For an orbit that is close to circular, the formula for calculating the mean orbital speed is:

1.) v = 2(pi)*r / T

where v is velocity, r is the length of the radius, and T is the time of orbit. Since we're calculating in units of seconds, we must...

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For an orbit that is close to circular, the formula for calculating the mean orbital speed is:

1.) v = 2(pi)*r / T

where v is velocity, r is the length of the radius, and T is the time of orbit. Since we're calculating in units of seconds, we must convert 18 days into the same units:

2.) (60 sec/1 min)*(60 min/1 hour)*(24 hour/1 day) = 86400 sec/day

3.) T = (86400 sec/day)* 18 days = 1.56*10^7 sec

4.) v = 2(3.1416) * (6.08*10^8 m) / (1.56*10^7 sec)

5.) v = (3.82 * 10^9 m) / (1.56*10^7 sec)

6.) v = 2.45 * 10^2 = 245 m/sec

Centripetal Acceleration is given by a = v^2 / r, so

7.) a = (245 m/sec)^2 / (6.08*10^8 m)

8.) a = (6.0025*10^4 m^2/sec^2) / (6.08*10^8 m)

9.) a = .9872*10^-4 = (9.872*10^-5 m/sec^2)