TABLE 827.— MISCELLANEOUS ASTRONOMICAL DATA* 729 



Aberration constant. — 20"47 (conventional value; work of Doolittle, Spencer Jones, 

 and others, indicates a value of 20'-' 50). 



Aphelion. — Point where earth is farthest from sun = 1.520 X 10 18 cm. 



Astronomical unit (A. U.)— Distance: mean distance earth to sun, 149,500,000 km. 

 (Conventional value, solar parallax 8'79 would give 149,700,000.) Mass: the combined 

 mass of the sun and earth which means, practically, the sun's mass = 1.987 X lO^g. 



Color index. — Ordinary stellar magnitudes are supposed to correspond to observations 

 with the normal eye. This is by no means easy to define, for the brightness of a red star 

 compared with a white, appears greater when the amount of light entering the eye is 

 increased for both in the same ratio (Purkinje effect) for low brightness. 



Owing to differences in the actual distribution of the energy with wavelength, the rela- 

 tive brightness of stars of different temperatures and colors measured with receptors 

 sensitive to different spectral regions vary greatly. 



On ordinary photographs, red stars appear much fainter than to the eye. If the measures 

 are calibrated so that the visual and photographic magnitudes average the same for spec- 

 tral class A, the difference for any other group of stars is called color index. This ranges 

 from about — 0"'.3 to + 1.8 for class M and reaches 5 m for the reddest stars of class N. 



The difference in color index between the two standard types, e.g., AO and KO is 

 called the color-equation. It varies over a wide range with the spectral sensitivity of the 

 receiver, very large and positive for the violet and ultraviolet and negative for the red and 

 infrared. 



Photoelectric devices, combined with screens and measurable transmission have at last 

 provided standard systems for stellar photometry of at least approximately definite physi- 

 cal significance for spectral regions ranging from the ultraviolet to the infrared. Radio- 

 metric magnitudes correspond to the measures of the whole observable energy radhtion. 



Bolometric magnitudes are supposed to represent the total energy radiation of all wave- 

 lengths, and must be found bv calculation. 



Date line. — Established by convention not far from the 180th meridian from Green- 

 wich. Where the line runs across a group of islands, the change of the date line is diverted 

 to one side so that the group has the same day. Ships crossing from the east, skip a day ; 

 going east, count the same day twice. 



Day. — Mean solar day =1,440 minutes = 86,400 seconds = 1.0027379 sidereal day. 

 Sidereal day (ordinary, two successive transits of vernal equinox, might be called equinoc- 

 tial day) =86,164.09054 mean solar seconds = 23 hr, 56 min. 4.09054 sec mean solar time. 



Two successive transits of same fixed star = 86,164.09967 mean solar seconds. 



Declination. — If 5 = declination, t, hour angle measured west from meridian, h, 

 altitude, <j>, latitude and A, azimuth measured from S. point through W. Then 



sin h = sin <p sin 8 -\- cos </> cos 8 cos t 

 cos h cos A = — cos <t> sin 5 -f sin <p cos 5 cos / >given 8, t, <p 

 cos h sin A = cos 8 sin t) 



sin 8 = sin </> sin h — cos <t> cos h cos A~\ 

 cos 8 cos t = cos sin h + sin <f> cos h cos A Sgiven h, A, <t> 

 cos 8 sin t = cos h sin A) 



Delaunay's y = sin 1/2 / = 0.04488716 ( Brown). 



Dip of horizon. — In minutes of arc = V elevation in ft (anproxi-rately). 



Earth. — Mean r = 6.3712X10" cm. Equatorial diameter = 12,756.78km ; polar diameter 

 = 12,713.82 km. Area = 5.101Xl0 18 cm 2 . Angular velocity = 72.9X10- 6 radians/sec. Volume 

 = 1.083 XI 27 cm 3 . Mass = 5.975Xl0 27 g. Density = 5.517 g/cm a . Mean distance to sun = 

 1.495Xl0 13 cm. Distance to the moon = 3.844Xl0 lo cm. Light traverses mean radius of 

 earth's orbit in 498.6 sec. Semimajor axis orbit = 1.4950X10 13 cm ; semi.ninor axis = 

 1.4948Xl0 13 cm. Viscosity = 10.9X10 1 " cgs. Velocity of equatorial point on earth, because 

 of rotation : 1,050 mi/hr = 1,550 ft/sec = 1,650 km/hr = 460 m/sec. In orbit : 18/5 mi/sec 

 = 30 km/sec. See Tables 831 and 833. Rotational energy = 2.16X10 30 erg. 



Earth's orbital velocity = 18.5 miles/second. 1,550 ft/sec (rotation at Equator). 



Eccentricity of earth's orbit = e = 0.01675104 - 4.180X10" 7 (/ - 1900) - 1.26X10"" 

 (/— 1900) 2 . 



Eccentricity of moon's orbit = e 2 = 0.05490056 (Brown). 



Gal. — Unit of gravity acceleration = 1 cm sec" 2 . 



General precession (westward movement of the equinoxes) = 50-2564 -fO-000222 

 C^ — 1900) per year (Newcomb). Probably requires correction of about H- O'-'Ol. See 



Gravitation constant = (6.670 ± 0.005) X 10" 8 dyne cm 2 g" 2 (Heyl, 1930). 

 Gravity, acceleration due to, g = 978.0495 cm sec 2 (conventional value at sea level at 

 equator. See Table 802). Unit, gal = 1 cm sec' 2 . 



* Prepared by G. M. Clemence, U. S. Naval Observatory. 



(continued) 



SMITHSONIAN PHYSICAL TABLES 



