VIII. Data Gathering Methods

A. Jupiter Diameter (JD) Method

1. Basics

This method uses apparent size of Jupiter as the unit of measure. While the apparent size of Jupiter does change appreciably over the course of a month of observations, the satellite separations vary in proportion, so the JD method is suitable for a rough unit of measure. It has been reported that Kepler used this measure in his efforts to determine the orbital periods of Jupiter’s satellites.

2. Estimating Spacings

The observer may want to practice estimating distances (suggested practice sessions with mock data prepared beforehand 11 ). Another helpful hint is to use a higher power eyepiece. The ratio of  Jupiter’s diameter to the moon spacing is relatively low ( below about 1:15 ), in a region wherein the mind can more easily estimate the separation.

The Quad-A observer using the JD method may have varying degrees of success in estimating the separations. Inaccuracies are expected, but the goal here is to improve your observing skills, not to  expect perfection. The fitting of the observed separations ( See Section IX) will tend to help smooth out the inaccuracies.

Mr. C. Warren, a Quad-A member, contributed a useful idea for Project Jupiter. Mr. Warren found a piece of stiff plastic screening material with a wide and even weave. He made a little tube of cardboard, and applied the plastic weave to the top of the tube with tape. He then placed that over his eyepiece. It worked quite well on top 12 of the eyepiece, as his Nagler™ has good eye-relief with an adjustable sleeve. He selected an eyepiece focal length such that Jupiter nearly filled one mesh square, making the estimates of the moon spacings simply a matter of counting the mesh lines. He did find that the plastic weave itself was a little heavy (about c JD), so he needed some minor repositioning of the mesh to ensure each moon was visible. No one asserts that this measuring-by-mesh method is precise 13 , but may be an improvement over unaided estimates of the JD spacings.

3. Need to use same eyepiece & scope

Because the separation of the satellite in any given observation is to be compared to other observations, there is a need to have consistent estimates from one observation to the next. The focal length of the eyepiece directly influences the FOV (as do other parameters that vary between eyepieces ). Better orbital period estimates are obtained with observations from the same telescope and eyepiece combination.

4. Eyepiece Selection

Use higher power eyepieces, as they generally reduce the field of view (FOV) so that the proportionality of the satellite separation to the FOV is small. This makes the estimating of the moon separation as easy as possible. Details of the planet surface features, while interesting, are not needed for this project. Select an eyepiece that as that enables the maximum separation to be still in the FOV during subsequent observations without having to change eyepieces.

Observers using a telescope lacking tracking capabilities will need to use a lower power eyepiece. That enables Jupiter and it’s selected satellite remain in the FOV long enough to enable the observer to make a reasonably estimate of the JD value to be logged ( It is harder to make a good estimate when the object moves more rapidly through the FOV.).

The apparent separation of the satellites from Jupiter will vary considerably over about six months (See Attachment G). Because of this the appropriate eyepiece (selected for FOV ) for subsequent uses of Project Jupiter (not measures during a set of observations) may change.

5. Data Logging

During each observing session estimate the separation, in units of Jupiter Diameters (JD), of the selected moon from the center of Jupiter. Avoid the tendency to "smooth" the data at this stage. If on observation #2 the separation is 4 JD, on observation #3 it is 6 JD, and on observation #4 it is 5 JD (went back down), that is OK.

Report what you observed, not what you think the separation should be! For each observation, record the separation measurement (in JD) on the data sheet (Attachment A). Note that when the moon is emerging ( or disappearing behind) Jupiter that the spacing is 0.5 JD