Nitration of naphthalene and anthracene
In the electrophilic substitution of polycyclic aromatics, when drawing resonance structures keeping as many benezene rings intact as possible is important.  Note too that a naphthalene ring isn't as "good" as two separate benzene rings. 

Look at the structures below, electrophilic attack at the 1 position in naphthalene (top row of drawing) allows you to draw 2 resonance structures with a benzene ring remaining intact (4 if we count structures with the double bonds simply shifted in the intact benzene ring); attack at the 2 position (middle row in figure) only has 1 resonance structure (2 if we count structures with the double bonds simply shifted in the intact benzene ring) with a full benzene ring intact.  Hence, electrophilic attack at the 1-poisition is preferred in naphthalene.

Applying the same considerations to anthracene, we see that only attack at the central ring position allows 2 full benzene rings to remain intact in the possible resonance structures. Consequently electrophilic attack at anthracene's 9-position is preferred.