The fraction of light that is reflected
in all directions by a surface.
A unit of distance employed in astronomy usually, for expressing distances within the solar system. 1AU is the mean distance of the Earth from the Sun. The value of this length is 149,598,500 Km or 92,956,000 miles.
The center of mass is that point of a material body or a system of bodies which moves as thought the system's total mass were concentrated in that point and all external forces were applied at that point. It is also called center of inertia.
To better understand this concept, let's
imagine a system made of 2 separated bodies with 2
different masses m2
and m1
= 2 * m2. The center of mass of this system is the point defined by the relation by the distances d1 and d2:

If we consider a system of two bodies, a bigger one called
main body and a second much smaller one, orbiting around it, the
escape velocity is the minimum speed that the smaller body must
acquire to escape permanently from the gravitational attraction
of the main body.
Friction is a general force which opposes to a motion when
this motion exists or to other forces which tend to produce such
motion.
It's the energy of a body in motion. It's very intuitive that speed and mass are both important parameters when calculating the energy of a moving body (it's every day experience that a 100 Km/h car and a 50 Km/h truck are both very dangerous bodies, from an impact point of view!!!). The kinetic energy can then be calculated with the formula:
where v is the speed and m is the mass of the moving object. 
When seen by two different observations points, a star seems in different apparent positions against the background sky. This is due to the fact that the star is much nearer than the background sky (made of what are called fixed stars). This is the phenomenon of parallax. 
Let's consider a light object (such as a planet or a comet) orbiting around a much heavier one (like the Sun, in the solar system). The orbit of this object, described by the three Keplero's laws, will be elliptical. In this orbit, two important geometrical points can be found, the perihelion (the point where the object is nearest to the sun) and the aphelion (the point of this orbit where the object is farthest).
The perihelion and aphelion distances can be defined and calculated not only for what concerns ellipse, but for all the other conics, as a function of the eccentricity e of the orbit (click here to know more about conics and eccentricity):
Sublimation of an element is the direct conversion from its solid to its gaseous state, without melting into a liquid. Sublimation can take place at temperature and pressure conditions that depend on the considered element.