Changes in the of the medium through which a wave travels can cause the wave to either speed up or slow down.

Changes in the elastic properties and density of the medium through which a wave travels can cause the wave to either speed up or slow down.

Elastic properties determine how rigid the material is, and the rigid the material the faster its particles can vibrate, which means that the wave can travel faster trough it. This explain why waves can travel way faster trough iron than trough rubber because iron is far more rigid than rubber.

Density on the other hand, is the mass of a substance per unit volume. The higher the density of a medium the higher the mass of it, and so the wave has to use more energy to travel trough it; thus slowing the wave down.

Neptune is the eighth planet from the sun. it was the first planet to get its existence predicted by  mathematical calculations  before it was actually seen through a telescope on sept. 23, 1846. irregularities in the orbit of  uranus  led french astronomer alexis bouvard to suggest that the gravitational pull from another celestial body might be responsible. german astronomer johann galle then relied on subsequent calculations to   spot neptune via telescope. previously, astronomer galileo galilei sketched the planet, but he mistook it for a star due to its slow motion. in accordance with all the other planets seen in the sky, this new world was given a name from greek and roman mythology — neptune, the roman god of the sea.