Parabolic Velocity

in astronomy, the velocity a body would need to make a parabola about the centre of attraction (also called ESCAPE VELOCITY).

Taken from Dictionary of Science

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The velocity which an object at a given point would require in order to describe a parabola about the center of attraction. This is also the escape velocity since it is the upper limit of velocity on a closed curve. It is obtained by multiplying the velocity of an object moving in a circular orbit by the square root of 2 (approx. 1.414). For example, if the Earth's mean velocity around the Sun of 29.8 km/sec. (18.5 miles/sec.), in a near circular orbit, were increased to 42 km/sec. (26 miles/sec.), the Earth would move to a parabolic path and escape from the solar system.

Taken from The Internet ENCYCLOPEDIA OF SCIENCE

Galactic Rotation

the rotation of the GALAXY, and all its gas, stars and dust, which increases towards its centre. The rotation near the SUN is approximately 250 kms^-1 and to make one galactic orbit takes the Sun in the region of 250 million years.

Taken from Dictionary of Science

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The rotation curve of a galaxy can be represented by a graph that plots the orbital velocity of the stars or gas in the galaxy on the y-axis against the distance from the center of the galaxy on the x-axis. Stars revolve around the center of galaxies at a constant speed over a large range of distances from the center of the galaxy. Thus they revolve much faster than would be expected if they were in a free Newtonian potential. The galaxy rotation problem is this discrepancy between the observed rotation speeds of matter in the disk portions of spiral galaxies and the predictions of Newtonian dynamics considering the visible mass. This discrepancy is currently thought to betray the presence of dark matter that permeates the galaxy and extends into the galaxy's halo. An alternative explanation is a modification of the laws of gravity, such as MOND (Modified Newtonian Dynamics).

Taken from Wikipedia

Galactic Plane

the plane which passes through the centre of the MILKY WAY, as far as that can be determined.

Taken from Dictionary of Science

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The plane in which the disk of the Milky Way Galaxy lies. By definition, one direction perpendicular to this plane is called above or north, and the opposite direction, also perpendicular to the galactic plane, is called below or south. From Earth, due galactic north is marked by the north galactic pole, which lies in Coma Berenices, near the bright star Arcturus, and due galactic south is marked by the south galactic pole, which lies in Sculptor.

Taken from The Internet ENCYCLOPEDIA OF SCIENCE

Zenith

the highest point over head -- the pole which is vertically above the observer with the latter in the centre of the horizon circle. Zenith is opposite to the NADIR in the CELESTIAL SPHERE.

Taken from Dictionary of Science

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In general terms, the zenith is the direction pointing directly "above" a particular location; that is, it is one of two vertical directions at the location, orthogonal to a horizontal flat surface there. The concept of "above" is more specifically defined in astronomy, geophysics and related sciences (e.g., meteorology) as the vertical direction opposite to the net gravitational force at a given location. The opposite direction, i.e. the direction of the gravitational force is called the nadir. The term zenith also refers to the highest point reached by a celestial body during its apparent orbit around a given point of observation. This sense of the word is often used to describe the location of the Sun, but it is only technically accurate for one latitude at a time and only possible at the low latitudes.

Strictly speaking, the zenith is only approximately contained in the local meridian plane because the latter is defined in terms of the rotational characteristics of the celestial body, not in terms of its gravitational field. The two coincide only for a perfectly rotationally symmetric body. On Earth, the axis of rotation is not fixed with respect to the planet (for example due to constant displacements of its fluid components) so that the local vertical direction, as defined by the gravity field, is itself changing direction in time (for instance due to lunar and solar tides).

Taken from Wikipedia