A modification of the Newton's law of gravitation below a critical acceleration of
about 1.2 x 10^{-8} cm s^{-2}, where the gravitational force scales
as 1/r instead of 1/r^{2}. Originally put forward to
describe the rotation curves of
galaxies with no need to assume any dark matter,
MOND is now tested at larger cosmological scales
(Milgrom, M. 1983, ApJ, 270, 365).

The use of → Newtonian mechanics to derive homogeneous
and isotropic solutions of → Einstein's field equations,
which represent models of expanding Universe. The Newtonian cosmology deviates from the
prediction of → general relativity in the general case of
anisotropic and inhomogeneous models.

The Newton's equations of motion, if they hold in any
→ reference frame,
they are valid also in any other reference frame moving with uniform
velocity relative to the first.

A telescope with a concave paraboloidal objective mirror and a small
plane mirror that reflects rays from the primary mirror
laterally outside the tube where the image is viewed with an
eyepiece.

An approximate version of → general relativity
that applies when the → gravitational field
is → weak, and the matter → velocity
is → small.
Post-Newtonian formalism successfully describes the gravitational field
of the solar system. It can also be applied to situations
involving compact bodies with strong internal gravity, provided
that the mutual gravity between bodies is weak.
It also provides a foundation to calculate the
→ gravitational waves emitted by
→ compact binary star systems, as well as their
orbital evolution under radiative losses.
The formalism proceeds from the Newtonian description and then,
step by step, adds correction terms that take into account the
effects of general relativity. The correction terms are ordered in a systematic way
(from the largest effects to the smallest ones), and the progression of ever smaller
corrections is called the → post-Newtonian expansion.