light-second light-year parsecs
Astronomical Units (A.U.)

Astronomical Distance megaConverter #39

Introduction and Overview
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Units of astronomical measure are different from common units and standards in that they refer primarily to things not of this earth. The size of outer space is so large that terrestrial units are insignificant . There are really only four units in wide use today, although as humans continue exploring space there will probably be more coined.

Humans are just beginning to explore space. As we move away from earth the units coined here will be less meaningful. No doubt new ones will be made that describe physical features in whatever new place we land. If the units are not needed to describe some physical phenomenon , then we already have a built in system capable of creating new units, called the metric system. 150 Gigameters is about an A.U. Ten Terameters is approximately a light-year. A parsec is about 32 Terameters. A Petameter is about 31 parsecs. Many other distance units can also be developed in the same fashion.

See converters #2 & #26 & #27 for standard conversions or ancient eras or foreign countries. For a more comprehensive treatment of measurements, find "NTC’s Encyclopedia of International Weights & Measures" by William D. Johnstone at your local library. For a better discussion of temperature, see any college physics textbook.

* Much of our written history still refers to things in common units. The Bible does not refer to meters or kilograms, but to cubits and stadia, or shekels and drachma. Wouldn't it be nice to know what they were talking about way back then? Now you can use megaConverter! For a more complete listing of ancient, foreign, and obsolete measures, download our 'megaSpreadsheet' of conversions in MS Excel format.

Glossary of Conversions:

The most common and useful is the light-year. This is the distance a ray of light will travel through vacuum in one standard earth year.

This is closely related to a light-year, and is the distance light will travel through vacuum in one second.

Astronomical Unit
Another unit commonly use is the Astronomical Unit or AU. This is the average distance from the earth to the sun. It is commonly used in measuring things the size of solar systems.

The last common unit of astronomical measure is the parsec. This unit is important because it was the basis of the method for determining the distance from earth to the nearer stars. Parallax is the feature of observation that when viewing an object from varying viewpoints, features in the background of the object tend to move with respect to the object. Hold your finger up in front of your face and briefly close one eye and then the other. You will note that objects in the background of your finger will seem to shift position. This is parallax. If you can determine the angle that the background objects shift and you know the distance between your eyes, you can calculate by geometry the distance to your finger. Likewise, if we use the earth’s orbital radius as the distance between our "eyes" we can measure much greater distances. Astronomers do this with the earth’s orbit and use 1 A.U. as the known distance. They then view the same star three months apart and measure the change in the background angle. A person can calculate that a parallax of one second of angle with a baseline length of 1 A.U. will yield a distance to the observed object of about 3.26 light-years. This distance is known as a parsec.

Note: Because of round-off errors, converting from very large units to very small units or vice-versa may not be accurate (or practical). Conversion factors can be found by converting a quantity of 1 unit to another unit several steps above or below the first. You may need to string several conversion factors together to find the factor from a very large unit to a very small unit, and then you can use a calculator with sufficient digits to find your answer.