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Atmospheric
refraction
Astronomical
twilight
This is the phenomenon and the value where the refraction caused by
the atmosphere of the earth causes the apparent position of the celestial
body to appear higher than its actual position. The refraction has been
corrected for the altitude lines, including the sunrise/sunset line and
moonrise/moonset line, which are displayed on the transparent dial.
The refraction amount of celestial objects (approx. 0.57°) that appear to
be on the horizon is called the horizontal refraction.
Twilight is the period after sunset or before sunrise when the sun's
afterglow or pre-glow is in the sky and appears as a half-light. In
particular, the period when the sun's altitude is -12° to -18° (18° under
the horizon) is called the astronomical twilight. If the sky is clear,
when the astronomical twilight finishes (or before it starts), stars
of magnitude 6 can be seen near the zenith. On this watch, the −18°
altitude line is displayed on the transparent dial with 35°N latitude as
the standard, to determine the astronomical twilight.
Brief explanation of astronomical terms
45
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