Definitions Fact-pak

Kepler's Laws

The planets move in ellipses around the Sun, which is at one focus of the ellipse. The planets sweep out equal areas in equal times, relative to the solar focus. The rate of rotation is proportional to the distance from the Sun, such that p2/a3 is a constant. (p is the period of revolution and a is the mean Solar distance)

Newton's Universal Law of Gravity

...states that all masses in the Universe are attracted to all other masses in the Universe and that this attractive force is proportional to the respective masses of the objects concerned and is inversely proportional to the square of the distances between them. This results in the formula: Fg=Gm1m2/r2

Where Fg is the gravitational force, m1 and m2 are the masses of two objects attracting one another, r is the distance apart of the two bodies (measured between their centres of gravity), G is the Gravitational Constant and its value is 6.67 x 10-11 Nm2kg-2.

Albedo How reflective an object is. It is the ratio of reflected to incoming light or radiation

Diurnal Motion The daily movement of astronomical objects across the sky

Siderial Time Time monitored with reference to the stellar background

Ecliptic The imaginary plane on which the Earth and Sun jointly lie

Ellipticity The amount or degree that something - eg an orbit or planet - varies from a circle towards an ellipse

Rift Valley A huge flat-bottomed, steep-sided depression in the landscape, often associated with tectonic activity

Caldera The crater of a volcano

Tidal Lock A process that makes planets and moon harmonise their rotation and orbit

Celestial Sphere The sky sphere around the Earth that contains the stellar background

Sol A Martian Day 24 hours 37 minutes 22 Seconds

Meridian A line of longitude on the celestial sphere

Precession The slow drift of the poles due to the gyroscopic action of the planet

Obliquity The angle of tilt of a planet's axis relative to the ecliptic

Eccentricity The varience of an elliptical orbit from a perfect circle

Solstice The limits of north and south apparent movement of the Sun

Vernal Equinox The moment in spring when the sun is over the equator

Autumnal Equinox The moment in autumn when the sun is over the equator

Aphelion Farthest point from the Sun for an object in solar orbit

Perihelion Closest approach to the Sun for an object in solar orbit

Apogee Farthest point in an orbit from the Earth

Perigee Closest approach in an orbit to the Earth

Stellar Magnitude The brightness of an object, usually a star, in the sky

Astronomical Unit AU The distance from Earth to the Sun

Light-year Distance light travels in one year

Tharsis Bulge A large area of Mars which rises several kilometres above the average surface level of the planet

Terraforming Remodelling and engineering a planet on a global scale to make it habitable

Asteroid A small rocky celestial body orbiting the Sun

Crustal Dichotomy Division between two parts of crust, which appear very different from each other (Mars)

Mare A large featureless area of of a planet or moon resembling an ocean (however, not containing liquid)

Planitia Literally plain

Mons Literally mountain or hill

Splat Crater Also known as rampart craters, these are formed by impact into wet or icy soil or regolith

Ray Crater Ray craters formed in dry soil or regolith, so called because material from them sprays out leaving ray-like trails

Regolith The top soil of a planet or moon

Plate Tectonics The movement of continents and ocean floors about the surface of the planet, caused by deep geological processes

Meteor All the individual grains of rock and dust in space are called "meteoroids" while they are in space; when they interact with the Earth and its atmosphere that they receive different names: meteors are very small, often smaller than sand grains, and burn up in the atmosphere, producing "shooting stars"

Meteorites Are meteoroids (see above), but are massive enough to reach the ground, they are stony, stony-iron or iron lumps

Mantle The material surrounding the core of a planet, beneath the crust. On terrestrial planets this is rocky, probably silicate

Core The centre of a planet. The three terrestrial planets have high-density iron-rich cores. Earth has a liquid metal outer core, containing about 10 percent by mass of non-metal, and a solid, probably iron-nickel, inner core

Van Allen Belts A zone of radiation around the Earth, where charged particles are trapped within the magnetic field. These particles are collected from the Solar Wind, Cosmic Rays, and Earth

The Site uses as far as possible the mathematical conventions of the SI System, which has been adopted by over 90% of countries. Figures and values are also given in many cases, in non-SI units but these should be considered as approximate conversions, and not true values.


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