What is the rate of change in longitude per year, in centimeters, for ZGN-6 between 1990 and 2010?

• A. 3.1
• B. 4.1
• C. 5.1
• D. 2.1

Answer: (A)

Longitude drift is read as an angular change over time, but the question asks for a linear rate, so you convert that angular change into a distance measured along the circle of latitude and then divide by the time interval.

First find how much longitude changed from 1990 to 2010, then divide by 20 years to get degrees per year. To turn that angular rate into a linear speed at the Earth's surface, use the length of one degree of longitude at ZGN-6’s latitude, which is (π/180) × R × cos(latitude). Multiply this distance per degree by the degrees-per-year you just computed to get the surface distance moved per year, and convert to centimeters.

Doing this with ZGN-6’s data for that 20-year span gives a small, east–west drift on the order of a few centimeters per year. The resulting rate is about 3.1 cm per year, which is the value that matches the observed change when you account for the latitude factor and the 20-year interval.