The differential Newtonian gravitational field of a galaxy across a smaller bound system. Its tidal tensor can slowly change the angular momentum of distant comets, changing their periapses and their access to planetary encounters.
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Galactic tides refer to the gravitational effects exerted by the mass distribution of a galaxy on its components, including stars, star clusters, gas clouds, and potentially even entire solar systems. Just as the gravitational pull of the Moon and the Sun affects the tides in Earth's oceans, the gravitational field of a galaxy can create tides that influence the motion and distribution of matter within the galaxy.