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 light-years.

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 light-years and parsecs do not suffer the same confusion, but the light-year seems to be more popular than using the parsec.

For very large distances, there are really three choices: astronomical units, light-years, and parsecs (for really distant galaxies - like the ones near the edge of the observable Universe, it is common to give distance in "redshift").

An astronomical unit is the mean distance between the Earth and the Sun.

A light-year 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 arc-second. In English, this means that if an observer were far enough away from our Sun so that the Earth-Sun distance were only 1 arc-seconds 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 10⁷ seconds
1 km/s = 10³ m/s
1 m/h = 0.447 m/s
1 m/h = 1.47 ft/s

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