Expert: Jayendra Upadhye - 1/21/2006

Question
Hi,
I need to know (it is a part ifa a project) how i can determine jupiters diameter
or it's angular diameter by observation.
all thanks are welcomed

Answer
Hi,
Number of approximate (With your resources, you can only arrive at approximations of things) methods jump to mind.
1 - Most of jupiters galilean satellites present their orbits edge on to us.
   Mark one particular satellite for observation.
Select the one with most circular orbit.
Note its orbital period as closely as possible. (that should be easy if you take one extremity of its orbit as starting point and count time for say 10 orbits. then take the average).
As the orbits are presented edge on to our view, the satellites all follow simple harmonic motion.
Using SHM (Simple Harmonic Motion) you can predict the satellite's speed across your field of vision on a point to point basis.
Observe the occultation time as the satellite traverses the face of jupiter. (This can be done using any telescope of moderate power).
Knowing the speed as a function of time, you can calculate the distance travelled by the satellite in its plane of motion using SHM maths.
That will be the diameter of jupiter to a fine approximation.

2 - The interferometer. (may be beyond your scope).
Michelson and morley, for their celebrated ether experiment had devised a 20 foot (?) interferometer and actually used that to calculate the diameter of betelguese.
   The same technique can be used to calculate the dia of jupiter, by using monochromatic light from the opposite limbs of its disc.
The interference pattern will tell you the diameter of jupiter.
I am not familiar with the optics involved but you surely will find it all on the web.

3 - knowing the magnification of your telescope, you will know what is the actual image size of jupiter, in your telescope. Knowing the distance to jupiter accurately, one can then predict the actual size of jupiter.

4 - To get the angle subtended by jupiter, you will require to have a travelling synchronized tracking telescope, with a cross hair mechanism and means to evaluate relative motion of telescope in degrees as you move the cross hair across the image of jupiter. The travel will tell you how many seconds of arc or degrees you traversed. (quite sophisticated technique required).

5 - Simplest and quite accurate method is to wait till jupiter is close to zenith (to avoid atmospheric distortions in motion), then fix the cross hair at one end of the jupiter image (non tracking ordinary telescope).
Wait for the earth to rotate and make the cross hair traverse the face of the image fully.
Knowing the time well (take 10 readings and take average),
will help as the earth turns 360 deg/(24*3600)  or 0.004166 deg/sec or the earth takes 4 minutes to turn a degree!

hope that suffices
jayen