In order to help standardize
the mathematics used in science, setting rules on
measurements is key. The failure of the Mars Climate
Orbiter in 1999 was the result of confusion over
what standard unit of measure was to be used.
There are 4 categories in which standard of
measure are applied: time, distance, mass and
temperature. At the bottom of the page are some
conversions:
Time:
Units of time is a simple matter, things occur in
a linear fashion and so a value of time is assigned.
Units of measure are: seconds, minutes, hours,
years, and so on. There is no "metric" or "English"
units of measure in time; however, there can be some
confusion as time coordinates are a part of the
equatorial measure of right ascension. For example,
the right ascension (RA) coordinate of the
star Vega
is 18 hours and 38 minutes. That has nothing to do
with a phenomenon that occurred at hour 18 and 38
minutes.
Time in relation to Relativity is another matter  covered in the Cosmology section.
Distance:
In astronomy, distances can be very large. While
on Earth we measure distance (and dimensions for
that matter) in either centimeters, meters, and
kilometers for the metric system and inches, feet,
and miles for the English system, distances to
distant galaxies is given in lightyears.
As a rule of thumb, science tends to lean more
towards the metric system so variations on the meter
(kilometer, centimeter, millimeter, and so on) are
common place. Measure of lightyears and parsecs do
not suffer the same confusion, but the lightyear
seems to be more popular than using the parsec.
For very large distances, there are really three
choices: astronomical units, lightyears, and
parsecs (for really distant galaxies  like the ones near the edge of the observable Universe, it is common to give distance in "redshift").
Astronomical Unit (AU) 
1.5x10^{11} meters 
LightYears (ly) 
9.5 x 10^{15} meters (63,240 AU) 
Parsecs (pc) 
3.1 x 10^{16} meters (206,265 AU) 
An astronomical unit is the mean distance between
the Earth and the
Sun.
A lightyear is the distance light travels in 1
year  ~300,000 m/s.
A parsec is the distance a nearby object, 1 AU
away "subtends" to one arcsecond. In English, this
means that if an observer were far enough away from
our
Sun so that the EarthSun distance were only 1
arcseconds across, the observer would be 1 parsec
away (a
parallax is the apparent shifting of an
object due to perception. Example: hold a pen at
arms length and alternate closing one eye at a time
 the apparent shift of the object is called a
parallax).
Also, for comparison purposes: 1 parsec = 3.26 ly.
Conversions:
1 inch = 2.54 cm
1 foot = 0.3048 meters
1 mile = 1.609 kilometers
1 micrometer = 1
µm = 10^{6}
meters
1 nanometer = 1 nm = 10^{9} meters
1 minute = 60 seconds
1 hour = 3600 seconds
1 day = 86,400 seconds
1 year = 3.156 x 10^{7} seconds
1 km/s = 10^{3} m/s
1 m/h = 0.447 m/s
1 m/h = 1.47 ft/s
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