One of the major principles of Einstein's theory of relativity
is that spacetime is curved around a massive object such as
a black hole.
In a geometry that is curved (as opposed to flat or
Euclidean), objects and photons followed curved paths rather
than straight lines, and the sum of the angles of a triangle
is not equal to 180^{o}.
According to Einstein's theory, the warping of spacetime
around the Sun causes the
observed advance of the perihelion of Mercury
and the bending of starlight as it passes close
to the Sun.

