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Home , Barye, Dyne, Electrostatic units, Erg, Gram, Statcoulomb

The centimeter-- system (CGS) is a system of physical units.

Mechanical CGS units

Dimension Unit Definition SI

length centimeter 1 cm 10−2 m

gram 1 g 10−3 kg

time second 1 s 1 s

1 dyn = 1 g·cm/s² 10−5 N

1 erg = 1 g·cm²/s² 10−7 J

power erg per second 1 erg/s = 1 g·cm²/s³ 10−7 W

1 Ba = 1 dyn/cm² = 10−1 Pa 1 g/(cm·s²)

While for most units the difference between cgs and SI are just powers of 10, the differ- ences in electromagnetic units are more involved. In SI, is defined via the magnetic force it exerts and charge is then defined as current multiplied with time. In the cgs system, (esu), charge is defined via the force it exerts on other charges, and current is then defined as charge per time. One consequence of this ap- proach is that ’s law does not contain a constant of proportionality. What this means specifically is that in cgs electrostatic units, the unit of charge or , is de- fined as such a quantity of charge that the Coulomb force constant is set to 1. That is, for two point charges, each with 1 statcoulomb spaced apart by 1 centimeter, the electrostatic force between them will be, by definition, precisely one dyne. This also has the effect of eliminating a separate dimension or fundamental unit for . In cgs electro- static units, a statcoulomb is the same as a centimeter times square root of dyne. Dimen- sionally in the cgs esu system, charge Q is equivalent to M1/2L3/2T-1 and not an independ- ent dimension of physical quantity.

Therefore: electrostatic unit of charge 1 esu = √(g·cm³/s²) = 3.33564 × 10−10 C and Coulomb’s law is just:

2 F = q1 q2 / r ASTRONOMICAL CONSTANTS AND CONVERSION FACTORS

SI VALUE CGS VALUE

30 33 Mass of sun (Mu) 1.989x10 kg 1.989x10 g 8 10 Radius of sun (Ru) 6.959x10 m 6.959x10 cm 26 33 -1 Luminosity of sun (Lu) 3.826x10 W 3.826x10 erg s 24 27 Mass of earth (Mr) 5.976x10 kg 5.976x10 g 6 8 Equatorial Radius of earth (Rr) 6.378x10 m 6.378x10 cm Astronomical unit (au) 1.496x1011 m 1.496x1013 cm Parsec (pc) 3.086x1016 m 3.086x1018 cm Tropical year 3.156x107 s 3.156x107 s

PLANETARY DATA Planet Semi-Major Axis Sidereal Period Orbital M/M R/R ¾ ¾ (au) (tropical yrs) Eccentricity Mercury .3871 .24085 .2056 0.0558 0.382 Venus .7233 .61521 .0068 0.8150 0.949 Earth 1.000 1.000039 .0167 1.0000 1.000 Mars 1.5237 1.88089 .0934 0.1074 0.532 Jupiter 5.2028 11.8622 .0483 317.893 11.27 9.538 29.4577 .0560 94.147 9.44 Uranus 19.191 84.013 .0461 14.54 4.10 Neptune 30.061 164.793 .0097 17.23 3.88 Pluto 39.529 248.54 .2482 .0022 0.180 SOLAR DATA

Apparent bolometric magnitude, mbol: -26.82

Apparent visual magnitude, mV (or V): -26.74

Absolute bolometric magnitude, Mbol: + 4.75 B - V colour index: + 0.65 Spectral Type and Luminosity Class: G2 V

Effective Temperature (Teff): 5770 °K

MISCELLANEOUS Hubble's Constant: 75 km s-1 Mpc-1 ASTRONOMY DATA SHEET UNITS Physical Quantity SI Unit CGS Value Mass 1 kg 103 g Length 1 m 102 cm Time 1 s 1 s Energy 1 (J) 107 erg Power 1 (W) 107 erg s-1 Force 1 (N) 105 dyne Velocity 1 m s-1 102 cm s-1 Pressure 1 (Pa) 10 dyn cm-2 1 kg m-3 10-3 g cm-3 Electric Charge 1 Coulomb (C) 2.998x109 esu Magnetic flux density 1 (T) 104

PHYSICAL CONSTANTS AND CONVERSION FACTORS SI Value CGS Value , c: 2.998x108 m s-1 2.998x1010 cm s-1 Gravitational constant, G: 6.670x10-11 N m2 kg-2 6.670x10-8 dyn cm2 g-2 , h: 6.626x10-34 J s 6.626x10-27 erg s , k: 1.381x10-23 J K-1 1.381x10-16 erg K-1 Electric charge, e: 1.602x10-19 C 4.803x10-10 esu Stefan Boltzmann constant, F: 5.669x10-8 W m-2 K-4 5.669x10-5 erg cm-2 K-4 s-1 Radiation density constant, a: 7.565x10-16 J m-3 K-4 7.565x10-15 erg cm-3 K-4 Mass of electron 9.1096x10-31 kg 9.1096x10-28 g Mass of proton 1.6726x10-27 kg 1.6726x10-24 g Mass of neutron 1.6749x10-27 kg 1.6749x10-24 g Mass of hydrogen atom 1.6735x10-27 kg 1.6735x10-24 g Mass of helium atom 6.6464x10-27 kg 6.6464x10-24 g 1 amu 1.66053x10-27 kg 1.66053x10-24 g 1 ev 1.602x10-19 J 1.602x10-12 erg 1 A 10-10 m 10-8 cm Wien's law constant 2.898x10-3 m K 0.2898 cm K