731 



TABLE 827.— MISCELLANEOUS ASTRONOMICAL DATA (concluded) 



Parallactic inequality moon = Q — 124.785" (Brown.) 



Parsec. — Distance of star whose parallax is 1 sec = 31 X 10 12 km = 19.2 X 10 12 miles 

 = 3.263 light years. 



Perihelion. — Point where earth is nearest sun = 1.4700 X 10' 3 cm. 



Planetary precession = X = 0.1247" (Newcomb). 



Pole of Milky Way = R. A., 12 hr 48 min ; Dec, + 27? 



Refraction. — r in. (") = [983 X (barometer in in.)/(460 + (°F)] tan Z, where Z = 

 zenith distance. Error < 1", Z < 75°, ordinary t and pressure. 



Solar diameter = 864,408 miles. 



Solar parallax = 8'.'80 (conventional value), 8'79 (Newcomb, Spencer Jones). 



Sun.— r = 6.965 X 10'° cm. Area = 6.093 X 10 22 cm 2 . Volume = 1.412 X 10 33 cm 3 . Mass 

 = 1.987 X 10" g. Density = 1.41 g/cm 3 . Mean distance to earth 1.495 X 10 13 cm. See 

 Table 831. 



Twilight. — There are three definitions of twilight : civil, nautical, and astronomical. 

 Civil twilight lasts until the sun is about 6° below the horizon, after which motor-car 

 lights must be turned on. Nautical twilight lasts until the sun is about 12° below the 

 horizon. This is the limit for observations of stars with the sea horizon. Astronomical 

 twilight is considered to end when the sky is dark in the zenith. It lasts until the sun is 

 about 18° below the horizon. For latitudes > 50° there is a faint twilight at midnight 

 in midsummer. 



Year. — Anomalistic (two successive passages of the perihelion) = 365.25964134 + 3.04 

 X 10" 8 (t — 1900) days. Eclipse (time taken by sun to pass from a node of the moon's 

 orbit to the same node) = 346.620031 + 3.2 X 10" 7 (t — 1900) days. Sidereal (from given 

 star to same star again) = 365.25636042 + 1.1 X 10"" (t — 1900) days. Tropical (ordinary) 

 (two successive passages of vernal equinox by sun) = 365.24219879 — 6.14 X 10~ 8 

 (t— 1900) days. 



TABLE 828.— ELEMENTS OF SOLAR MOTION* 



Because of the asymmetry in stellar motions (Table 876), determinations of the speed 

 and direction of the sun's motion are very sensitive to the selection of stars to which it is 

 referred. Ideally we wish to refer the sun's motion to the circular velocity with respect to 

 the galactic center ; this may be called the basic solar motion. It is possible to determine 

 this basic solar motion from detailed studies of the distribution of motions among nearby 

 stars and it is found that such a determination made from the giant K stars is in excel- 

 lent agreement with an independent determination from the A stars (Janssen and Vyssot- 

 sky). This value is given in the last line of the table. The figures listed for the first five 

 groups are smoothed values obtained from a combination of the best observational re- 

 sults. 2 ™ The values for the next four groups come from investigations made at Leiden, 

 Mount Wilson, and- McCormick Observatories. The solar motion with respect to B stars, 

 f-stars, and Cepheids is difficult to determine satisfactorily because of uneven distribu- 

 tion in space, very small proper motions, etc. 



Coordinates of the apex 



Stellar group Solar 



of reference velocity 



B8 to A3 16 km/sec 



A5 to F2 17 



F5 to GO 18 



K0 to K2 20 



gK5 to gM8 22 



dK8 to dM5 23 



Irregular var 35 



Long-period var 54 



Cluster-type var 130 



Basic solar motion 15 



* Prepared by A. N. Vyssotsky, University of Virginia. 

 266 Astron. Journ., vol. S3, p. 87, 1948. 



SMITHSONIAN PHYSICAL TABLES 



