**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).