Doubt regarding variation of a semi-axis

[JKepler]

Good afternoon,

I was wondering if I'm making these calculations right regarding the computation of the semi-major axis a in a elliptical orbit.

If the body is at a distance of R and has a velocity V, then a will be:

a = 1 / (2 / R - V^2 / mu)

But supose that I use a different V', so that V'^2 - V^2 = error
I'll have then:

a = 1 / (2 / R - V^2 / mu - error / mu)

I've made a simple test, with the Moon's orbit, and placed an error of about 15 meters. Suprisely I've obtained an a value of about 5400 meters of diference between the 2 semi-major axis calculated.

Can this be right?

Regards,

JKepler

 

[Keithth G]

The equation for the semi-major axis is correct.

However, I'm a bit confused by your 'error': the physical dimensions of v^2 are (metres^2 * seconds^-2). And yet you say that you have incorporate an error of 15 metres?