Hold your arm straight out in front of you and look at your hand against
the background. Close first the right and then the left eye. Watch how
the hand moves relative to the background!
The reason is that you look at the hand from slightly different angles.
The eyes are only a few centimeters apart, but that suffice to give
slightly different viewing angles of the hand.
The same effect can be used to measure really big distances in space!
If we study an object in space from two different locations on the Earth,
for instance Norway and Hawaii, there perspective is a bit different.
How much the object "moves" against the background depends on two factors:
 The diameter of the Earth (that is known quite accurately)
 The distance of the object
The farther away the object is, the smaller the angle (or movement) becomes
when we change our viewing position. The angle is called the parallax
of the object.
This illustration demonstrates the parallax of the Sun:

Imagine looking at the Sun from the North pole and Equator repectively.
Then we see the Sun in slightly different perspectives. The acute angle ASC
is called the parallax of the Sun.
The lenght of AC is equal to the radius of the Earth, which we denote
R. The angle at point A (the North pole) is 90 degrees. If we can
find the parallax (the acute angle), we can therefore easily calculate the
distance CS  the distance from the Earth to the Sun!
For those familiar with trigonometric functions , the solar distance is
CS=R/sin(parallax of the Sun).
Illustration: Astronomy.no 
We can therefore determine the distance to the Sun "just" by measuring
its parallax. Unfortunately, the angle is so tiny (due to the large distance
from the Sun to the Earth) that it is hard to measure. Due to the enormous
brightness of the Sun, it is not possible to see objects in the background
that could have been used as a reference to measure the parallax.
MORE INFORMATION
Major celestial events in Norway 20102015