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Measurements and Terms


A number of terms are used which may not be familiar to the student. Hopefully, the material on this page will help.


Inches and feet -- these are familiar to Americans, but not used as much around the world or scientifically. There are twelve inches in a foot and three feet in a yard. It takes 5280 feet to make a mile. This system is based on the number 12 -- a yard is 3 x 12 inches, or 36 inches. A mile is 440 x 12 feet, etc.

The metric system -- this is the system much more widely used around the world and in science. It is based on the number 10.

one meter is about 3.28 feet (a little more than a yard. A yard is 36 inches and a meter is 39.4 inches)

one centimeter is 1/100 of a meter (think of the width of two pencils, side by side for an approximation)

one millimeter is 1/1000 of a meter (think of an ink dot on a paper for an approximation)

going the other way, one kilometer is 1000 meters (about 5/8 of a mile)

Astronomical Units -- An astronomical unit is abbreviated to "AU." One AU is the distance from the earth to the sun, or 93,000,000 miles. In our solar system, we define distances in terms of astronomical units.

But that is WAY too small a distance unit when we get out of our solar system. That is when we start to talk about "light years."

Light Years -- because of the word 'year,' a light year sounds like it must be a unit of time, but it isn't. It is how far light can travel in one year. At its present speed, light can travel 186,000 miles PER SECOND! So 186,000 miles is one "light second." One light year is then that distance times sixty second in a minute, times sixty minutes in an hour, times 24 hours in a day, times 365 days in a year. So one light year is 5.87849981 × 1012 miles. or about 5,880,000,000,000 miles. We are 28,000 light years away from the center of our own galaxy. THAT is why we speak in terms of light years and not miles. The term "light year" is often abbreviated LY when talking about astronomical distances.

A parsec is a distance of 3.26 light years. 

Star Magnitudes (Brightness)

The lower the number, the brighter the object. The magnitude system dates back to about 2000 years ago in Greece and possibly earlier. Stars were classified by their apparent brightness, which they considered the same as size, as well as distance, at that time. A very bright star such as Sirius would then be thought of as much larger and/or closer than many others. Those that were bright and considered closest were the first magnitude stars and the fainter stars, which would be considered either farther away or smaller (or both) went up to the sixth magnitude. So stars were classified according to six magnitudes.

In 1856 the system was refined with the use of early photometers. Magnitude 1 was then defined as being 2.5 times brighter than a magnitude 2 star, which was then 2.5 times brighter than a magnitude 3 star, and so on. This resulted, mathematically, in a difference in brightness of 100 between a magnitude 1 and a magnitude 6 star.

As it stands, currently, the bright star Arcturus has been assigned a magnitude 0 classification. Sirius is now a -1.46, being brighter than Arcturus. Using this method, our sun has an apparent magnitude of -27. A full moon is -13, and Venus, the brightest object inthe sky, can get as bright as magnitude -5. Jupiter, on a clear night, is -3.


There are three temperature scales: Fahrenheit, Celsius, and Kelvin. They are each different.

Fahrenheit is used in the United States primarily. It is formulated by a German physicist Daniel Gabriel Fahrenheit in the early 18th century. In the winter of 1708-1709, he chose the coldest air temperature in the city of Gdansk. He defined that as zero. He defined 100 as what he considered to be the "core body temperature" of a human being. More recently his temperature scale was redefined more precisely so that 32 was freezing point of water and 212 the boiling point. This was approximately what he had had.

Celsius -- which used to be called "centigrade." This is based on 0 for the freezing point of wate and 100 for its boiling point.

Kelvin -- In this temperature scale, 0 is absolute zero, which means the point at which there is no temperature radiation at all. At this cold a temperature there is very little movement on the atomic level. On this scale, water freezes at 273.16 and water boils at 100 degrees more, or 373.16 degrees.

The Kelvin scale is used in science, but not in weather reporting. Celsius is used both in science and weather reporting/medical uses around the world. Fahrenheit is used only in a few places and only for weather reporting and medical uses.

In both Celsius and Kelvin, there is a hundred degree difference between the point at which water freezes and that at which it boils. That means that they each judge one degree to be the same amount of temperature difference. Fahrenheit has 180 degrees between water's freezing and boiling points, so the degrees on the Fahrenheit scale measure much smaller increments of temperature than the other two do.

This makes converting one scale to another a little difficult, but there are conversion tables on the net.


"Velocity" is defined as the rate of motion in a straight line. The straight line part is important. This makes velocity different from speed, which doesn't care which direction it is going in or any change of direction that might occur. Both are measured in the same units: miles per hour or kilometers per hour; miles per second, kilometers per second, etc. In astronomical considerations, it is more normal to talk about speeds per second.

You can drive a circular race track at 60 miles per hour, and that is your speed. But it is not your velocity, because you are going in a curve.


Mass means how much stuff is in something. We often refer to it as weight if we are talking about something on Earth. It can be measured in tons, pounds, ounces, grams, kilograms, etc. (1 kilogram = 2.205 pounds)


Volume is how big something is in all three dimensions: width, height and depth. Volume is measured in cubic units, either in the feet/inches way or in the metric way.


Technically and mathematically, density equals mass divided by volume. In easy to understand terms, it refers to how tightly packed the material is in that particular volume. (The density of water is assigned the number "1" and other densities are then scaled accordingly.)

Atmospheric Pressure

Atmospheric pressure is measured in 'bars,' a metric unit. Air pressure at sea level on earth is about 14.5 pounds per square inch. That is considered one bar of atmospheric pressure.

Some Other Terms

Rotation Rate-- The time it takes an object to go around once on its axis. The Earth's rotation rate is one day.

Revolution -- The time it takes an object to go around another object one time in its orbit. The Earth takes one year to go around the sun one full time, so its revolution rate is one year.

Sidereal period -- The time required for a body within the solar system to complete one revolution with respect to the fixed stars (in other words, as seen from some point outside the solar system)

Synodic period -- The time required for a body within the solar system to return to the same, or approximately the same, position relative to the sun as seen by an observer on Earth.

Example for sidereal vs. synodic: When the moon goes around the earth, we see it from earth, in its synodic period, as requiring the time from one full moon to the next. This is actually 29.5 days. But the sidereal, or true, time is 27.3 days. Why the difference? Because the earth is moving around the sun and its movement means there is a slight difference in the relative positions of the three bodies (sun, earth, moon) between different times. A very good animation showing what happens is here.

Gravitational Lensing -- According to Einstein, a massive object in space should exert enough gravitational pull to actually bend light rays. In this way the object is acting like a giant lens. As a result, we would 'see' the object where it is not, because we are accustomed to viewing things in a straight line. Here is a diagram of what happens:

gravitational lensing

Often, as shown above, we should be able to see multiple images, none of which are where the object is.

Here is a photograph of what is seen:

lensed quasar

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