- ...
SLALIB1
- The name isn't an acronym;
it just stands for ``Subprogram Library A''.
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- ... papers;2
- One frame not included in
Figure 1 is that of the Hipparcos catalogue. This is currently the
best available implementation in the optical of the International
Celestial Reference System (ICRS), which is based on extragalactic
radio sources observed by VLBI. The distinction between FK5 J2000
and Hipparcos coordinates only becomes important when accuracies of
50 mas or better are required. More details are given in
Section 4.14.
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- ...equinox.3
- With
the introduction of the International Celestial Reference System (ICRS), the
connection between (i) star coordinates and (ii) the Earth's orientation
and orbit has been broken. However, the orientation of the
International Celestial Reference Frame (ICRF) axes was, for convenience,
chosen to match J2000 FK5, and for most practical purposes ICRF coordinates
(for example entries in the Hipparcos catalogue) can be regarded as
synonymous with J2000 FK5. See Section 4.14 for further details.
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- ... used.4
- An equinox is, however, not required for coordinates
in the International Celestial Reference System. Such coordinates must
be labelled simply ``ICRS'', or the specific catalogue can be mentioned,
such as ``Hipparcos''; constructions such as ``Hipparcos, J2000'' are
redundant and misleading.
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- ... decreasing,5
- The Earth is slowing
down because of tidal effects. The SI
second reflects the length-of-day in the mid-19th century, when
the astronomical observations that established modern timekeeping
were being made. Since then,
the average length-of-day has increased by roughly 2 ms.
Superimposed in this gradual slowdown are
variations (seasonal and decadal) that are geophysical in origin,
notably due to large scale movements of water and atmosphere.
Because of
conservation of angular momentum, as the Earth's rotation-rate
decreases, the Moon moves farther away. In 50 billion years the
distance of the Moon will be at a maximum, 44% greater than now, at
which stage day and month will both equal 47 present days.
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