Units and Distances
How
Distances are Measured in Astronomy
Dealing
with the numbers involved with the distances to the stars or even with those
found in the solar system can be hard going. Astronomers make their lives
easier by using a number of rulers (units of distance) for the distances and
although they have some strange names they can be very useful for comparing
the distances to stars, other galaxies and even the planets in our solar
system.
One AU is
the distance that the average distance that the Earth orbits the Sun at. The
AUis most commonly used for the distances of objects with in our solar
system. Pluto, the last planet in the solar system is found at an average
distance of 39.47 au from the Sun. Sedna the new body nearly as large as a
planet found beyond Pluto is never nearer to the Sun than 76AU and then goes
to 880AU from the Sun in its giant elliptical orbit.
One of
the most common rulers is the light year. The light year is is the distance
that light travel in one year (365 1/4 days). It is most commonly used for
the distances to stars and other galaxies.
The
nearest star is 4.2 light years away from our sun. We are 8.3 light minutes
away form the Sun. the distance to the outer most planet Pluto is about 13
light hours.
Some
other interesting distances in light years:
|
Object |
Distance in light years |
|
Nearest Star (Proxima Centuri) |
4.2 |
|
Sirius the dog star (the brightest star in the sky) |
8.6 |
|
Center of the galaxy |
approximately 30 000 |
|
Andromeda (one of our nearest neighbouring galaxies) |
approximately 2 million |
|
The stars of Orion. (Betelgeuse and Rigel) |
1400 light years |
Astronomers started measuring distances from the amount that a star moves as the Earth goes from one side of the Sun to the other. Try moving your head and you will see that the position of everything around you changes. One parsec is derived from the smallest angle measurement of 1/3,600th of a degree or an arc second that is the angle that a star at this distance would appear to move in 6 months as the Earth journeys around the Sun. A parsec is 3.2616 light years or 30,857,000,000,000 km.. Two parsecs is 6.5532 light years or twice the distance, it is not a measure of change in angles of the stars.
Below is
a conversion table for some useful astronomical distance units
Kilometers (km)
Astronomical units (AU)
Light Years (l.y.)
Parsec (pc)
kilometers (km)
1
149.6 million
9,460,000,000,000
30,857,000,000,000
Astronomical units (AU)
0.0000000067
1
63,240
206,263
Light Years (l.y.)
0.00000000000011
0.000016
1
3.2616
Parsec (pc)
0.000000000000033
0.0000048
0.3066
1