Beat frequency of strobe lights

[begbeg42]

Homework Statement

The strobe lights on top of two tall radio towers both have a period of about 1.00 s. When turned on simultaneously, a keen observer notes that during an 813 s interval, the lights gradually fall out of synchronization and then come back into flashing synchronously. What is the interval ΔT between the periods of the two beacons?

Homework Equations

abs(frequency1-frequency2)=beat frequency

The Attempt at a Solution

it would appear that the two strobe lights have the same frequency, frequency=1/period=1 Hz...I don't know what to make of the 813 interval or what exactly delta-T refers to physically

I know the beat frequency involves the superimposition of these two light waves...

 

[begbeg42]

i have
299,792,458 m/s=speed of light=frequency(wavelength)
is this relevant?

also is the frequency in this time interval 813 Hz? the period is 1 s i.e. the light completes an oscillation every second so in a 813 s time interval there 813 oscillations?

i know that initially f1-f2=0
then abs(f1-f2) doesn't = 0
then it does again...
however I am not sure what delta-T refers to/asks for... and not sure how to work this

 

[begbeg42]

nothing? help please!