Glossary
Accumulated clock difference
The total time gained or lost by one ideal clock relative to another. Voyager Relativity obtains it by integrating the instantaneous clock-rate difference from launch to the present.
Astronomical unit (AU)
A unit of length defined as exactly 149,597,870.7 kilometers, approximately the average Earth-Sun distance.
Cartesian state vector
A set of three position coordinates and three velocity components that describes an object's state at a particular time.
Deep space
Regions of space far beyond Earth and its immediate surroundings.
Fractional clock-rate difference
The dimensionless difference between the rates of two clocks. Multiplying it by the duration of a day converts it to time gained or lost per day.
General relativity
Einstein's theory of gravity as the curvature of spacetime. In the weak-field approximation used here, a clock at a less negative gravitational potential runs faster than one deeper in a gravity well.
Gravitational parameter (GM or μ)
The product of the gravitational constant and a body's mass. It is used to calculate that body's gravitational potential and acceleration.
Gravitational potential
Potential energy per unit mass at a location. With zero chosen at infinite distance, the Newtonian potential of an isolated body is negative and becomes more negative closer to the body.
Heliocentric
Measured or described relative to the center of the Sun.
Heliopause
The boundary where the outward solar wind is halted by the interstellar medium. The Voyager mission uses its crossing to identify entry into interstellar space.
Horizons
NASA/JPL's system for generating ephemerides: calculated positions, velocities and other data for Solar System bodies and spacecraft.
ICRF
The International Celestial Reference Frame, a standard celestial reference frame used to express the state vectors in this project.
Ideal clock
A conceptual perfect clock whose rate follows the relativistic model exactly, without mechanical, electronic or environmental imperfections.
Interstellar space
The region between stars. Voyager 1 and Voyager 2 are regarded as having entered interstellar space after crossing the heliopause, although neither has left the Solar System in every definition of that term.
One-way signal time
The approximate time a radio signal takes to travel between Earth and Voyager, based on their current distance and the speed of light.
Proper time
The elapsed time measured by a clock along its own path through spacetime.
Radioisotope thermoelectric generator (RTG)
A long-lived electrical power source that converts heat from radioactive decay into electricity. Each Voyager carries three RTGs.
Reference clock
The clock against which another clock is compared. Here it is an ideal, non-rotating clock at the Earth's mean surface radius, moving with the Earth's center around the Sun.
Reference frame
A coordinate system and state of motion used to describe positions, velocities and elapsed time.
Special relativity
Einstein's theory of space and time in the absence of gravity. Relative motion causes a moving clock to run more slowly in a suitable comparison frame.
Time dilation
A difference in elapsed time between clocks caused by relative motion, gravity or both.
Trajectory
The path followed by a spacecraft through space over time.
Trapezoidal integration
A numerical method that approximates an accumulated quantity between samples by averaging consecutive values and multiplying by the time interval.
Weak-field approximation
An approximation to general relativity valid when gravitational fields are weak and speeds are much lower than the speed of light. It keeps the leading gravitational-potential and motion terms used by this project.