The Science Guys

May 2001

What is the Astronomical Unit?

When we make a measurement, we use a number and a name called the unit of the measurement. We weigh 150 pounds; we buy 20 oz of a liquid; it is 150 km to the next town; we are going 50 mph; and so forth. There is always a number and some unit (a group of words). Long ago the human body was used as a reference. A cubit was the distance from the elbow to the tip of the index finger; the foot was the length of a person’s foot; a fathom was the length of a man’s outstretched arms. Obviously this varied from person to person and there was little standardization. In 1670 a Frenchman, Gabriel Mouton, proposed standardizing the length to relate to a physical measurement of the Earth. Thus began the decimal metric system. We have continued to refine and change our units, seeking to use those with more permanence. In 1875 the "Treaty of the Meter" was signed by the major countries of the world (including the United States), making the meter the official world standard for length.

Whenever we make a measurement, whether in science or everyday life, we choose a unit appropriate to the scale of the object being measured. You would measure the length of a bug in inches, or the distance to Memphis in miles. You could measure insects in miles and highway distances in inches, and be technically correct. But it’s awfully cumbersome to say that a beetle is 0.000016 miles long or that Memphis is 5 069 000 inches from Jackson.

As we measure larger and larger sizes, different units are applied. The solar system is extremely large and the mile is just too small of a unit to use for measuring interplanetary distances. Instead scientists devised another unit, called the astronomical unit (AU), which is convenient to use for making measurements within our solar system. One astronomical unit is defined as the average distance from the Earth to the Sun, approximately 93,000,000 miles. Then if we say an object is 35 AU from the Earth we mean it is 35 times as far from the Earth as the Earth is from the Sun or 35 times 93 million miles.

In reading astronomy people encounter the unit called a Light Year, which is the distance light travels in one year (about 6 trillion miles). You might wonder why we don’t use this unit when discussing the solar system. The Light Year is about 64,500 times larger than the Astronomical Unit, too large to be appropriate for an object the size of our solar system. The Light Year is fine for measuring distances to stars or other galaxies but not for measuring distances within our own solar system. An object 35 AU from Earth would only be a few thousandths of a Light Year away. Thus the Light Year is just not a practical unit for our solar system.

Astronomers use another distance unit, the parsec, which represents 3.26 light years or about 20 trillion miles. It is more difficult to understand and is linked to the measurement of angles and geometry. Nearby stars shift their position relative to those that are much farther away. That shift is measured as a tiny angle, and is correlated to the distance from the earth to the star. You can demonstrate the angular shift, or parallax, yourself. Place a finger directly in front of your nose. Then open and close each eye, one at a time, and you’ll notice your finger shifting relative to objects at the back of the room.