Timeline Questions re Snape's and Trewlaney's Appointments

[gelite67]

No: HPFGUIDX 150914

In OOP, Harry turns 15; Trewlawney said she'd been teaching for 16 
years; and Snape said he'd been teaching for 14 years.

So, Trewlawney started teaching the year before Harry was born?  And 
Snape the year after?  

I'd always thought that Snape started teaching right about the time 
of LV downfall, when Harry was one.  Is that right?  

So, we have a two-year gap b/w ST's appointment and Snape's?  We know 
that Snape overheard the first prophecy during ST's interview and we 
know that Snape "defected" to DD's side before LV's downfall, but how 
long before?  I can't believe DD caught Snape eavesdropping two years 
before and just let him go -- I guess I don't understand what Snape 
was doing during the time b/w the eavesdropping and his own 
appointment.  

I mean, I know he was busy being a DE, and at some point, a spy for 
DD, but I'm not clear on what happened when. 

I also wonder if Snape would have been appointed to HW had DD not 
caught him eavesdropping and had LV not taken a turn for the worse in 
GH.  (I assume even DEs need paying jobs.)

Also, why was Snape eavesdropping in the first place?  Probably on 
LV 's orders, but why?  ST didn't have a rep for being a successful 
seer, after all.

Angie (who's never been good with "the math" and hoping this makes 
sense)