Some practical units of length, mass, and time

Length	Mass	Time
Light year   = distance travelled by light in one yar in vaccum. 1ly =9.46 x 10 ¹⁵m 1 astronomical unit (A. U.)   = 1.5 x 10 ¹¹m 1parsec  = 3.26 ly         =3.08x10¹⁶ m 1 nautical mile or seamile             = 6020 ft. 1 micron  = 1μm  = 10⁻⁶ m 1 angstron (A˚) = 10⁻¹⁵ m	1 quintol = 10² kg 1 metric ton = 10³ kg 1 atomic mass unit (amu) or Dalton  = 1.66 x 10⁻²⁷ kg 1 slug  = 14.59 kg 1pound = 0.4537 kg 1 Chandrashar limit    = 1.4  times the mass of sun   = 2.8 x 10³⁰ kg	1 solar day   = 86400 sec. 1 year = 365 ½ solar days 1lunar month = 27.3 solar days, Tropical year = It is the year in which total solar eclipse occurs. Leap year = it is the year in which the month of February is of 29 days.

Prefixes used in metric system

Prefix	symbol	Multiplier	Prefix	Symbol	Multiplier
deci	d	10⁻¹	deca	da	10¹
centi	c	10⁻²	hecto	h	10²
milli	m	10⁻³	kilo	k	10³
micro		10⁻⁶	mega	M	10⁶
nano	n	10⁻⁹	giga	G	10⁹
pico	P	10⁻¹²	Tera	T	10¹²
femto	f	10⁻¹⁵	peta	P	10¹⁵
atto	a	10⁻¹⁸	exa	E	10¹⁸
zepto	z	10⁻²¹	zetta	Z	10²¹
yocto	y	10⁻²⁴	yotta	Y	10²⁴