Math question.... almost finished but uncertain on one part
Ted is watching a boat from the top of a lighthouse. The boat is approaching the lighthouse directly. When first noticed, the angle of depression to the boat is 16°18'. When the boat stops, the angle of depression is 48°51'. The lighthouse is 200 feet tall. How far did the boat travel from when it was first noticed until it stopped? Round your answer to the hundredths place.

Somebody else had helped me with this and I understand it but why the 90°?
200 ∗ (90° − 16°18′) = 683.95
200 ∗ (90° − 48°51) = 174.78
Find difference:
Length = 683.95 – 174.78 = 509.17
Answer: 509.17 meters

Question

Math question.... almost finished but uncertain on one part 
Ted is watching a boat from the top of a lighthouse. The boat is approaching the lighthouse directly. When first noticed, the angle of depression to the boat is 16°18'. When the boat stops, the angle of depression is 48°51'. The lighthouse is 200 feet tall. How far did the boat travel from when it was first noticed until it stopped? Round your answer to the hundredths place.

Somebody else had helped me with this and I understand it but why the 90°? 
200 ∗ (90° − 16°18′) = 683.95
200 ∗ (90° − 48°51) = 174.78
Find difference:
Length = 683.95 – 174.78 = 509.17
Answer: 509.17 meters