The resistor is require for matching the two LDR (East west or North South) so that when the sun is exactly at the top of both they both (LDR) gives same output. (No two LDR's are identical they always give you slight difference in the Dark and Light conditions):

Following Formula was used:

LDR resistance(Ohms) = SQRT ( Rdark x Rnoon)

Where Rdark = Resistance of the LDR in night (No light condition) =247 Ohm (in my case)

Rnoon= Resistance of LDR at high noon (direct under the sunlight) =39000Ohm (in my case)

LDR resistance = SQRT (247 x 39000)

= 3101 Ohm = 3.1 ohm

=3.2 is the nearest available resistance

you can read my blog:

http://my-diysolarwind.blogspot.com/