In this activity you will:
1)   Observe Jupiter and its moons at least once a day.
2)   Plot the positions of the moons using graph paper or computer.
3)   Determine which moon is which.
4)   Determine how long it takes each moon to go around Jupiter.
5)   Use your observations to determine the mass of Jupiter (extension).

Part One:  Observing Jupiter and its Moons

If you are taking your own observations (recommended):

Obtain at least one image each day with the MicroObservatory.  It is 
best if the images are taken with the same telescope, and if they are 
taken at the same time of day.

You can use the Table of objects in the Telescope Control Panel to 
select Jupiter as your target.  Use the main telescope, not the finder
scope.  You may need to refer to Sky and Telescope or Astronomy 
magazines or to a planetarium computer program such as Voyager 
(for the Mac) or Redshift (for the PC) to find the approximate time 
Jupiter is visible.

Jupiter will always be overexposed on your images.  This will cause 
the planet to look like a bright circle with a vertical line through it.  

Recommended exposure times for Jupiter are very short, less than 1 
second.  Try 0.2 seconds first.  If the weather is very hazy, you might 
need to increase the exposure time to see all the moons.

If you cannot take your own data for reasons of time or weather:

Use the data set in the permanent Image Archive.  Not all are perfect 
images; some days are missing and some were less than perfectly
clear.  Galileo had these problems too!

Intermediate Step:  Saving and Looking at Images

Once you have gotten useful images, you need to store them in a format which is 
readable by your software.  For this exercise, NIH image (free shareware which 
can be downloaded from the MicroObservatory home page) has been used.  See 
the documentation on image processing for other options.  

To use NIH Image:  
1.  Store your MicroObservatory Images as GIF files.
2.  Use Graphic Converter (which can be downloaded from the 
MicroObservatory home page) to convert the GIF file to a PICT file.
3.  Open the PICT file using NIH Image.
4.  Use the "Invert" command to change to dark planet and stars on a light 
background.  This will save on toner or ink if you decide to print it out.

Part Two:  Which Moon is Which?

Once you have obtained  images you need to figure out which of the satellites is which.  
This is not as easy as it might seem at first from MicroObservatory observations alone.  
The satellites shift in position from night to night, and so in order to use your observations 
you must obtain at least several nights worth of data.  It will then be necessary to overlay 
them in some manner to look for trends in the motion of the satellites.

There is a way to "cheat" by looking up the answers.  The Observer's Page in Sky and 
Telescope magazine for each month has a diagram showing the positions of the moons for 
each day of the month.  If you decide to use this, be sure you have the directions aligned 
the same way for your MicroObservatory Images and the Sky and Telescope diagram.

Method 1:  Manual Cut and Paste

In this method you run off a printer copy of each image, cut out the part which includes 
Jupiter and the moons, and manually paste each night's image on a single sheet of paper, 
one above the other.  You can then look for patterns of motion.  Keep in mind that the 
periods of motion around Jupiter range from 1.8 days (Io) to 16.7 days (Callisto).

[Scanned copy will be included here]

Method 2:  Cutting and Pasting Using NIH Image

You can use the NIH Image shareware to cut and paste portions of your Jupiter images.  
To do this, use the "Open" command frome File menu to open one of the images you want 
to study.  Use the mouse to drag and select a smaller area of the image and choose "Copy" 
from the Edit menu.  Open a new window and paste in the small image.  Now open a 
second full Jupiter image, drag and select the area around Jupiter and its moons, and go 
back to the new window.  Use the "Paste" command.  Go over to the "Windows" menu 
and click on "Show Paste Controls".  When that window opens, under "Transfer Mode", 
choose "Blend".  The second subimage will appear in the new window as though it is on a 
transparency.  You will be able to drag it around and line it up above or below the first 
subimage.   You can do this repeatedly to get a series of images of Jupiter and its moons on 
the same computer screen (or piece of paper if you print it out).

Some Hints on Identifying the moons:

1.  The moons are not all the same brightness.  Their brightness depends on the amount of 
light they reflect, and that, in turn, depends both on the size of the moon and the fraction of 
light its surface material reflects (known as its albedo).  Europa is, in size, the smallest 
moon, but the water ice composition of its surface gives it a high albedo.  Ganymede has a 
dark surface composition (whether this surface material is like that on the inside of the 
planet or due to impacts of comets and asteroids is not clear), but its diameter is very large. 
Thus these two moons are generally brighter than Io and Callisto.

[Data Table of Diameters, Compositions, Albedos, and Periods will be included]