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