Measurements
Astronomers frequently need to measure very long or very short distances. These distances are not commonly used for everyday civil activities. They also need to measure mass instead of weight because of the dependency on earth for the measure of weight. For convenience, the metric system is included here along with the less commonly used measures.
The metric system is the most natural system to use with astronomy. It was originally created by a group of French scientists in the 1790s and has been revised several times. The official name of this system is the International System of Units. The metric system has been accepted by most countries.
Abbreviations
The metric system contains many different types of measure. The abbreviations for these are listed below. They are explained in the next section of this page.
| Symbol | Measure |
| m | meter |
| l | liter |
| g | gram |
| t | metric ton |
| C | Celsius |
The metric system can be used to describe very large or small units of measure. Prefixes can be added to most units of measure to increase or decrease its size. One exception is Celsius, the metric system’s measure of temperature which never has a prefix. The abbreviations are listed below with their meaning.
| Symbol | Prefix | Increase or Decrease in Measure | Name of Number |
| E | exa | 1,000,000,000,000,000,000 | One quintillion |
| P | peta | 1,000,000,000,000,000 | One quadrillion |
| T | tera | 1,000,000,000,000 | One trillion |
| G | giga | 1,000,000,000 | One billion |
| M | mega | 1,000,000 | One million |
| k | kilo | 1,000 | One thousand |
| h | hecto | 100 | One hundred |
| da | deka | 10 | Ten |
| d | deci | 0.1 | One tenth |
| c | centi | 0.01 | One hundredth |
| m | milli | 0.001 | One thousandth |
 |
micro | 0.000 001 | One millionth |
| n | nano | 0.000 000 001 | One billionth |
| p | pico | 0.000 000 000 001 | One trillionth |
| f | femto | 0.000 000 000 000 001 | One quadrillionth |
| a | atto | 0.000 000 000 000 000 001 | One quintillionth |
For example, mm means millimeter and is one thousandth of a meter.
Length
The metric system is based on a unit of length called the meter. The meter is slightly longer than a yard and is divided into small increments. For conversion purposes, 2.54 centimeters is equal to one inch in the U.S. customary system.
The most common variations of this length are the millimeter, centimeter, meter, and kilometer.
Surface Area
The surface area of an object can be measured by using the length measurements squared. A square meter is a large square which is one meter long by one meter wide.
Volume
The volume of an object is the space the object occupies or encloses and is measured in liters or any length measurement cubed. A cubic meter is a box which is one meter high, one meter wide, and one meter long. A liter is equal to one cubic decimeter. One cubic centimeter is equal to one milliliter.
Mass
The mass of an object is the amount of matter the object contains. It is measured in grams. A gram is approximately equal to the mass of one cubic centimeter of pure water at 4 degrees Celsius. Sometimes grams are used for weight, but weight and mass are only equal at sea level on earth. A metric ton is equal to 1000 kilograms.
Temperature
The temperature of an object is the measure of how hot it is. Celsius is used in the metric system. At sea level, water freezes at 0 degrees Celsius and boils at 100 degrees Celsius.
Some people have a need to measure temperature in relation to absolute zero, the lowest temperature possible. This scale is known as the Kelvin scale. Absolute zero is 0 degrees Kelvin or -273 degrees Celsius. To convert a temperature from Celsius to Kelvin, add 273 degrees.
Astronomers have a need to measure extremely large or small distances. Some of these units are explained here.
An astronomical unit (AU) is the average distance from the earth to the sun. It is equal to about 149,597,870 kilometers.
Angstroms are used to measure the length of light waves or other electromagnetic waves. One angstrom is equal to 0.000,000,1 millimeters.
Light years are used to measure extremely large distances. One light year is the distance a beam of light travels through a vacuum in one year. It is equal to about 9.46 trillion kilometers. The speed of light through a vacuum is 299,792 km per second.
One parsec is equal to 63,290 AU or 9.461 trillion km. This is used for parallaxing measurements and is not used extensively in this site.
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