: the tidal influence from the Moon. Earth's Moon is a relatively large companion (at 1.23%[1] by mass). Both bodies influence each other tidally, and that influence saps away rotational energy and also Moon's orbital energy; the Moon already got tidally locked to Earth. Aside of that there's a (smaller) tidal influence from the Sun, which again saps Earth's rotational energy.
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[1] https://www.wolframalpha.com/input/?i=mass+of+the+moon+%2F+m...