# Homework Help: Proper motion of stars

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1. Dec 8, 2016

### Daniel P

Hi

I have a question about "proper motion of stars".
At this page (http://www.astronexus.com/a-a/motions-long-term) there are a lot of examples how you can measuring the proper motion of stars.

But at "Point 4 - Turn these velocities into Cartesian velocities" i have some different results.

vR = -5.19 Km/sec
vTA = -58.3 Km/sec
vTD = -106.6 Km/sec
α = 14.2612
δ = 19.1873

===========================================================================
should!

vx = ( vR * cos δ * cos α ) - (vTA*sin α ) - (vTD * sin δ * cos α ) = -57.7 Km/sec
vy = ( vR * cos δ * sin α ) + (vTA * cos α ) - (vTD * sin δ sin α ) = +31.5 Km/sec
vz = vR * sin δ + vTD* cos δ = -102.3 Km/sec

===========================================================================

My results: vx = +54.1, xy = +37.5 and xz = -102.4
You can see the results one and two don't agree with my calculation, the result three agrees (depents on the accuracy of "my" data!)

Thank you very much,
DANIEL

Last edited: Dec 8, 2016
2. Dec 8, 2016

### phyzguy

Did you see that α is in hours, not degrees? Did you convert it to degrees before taking the sin and cos?

3. Dec 8, 2016

### Daniel P

Hi

now i have seen that i must convert α = 14.2612 Hours in degrees (=213.915°)! So far so good!

but if i calculate Vx = -5.19*COS(19.1873)*COS(213.915)-(-58.34*SIN(213.915))-(-106.6*SIN(19.1873)*COS(213.915)) = +45.7 instead -57.7 Km/sec.
and if i calculate Vy = -5.19*COS(19.1873)*SIN(213.915)+(-58.34*COS(213.915)-(-106.6*SIN(19.1873)*SIN(213.915)) = -47.7 instead +31.5 Km/sec

i calculate it with Excel!

query: http://www.astronexus.com/a-a/motions-long-term

thank you again, phyzguy!

4. Dec 8, 2016

### phyzguy

You must have made a mistake. See below:
Code (Text):

from pylab import *
alpha = 14.2612 * 15.0 * pi / 180.0
delta = 19.1873 * pi / 180.0
vR = -5.0
vTA = -58.3
vTD = -106.6
vx = (vR * cos(delta) * cos(alpha)) - (vTA * sin(alpha)) - (vTD * sin(delta) * cos(alpha))
vy = (vR * cos(delta) * sin(alpha)) + (vTA * cos(alpha)) - (vTD * sin(delta) * sin(alpha))
vz = vR * sin(delta) + vTD * cos(delta)
print vx, vy, vz

-57.6862773004 31.464877787 -102.321575814

5. Dec 9, 2016

### Daniel P

Ohh ok now i have the right result!!! i forgot the ("Pi/180")
Tthank you very much!