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.
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Flamsteed designations for stars are similar to Bayer designations, except that they use numbers instead of Greek and Roman letters.
Sagittarius is one of the constellations of the zodiac.
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies.
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A star is a luminous sphere of plasma held together by its own gravity.
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