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.

A European Space Agency's satellite, launched on 10 December 1999 and
designed for the observation of → X-rays emitted by
astronomical objects. The satellite carries three very advanced X-ray telescopes.
The three corresponding European Photon Imaging Cameras (EPIC) are sensitive over
the energy range 0.2 keV to 12 keV. Other instruments on-board are two
reflection grating spectrometers which are sensitive below about 2 keV, and
a 30 cm diameter → Ritchey-Chretien
optical/UV telescope. The telescope moves in a highly elliptical orbit,
traveling out to nearly one third of the distance to the Moon and enabling long,
uninterrupted observations of faint → X-ray sources.
The original mission lifetime was two years, it has now been extended
for further observations until at least 2010.
Among recent results obtained using XMM-Newton one can mention
an intermediate-mass black hole of over 500 solar masses in the galaxy ESO 243-49
(Nature 460, 73, 2009) and
broad line emission from iron K- and L-shell transitions in the active galaxy 1H 0707-495
(Nature 459, 540, 2009). See also
→ X-ray astronomy.

XMM, from "X-ray Multi-Mirror;" Newton, in honor of Sir Isaac Newton,
→ newton.