When you enter the simulation you will see a moving image looking like this.
The main part of the screen shows a planet (the red dot) moving on the combination of a deferent (the large circle) and two epicycles (the two smaller circles). The black dot just below the center of the deferent is the Earth. It is off-center because this is an eccentric deferent. Its geometric center is marked by the green dot. And this deferent also has an equant point: it is marked by the cyan dot. You should be able to see the planet change speed on its path round the large deferent, as it keeps the same angular velocity from the point of view of the equant point. The path of the planet through the ether is shown by the blue line.
This is not all you see. Above the moving circles is a notice of the angle of the planet as seen from the Earth, and a linear scale extending through 360 degrees on which a moving red dot appears to trace out that angle. This dot changes size all the time, to indicate how the motion of the planet on its epicycles would make it seem brighter or dimmer to an observer on the Earth.
At the foot of the screen you can see six buttons.
Three of them have very simple functions.
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Reset |
This button will renew the display, starting the planet's motion afresh. |
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Pause |
This button will freeze the planet temporarily. Click again to resume the planet's motion. |
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Help |
Press this button if you are at a loss for what to do. |
The other three buttons have more complex functions, allowing you to take on the task of Ptolemaic astronomy.
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Click on this button to change how your model is displayed. |
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Click on this button to add or delete orbs, or to change those already present. |
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Click on this button to decide the kind of problem you are trying to solve. |
You now see the new dialog box shown here, called Display settings. This dialog box allows you to change key elements of the simulation's screen display.
To the left are four boxes in which you can enter values affecting the appearance of the display.
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Trail Length |
You can set the blue line marking the planet's path to any length up to 10000 units. Setting a longer value will make the simulation provide a record of the planet's motions over a series of cycles. |
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Time Step |
This sets the time interval between successive points at which the simulation displays the planet's position. Set it to a low value for a smooth, comprehensible path. |
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Delay |
This sets the rate at which the planet moves on the screen. Experiment with this value to get the best results: different computers will need different values. You may well have found that the original screen showed a planet moving at dizzying speed, and by using this box you can slow it down. |
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Number of Steps between Trail Locations |
This sets the number of animation frames between points recorded in the trail. Setting a large number results in a trail made up of straight lines connecting distant points. |
To the right there are five check-boxes that determine major elements of the display.
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Path Lines? |
Checking this box will make the planet leave a blue trail line. If the box is unchecked, no trail will be left. |
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Path Dots? |
Checking this box will make the planet leave a trail of dots instead of a line. This is useful because the resulting density of dots at any point records the relative speed of the planet as it moves through the ether. |
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Draw orbs? |
Checking this box makes the system display the deferent and epicycles. If it is unchecked, these will vanish, leaving the planet alone to pursue its mysterious path. |
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Draw Angle from Earth? |
Checking this box will make the system reveal the angle of the planet at every moment as viewed by an observer on earth. If unchecked, this part of the display will vanish. |
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Draw Planet to Match? |
Checking this box will make the system display on the linear scale at the top of the screen the observed path of a planet that the astronomer must attempt to model. If unchecked, no such planet appears. |
Once you have made your selections, click on OK to proceed back to the simulation. Your choices will take effect immediately.
A new dialog box appears, entitled Change system.
This dialog box allows you to add new orbs and to alter the settings of existing ones. It contains a window with the names of existing orbs, any of which can be selected with the mouse. It also contains five buttons.
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Done |
Click on this button when all your changes are complete |
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New |
Click on this button to make a new epicycle. A new dialog box will then appear, in which you can set all its characteristics. |
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Modify |
Click on this button to change the settings of an existing epicycle, or of the deferent. You will need first to select one of these by clicking on its name in the box above. Once you have done that, a new dialog box will guide you through the procedure for changing settings. |
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Delete |
Click on this button to delete an epicycle. You will need to select the epicycle to be removed first by clicking on its name in the box above. |
When you are ready, click on DONE to see the results of your decisions.
You now see a new dialog box, entitled new Orb.
This contains a series of options allowing you to set the characteristics of the epicycle or deferent you are creating.
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Orbit name |
You can, if you wish, name your epicycle. Otherwise the computer will provide a name. |
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Starting angle |
Enter the angle, in degrees, from which your planet will begin to move when you restart the simulation. The default, zero, is the three-o'clock position, and positive values move anticlockwise from there. |
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Revolutions per year |
How fast will your epicycle rotate? You can set any value. |
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Orbit radius |
How large is your epicycle? You can set any value up to the size of the deferent. |
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Use Eccentric? |
You can, if you wish, make your epicycle an eccentric. If you check this box you should then enter x and y coordinates for the circle's eccentricity in the boxes provided. |
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Use Equant? |
You can, if you wish, make your epicycle move on an equant point. If so, check this box and enter the coordinates of the equant point in the boxes provided. |
When you are ready, click on OK to return to the previous Dialog Box.
You now see a new dialog box, entitled Modify Orb.
This contains a series of options allowing you to set the characteristics of the epicycle or deferent you are changing.
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Orbit name |
This is the name of your orb. You can change it if you wish. |
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Starting angle |
Enter the angle, in degrees, from which your planet will begin to move when you restart the simulation. The default, zero, is the three-o'clock position, and positive values move anticlockwise from there. |
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Revolutions per year |
How fast will your epicycle rotate? You can set any value. |
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Orbit radius |
How large is your epicycle? You can set any value up to the size of the deferent. |
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Use Eccentric? |
You can, if you wish, make your epicycle an eccentric. If you check this box you should then enter x and y coordinates for the circle's eccentricity in the boxes provided. |
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Use Equant? |
You can, if you wish, make your epicycle move on an equant point. If so, check this box and enter the coordinates of the equant point in the boxes provided. |
When you are ready, click on OK to return to the previous Dialog Box.
This option lets you decide whether you are modelling the motion of a fixed star, the Sun, or a planet.
You see a simple dialog box asking whether you wish your new problem to be relatively easy, like a star, rather more difficult, like the Sun, or positively fiendish, like a planet. Click on one of the three.
Then click on OK for the new problem to begin.