The "parallax effect" sounds like the title of a sci-fi
movie. Read on, and see what it really is.

Draw vertical lines at 1 inch intervals on a chalkboard.
Number the lines. Hold a pencil upright at arm's length and line the pencil up with one of the lines. Without moving the pencil or the observer's head, look at your pencil and line it up with one of the lines -- first with one eye and then with the other -- and note how many vertical lines it seems to shift.

The apparent shift of the position of the pencil is called
the parallax effect.

Check the effect when the pencil is closer or further from
your eyes to see if the parallax effect is greater or
smaller.

Now fasten a pencil upright on a desk. On another desk
further from the chalkboard, fasten two more pencils in
upright positions 6 inches apart. The added pencils can be
used as sighting points.

Sight on the single pencil from one of the added pencils, and note which vertical line it is near. Next, sight from the second added pencil, and record the parallax effect (the number of vertical lines the pencil seems to shift). Repeat the procedure with the pencils 12 and 18 inches apart.

From these experiences, you will find that two factors
influence the parallax effect:

1) the further the object from the observer, the smaller the parallax, and

2) the greater the distance between sighting points, the
greater the parallax.

Astronomers can measure the distance to the moon in a similar way. The stars that are billions of miles away provide the background (much like the stationary vertical lines on the chalkboard).

Observers on opposite sides of the earth sight on the moon
(much like the sightings from the two pencils) in relation to the stars.

Astronomers have found that there is a difference of 1 degree between the background star and the moon for each observer; therefore, the base angle for each is 89 degrees, and the parallax angle for the moon is 2 degrees.

Draw this to scale by drawing a line that is 1 yard long,
making a 2 degree angle at one end, and continuing this line until the two lines are exactly 1 inch apart (about 30
inches). Since this inch represents the diameter of the
earth (about 7,900 miles), the scale is 1 inch = 7,900
miles. From this, calculate the distance to the moon.

The encyclopedia defines parallax as "any alteration in the
relative apparent positions of objects produced by a shift in the position of the observer. Stellar parallax is the
apparent displacement of a nearby star against the background of more distant stars resulting from the motion of the earth in its orbit around the sun; formally, the parallax of a star is the angle at the star that is subtended by the mean distance (1 astronomical unit) between the earth and the sun."

"A star's distance (d) in parsecs is thus the reciprocal of its parallax (p) in seconds of arc (or d = 1/p). Geocentric parallax, used to determine the distances of solar system objects, is measured similarly; the diameter of the earth, rather than that of its orbit, however, is used as the baseline."