Class 1 : General notes & Distances Measuring
General notes:
What is a star?
What is a planet?
What is a galaxy?
The universe ..
What are the constellations ?
Measuring the radius of the earth:
5,000 stadia  800km
Ship at sea:
x
x
d
d
r
r
r
Some useful distances:
R = 6.4x108 cm
R = 6.96x1010 cm
Rgallaxy = 3x1022 cm
1 A.U. = 1.5x1013 cm
1 pc = ? cm
1 ly = ? cm
1 radian 
1" 
180

 2.063  105 "
1

 1arc sec.(in radians )
3600sec 180
Parallax method:
By measuring the shift across the sky (in arc. Sec.) and knowing the distance the earth
has moved around the sun, we can calculate the distance to the star.
Ө
Example
Proxima Centauri, the nearest star (other than the sun), has an observed shift in position
of 0.76 seconds of arc, much too small to be measured without a telescope.
d
d
R
sin(  rad )  
1AU
 4  1018 cm
0
.
77





 3600 180 