a The cooler star in this system is one of the largest known stars, as large as the orbit of Mars. b The fainter component in this system is a white dwarf star, about the size of the Earth.

c There are altogether six stars in the Castor system;a faint visual component, Castor C, is an eclipsing binary of which both components are flare stars. d The third component of this system is Proxima Centauri, a faint red dwarf and flare star.

will not set at all for an interval of time lasting from about one day near the circles to slightly more than six months at the poles. The interval of twilight (the time between sunrise and when the Sun is a specified number of degrees below the horizon) is lengthy at high-latitude sites. There are three types of twilight:

(1) Civil twilight, for terrestrial visibility purposes, occurs when hSun > -6°, i.e., when the Sun is 6° or less below the horizon

(3) Astronomical twilight, when hSun > -18°

Note that the inequalities are algebraic so that the solar altitude must be less negative than the limit shown for a given category of twilight to apply. At any particular latitude, the length of time of twilight depends on the latitude of the observer and the declination of the Sun. The duration of twilight can be calculated with applications of (2.5). The hour angle of the Sun at the beginning (or end) of twilight can be compared with the time of sunrise (or sunset), and the difference in hour angle, in time measure, is the duration of twilight. Glare, or the scattering of light of a bright object, reduces the visibility of a fainter object in its vicinity still further. The full moon makes naked eye observations of

Table 3.2. Brightness and color in the solar system.

Object Vmax B - V

Table 3.2. Brightness and color in the solar system.

Object Vmax B - V


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