What is a point of time selected as a fixed reference in astronomy called?

What is a point of time selected as a fixed reference in astronomy called?

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  • Anonymous
    1 decade ago
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    Vernal equinox

    "...0 o'clock siderial time is defined as the instance when the vernal equinox passes through the meridian. This definition can be generalized to: ``Siderial time is the hour angle of the vernal equinox.'' Of course, the vernal equinox is a fictitious point on the celestial sphere and cannot be observed directly. From the known coordinates of observed stars, however, the location of the vernal equinox can be deduced. From the above it is also clear that a siderial day is the interval between two successive passages of the vernal equinox through the meridian. According to this final definition, a siderial day is shorter by about 9 milliseconds than the approximation given at the beginning. This is a consequence of the fact that due to Earth's precession the vernal equinox is moving with respect to the stars.

    The above definition refers to the local meridian and therefore leads to a siderial time that is dependent on the place of observation. To define a global standard of siderial time, one refers to the meridian of Greenwich and calls the time scale so derived the ``Greenwich Mean Siderial Time'' (GMST). To convert between GMST and local siderial time, the geographical longitude of the observation site must be known.

    ..."

    http://www.maa.mhn.de/Scholar/times.html

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