730 



TABLE 827.— MISCELLANEOUS ASTRONOMICAL DATA (continued) 



Heat index. — Radiometric (heat or bolometric), zero taken to agree with Class AO, 

 (radiometric — visual magnitude) = heat index, + for red stars. 



Horizon. — Distance at sea is approximately, miles = V (3/2) height in feet. Local 

 refraction (mirage) may introduce large percentage changes in either direction for obser- 

 vations from altitudes of 30 feet or less. 



Inclination of moon's orbit = / = 5°8'43.5" (Brown). 



Julian period, 1950 = 6663.— January 1, 1950, Julian-day number = 2433283. 



Latitude variation. — The direction of the axis of the earth in space changes approxi- 

 mately 20''5 per year owing to precession. The change is roughly periodic in 25,800 years 

 with an amplitude of 23?5. This does not affect terrestrial latitudes, but a variation in 

 them is caused by a shift of the earth's body about this axis. The two ascertained com- 

 ponents of the polar motion have periods of 1.00 and nearly 1.20 years (the annual and 

 Chandlerian components, respectively), so that the oscillations in X and Y, as well as the 

 resultant total motion have variations in amplitude with a "beat period" of about 6 years. 

 In contrast to the annual terms, Chandler's term shows striking variations in amplitude. 

 There is, further, a variation in the period of the Chandlerian term (1.18, 1.20, 1.17, 1.15, 

 1.19 years) which appears nearly proportional to the corresponding amplitude variations 

 according to the relation P = 0.185 A +1.128, where P is the period in years and A the 

 amplitude in O'-'Ol units. (See T. Nicolini, appendix to Commission 19 Report, Trans. Int. 

 Astron. Union, Zurich, 1948.) 



Light, velocity of. — (Mean value) in vacuo, 299773 ± 10 km sec" 1 (Dorsey). 



299792.5 ± 0.8 km sec' 1 (Bearden). 

 299776 ± 0.00004 km sec" 1 (Birge). 



Light year. — The distance light travels in 1 year = 9.5 X 10" kilometers = 5.9 X 10" 

 miles. Light traverses mean radius of earth's orbit in 498.6 sec. 



Lunar inequality of earth = L = 6.454" 



Lunar node d = daily motion = — 0?052954. 



Lunar parallax = 3422.70" (Brown). 



Lunar perigee, daily motion = + 0? 11 1404. 



Lunar-solar precession = />' = 50.3714" per year (De Sitter, 1927). Of this 0.0191", 

 Einstein, orbital motion earth. 



Magnitudes. — The observed intensity of light received on the earth from astronomical 

 bodies ranges over a factor exceeding 10 19 . It is therefore expressed on a logarithmic 

 scale — the system of stellar magnitudes. This system, which was adopted by Hipparchus 

 more than 2,000 years ago, is closely represented by the equation 



w = 2.5 log™ (lo/l) 



where / is the observed light and U a standard value corresponding roughly to the light 

 of Arcturus or Vega. Decrease of light by a factor of 100 increases the stellar magnitude 

 by 5.00; hence the brightest objects have negative magnitudes. (Sun: — 26.8; mean full 

 moon: — 12.5; Venus at brightest: — 4.3; Jupiter at opposition: — 2.3; Sirius : — 1.6; 

 Vega : + 0.2 ; Polaris-: +2.1). The faintest stars visible to the naked eye on a clear dark 

 night are of about the sixth magnitude (though on a perfectly black background the limit 

 for a single luminous point approaches the eighth magnitude). The faintest stars visible 

 with a telescope of aperture A (in inches) is one approximately of magnitude 9 + 5 

 logio A. The magnitude of the faintest stars which can be photographed with the 200-inch 

 telescope is about + 22.7. The apparent magnitude of a standard candle at a distance of 

 1 meter is — 14.2. 



Absolute magnitude, M, is that which the body would exhibit if placed at a distance of 

 10 parsecs, and corresponds to its actual luminosity. For a star of magnitude m, and 

 parallax />, in seconds of arc 



M = m + S + 5\ogp 



For the sun, M = + 4.7. The brightest stars probably exceed M = — 7 and the faintest 

 observed value is M = + 18, a range of 10 10 . The full moon (could it be observed without 

 interference from the standard distance) would have M = + 32 and a standard candle 

 + 72.8. 



Mean distance earth to moon = 60.2678 terrestrial radii. 



= 384,411 kilometers = 238,862 miles. (See Table 834.) 



Mean distance earth to sun = 149,500,000 kilometers = 92,900.000 miles. (See As- 

 tronomical unit.) See Table 833. 



Month. — Sidereal = 27.321661 days, synodical (ordinary) =29.530588 days (Brown). 



Nutation constant (periodic motion of celestial pole) =9.21", conventional value; 

 9.207'.' Principal in long = A <t> = (—17.234" — .017" T) sin fi; principal term in obliq- 

 uity = A e = (+ 9.210 + .0009 T) cos fi (Newcomb). T centuries from 1900. 



Obliquity of ecliptic = 23°27'8.26" — 0.4684 (t — 1900)" (Newcomb). 



(continued) 



SMITHSONIAN PHYSICAL TABLES 



