The system works the same for very small numbers: write the number as a multiplier and a power of ten with a negative exponent. The number 0.005 would be expressed as 5.0 x 10^{3}. Here
we count the places to the right of the decimal point in order to determine the exponent.
B. The Scale of Things: Astronomical Distances
Astronomers use much the same measurements as do other scientists. Distances are measured in kilometers (abbreviated km); mass is measured in kilograms (abbreviated kg
), and time is measured in seconds (abbreviated s). Because of the great distances covered in space, however, other units of measure have been devised.
The astronomical unit (abbreviated AU) is used when measuring distances within our Solar System. One AU is equal to the average distance between the Sun and Earth, so,
1 AU = 1.496 x 10^{8 }km = 93 million miles
The light year (abbreviated ly) is used when measuring distances outside our Solar System. One ly is equal to the distance light travels in one year or 9.46 x 10^{12} km (9,460,000,000,000 km in long hand). The star closest to the Earth, Proxima Centauri is approximately 4.3 ly away.
The parsec (abbreviated pc) is the final distance measurement we’ll discuss. This is used to measure the farthest distances. One parsec is equal to 3.26 ly (3.09 x 10^{13} km). The distance to Proxima Centauri is 1.32 pc.
Let’s put it all together once more from astronomical unit to light year to parsec and back.
1 AU = 1.58 x 10^{5} ly = 4.85 x 10^{6} pc
1 pc = 3.26 ly = 206,265 AU
By now you should appreciate the use of scientific notation instead of long hand for writing astronomical distances. Keep in mind that for even greater distances you can use the
prefixes kilo and mega, meaning 1000 and 1,000,000, respectively (one example of this is the distance from Earth to the center of our galaxy, that’s equal to 8 kiloparsecs).
C. Lost (and Found) in Space: Astronomical Coordinate Systems
Ancient civilizations believed the stars, planets, and other heavenly bodies existed at the same distance from Earth, a somewhat twodimensional approach. All of these points of
light were thought to rest on a transparent sphere, with Earth at its center, and was referred to as the “celestial sphere”. While we are aware that space is threedimensional, we still
use the idea of the celestial sphere as a means of charting the night sky.
In order to construct a meaningful coordinate system (sometimes referred to as a “referent system”) a few more details are added. The celestial equator is a projection of
Earth’s equator onto the sphere, and the celestial meridian is a projection of Earth’s meridian onto the sphere. The celestial meridian is an imaginary line drawn from the celestial north pole, through a point overhead referred to as the zenith, then to the celestial south pole.
Remember, the poles are the points of rotation for Earth.
Now, let’s add the finishing touches. By drawing a series of lines, both northsouth and eastwest, we’ve added what appear to be lines of longitude and latitude (refer to a globe
of Earth to refresh your memory) to our celestial sphere. The lines of latitude are declination and the lines of longitude are right ascension. These are the references we
utilize when locating objects in space. (See illustration below.)
One more detail and our system will be complete. Declination (abbreviated D) is measured in degrees, arcminutes, and arcseconds. For example, 8^{o} 12^{’} 06^{”} is a point of declination and is read as “eight degrees, twelve arcminutes, six arcseconds”. Right ascension (abbreviated RA) is measured in hours, minutes, and seconds. For example, 5^{h } 14^{m} 32.2^{s} is a point of right ascension and is read as “five hours, fourteen minutes, thirty two point two seconds. Putting these two coordinates together gives us the position of the star Rigel in the constellation Orion.
