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make it challenging to accurately measure the periods of the moons' orbits and their semi-major axes. The following procedure may seem convoluted, but it allows us to overcome the handicaps of our moving Earth-based perspective. Read ALL of the text below up to and including the next question before starting to make any measurements. You will measure the angular separation of each moon from the centre of Jupiter at various times during its orbit. Before starting, experiment with the PACE of the simulation by using the P key and the arrow keys. Starting with the innermost moon Io, select a speed for the simulation in which an orbit is completed in a reasonable time, say, 30 seconds to a minute. Then freeze the simulation by pressing the L key, which will label the moons.
54Carefully lay the plastic transparency provided to you by the TA over the screen (in ‘landscape’format, with the long end horizontal) and with a felt pen mark the position of the center of Jupiter's disk and the moon. Use the area near the top of the sheet so you have room for three more sets of measurements below. Number the position of the moon. Record the hour and date corresponding to each numbered measurement. Keeping the transparency in place, advance the simulation by pressing L and then L again after the moon has shifted noticeably in its orbit. Repeat until you have measured both points of maximum elongation in the orbit and at least one position on each side of each maximum elongation (a minimum of six measurements in total). Your observations should resemble Figure 4, although the exact positions of the moon will differ in your drawing. Figure 4A hypothetical example of what your drawing of the Jovian moon's positions would look like. Each numbered position corresponds to a date and time which is recorded at the right (month/day/hour/year). Positions 2 and 5 are the maximum elongations; the moon will move no further from Jupiter than these points. You can compare this side view of the moon's orbit with the overhead view in Figure 5.
55IMPORTANT ADVICE: ¾The monitors are fragile! UBC spent a large sum of money to purchase flat-screen monitors for this lab. Please respect them by not poking the monitors with your fingers or pens more than is absolutely necessary. (c: ¾Please DO NOT use glueto fix your transparency to the computer screen. Rather, use two small pieces of tape to hold the two upper corners to the top of the monitor. ¾It is essential that your measurements be made consecutively within a single orbit, since you will be measuring time differences between your observations to derive the orbital period of the moon. The best approach is to choose a point when a moon is approaching one of its maximum elongations, then advance to the time of maximum elongation, then choose a point after maximum but on the same side of Jupiter. Then let the moon move to the other side, and measure points before, at and after the opposite maximum elongation (as shown in Figure 4). If you miss a point, just use P