Next: Small-angle formula
When an object is viewed from two widely separated positions, the
object appears to move with respect to more distant objects. This is true
for observations on earth and also for observations of stars.
The basic equation for parallax is:
Note that this equation has a different form from Equation 2 
. This
is because the parallactic angle, 
is defined differently from
the angle B in Equation 2 . Note also the factor of 2 in the
denominator which comes from the definition of the parallactic angle given
on page 24.
Example. An object shows a parallax of 
when observed with
a baseline of 100 meters. How far away is this object? First you must
convert the angle from arc minutes to degrees: 
. Now plug these values
into Equation 3 :
Distance to Object = ( Baseline / 2 tan 
P) = (1000 m / 2 tan 0.75 o ) = 1000 m / 0.0262 = 3820 m.
One can also set up a proportionality to solve certain types of problems. If one wished, for example, to compare the parallax of a star when observed from Earth from its parallax when observed from Mars, one would set up and solve a ratio:
The distance to the object cancels out and the baseline in each case is the distance from the planet to the Sun.
Next: Small-angle formula