Epoch (astronomy)

Epoch (astronomy)

In astronomy, an epoch is a moment in time used as a reference point for some time-varying astronomical quantity, such as celestial coordinates, or elliptical orbital elements of a celestial body, where these are (as usual) subject to perturbations and vary with time. The time-varying astronomical quantities might include, for example, the mean longitude or mean anomaly of a body, or of the node of its orbit relative to a reference-plane, or of the direction of the apogee or aphelion of its orbit, or the size of the major axis of its orbit.

The main uses of astronomical quantities specified in this way include their use to calculate other parameters of relevant motions, e.g. in order to predict future positions and velocities. The applied tools of the mathematics disciplines of celestial mechanics or its subfield orbital mechanics (for predicting orbital paths and positions for bodies in motion under the gravitational effects of other bodies) can for example be used to generate an ephemeris, which can be presented as a table of values giving positions and velocities of astronomical objects in the sky at a given time or times.

Astronomical quantities can be specified in any of several ways, for example, as a polynomial function of the time-interval, counted from an epoch as temporal point of origin. (This is a common current way of using an epoch.) Otherwise, the time-varying astronomical quantity can be expressed by a constant, equal to the measure that it had at the epoch, leaving the law of its variation according to time interval from the epoch to be specified in some other way—for example, by a table, as was common practice during the 17th and 18th centuries.

The word epoch was often used in an alternative way in older astronomical literature, e.g. during the 18th century, in connection with astronomical tables. At that time it was customary to denote as "epochs", not the standard date and time of origin for time-varying astronomical quantities, but rather the values at that date and time of those time-varying quantities themselves. In accordance with that alternative historical usage, an expression such as 'correcting the epochs' would refer to the adjustment, usually by a small amount, of the values of the tabulated astronomical quantities applicable to a fixed standard date and time of reference (and not, as might be expected from current usage, to a change from one date and time of reference to a different date and time).