Apogee:The most distant point from Earth reached by a body in an elliptical orbit with Earth at the primary focus.
Attitude of a spacecraft: The orientation of the spacecraft in space, with respect to some chosen coordinate system.
Celestial equator: The projection of the equatorial plane of Earth on the celestial sphere.
Celestial sphere: An imaginary sphere of infinite radius on which celestial objects appear projected.
Center of mass: The point within a body at which all the mass could be located without changing its dynamical behavior.
Conjunction of planets: The position of the planets when they are on the same right ascension circle on the celestial sphere (in other words, when they appear closest together in the sky).
Cosmology: The study of the evolution of the cosmos or universe.
Declination: The analog, on the celestial sphere, of latitude circles on Earth.
Direction cosines:The cosines of the angles made by a vector in space with each of the three positive coordinate axes.
Ecliptic:The path described by the center of the Sun on the geocentric celestial sphere during the course of a year.
Electromagnetic spectrum: Radiation of various wavelengths emitted in the form of waves carrying rapidly varying electric and magnetic fields (light is an example of a portion of the spectrum).
Ephemeris:A list of the successive positions of a celestial object on the geocentric celestial sphere for a series of equally spaced times.
Geocentric:Concentric with Earth.
Iteration:A repetitive mathematical procedure, on an initially chosen trial value, which can produce improved values of a desired quantity.
Jovian: Relating to the planet Jupiter.
Julian day: The number of days, and fraction of a day, measured from noon on 1 January of the year 4713 B.C.
Orbital period:The time it takes for an object to complete one orbit.
Perigee:The closest point to Earth reached by a body in an elliptical orbit with Earth at the primary focus.
Photon:The smallest unit (or "particle") of electromagnetic radiation, carrying one quantum of energy.
Pitch:An angular rotation of an aircraft or spacecraft around an axis through the wings (which has the effect of moving the nose up or down).
Right ascension: The analog, on the celestial sphere, of the longitude circles on Earth.
Right-handed three-dimensional coordinate system:
A convention of three basis vectors most simply represented by the thumb, index, and middle fingers of the right hand when extended at right angles to each other; the x-axis is along the thumb, the y-axis along the index finger, and the z-axis along the middle finger.
Roll: An angular rotation of an aircraft or spacecraft around an axis along its length (which has the effect of tipping its wings).
Watt: A unit of power. (In the mks system of units, 1 watt = 1 joule per second.)
Yaw: An angular rotation of an aircraft or spacecraft around an axis perpendicular to its body (which has the effect of moving the nose from side to side).