GRAVITY V1.5 ============ The Program ญญญญญญญญญญญ Gravity is a tool which simulates the motion of stars, planets, or any other objects (up to a maximum of twelve) you may care to define. When the Program is first run, it will try to read the `GRAVITY.cfg0' file which should be in the `:Gravity' or current directory. This file contains the information for the demonstration solar-system, which simulates the motion of Mercury, Venus and Earth around the Sun. Also loading, you will be asked to choose a screen mode. Screens available are Lo-Res, Hi-Res and Hi-Res InterL. These open screens of 320x256, 640x256 and 640x512 (interlace) respectively. Pressing RETURN will give you the default screen mode (Lo-Res). You will then be presented with the title screen. Press any key to begin, and you will be shown the Editing Screen. From here, it is possible to change any of the details about an object, or create new ones. etc. Press SPACE to start the simulation. If everything has loaded O.K, you will now see 4 dots on the screen. The one in the middle is the sun, and the four that orbit it are the planets. Each object has an identifying letter: The Sun is `A', Mercury is `B', Venus is `C', and the Earth is `D'. The amount of time that has passed is shown in the top left hand corner. It is shown in years and days. In this program, one year is equal to exactly 365.25 days. There are several functions that can be performed while the simulation is running. Here is a list of keys that invoke them: P.. This will pause the simulation. Press P again to continue. N.. If you have paused the simulation, you may press N to advance one frame only. A-L.. Any of these keys will display information about the corresponding object. ie, if you press `B' for example, then information will be shown on Object `B'. Shift A-L. By holding down SHIFT and pressing a key A to L you can centre the screen on any of the twelve objects. Press the same keys again to cancel the centring. If an object is currently being centred on, then `Ctr:X' will be shown in the top-right corner, where X is the object involved. Ctrl A-L. By holding down Ctrl and pressing a key A to L, you can highlight the corresponding object. The highlighted object has a box drawn around it. You can cancel the highlighting by pressing the same keys again. If an object has been highlighted, `Hi:X' will be shown in the top-right corner of the screen, where X is the highlighted object. Z... Pressing this key will cause the computer to display information about the total Potential Energy, Kinetic Energy and Total Energy within the system. The total energy should remain constant, if the simulation is accurate. Q... Pressing this key will bring up a screen showing the list controls, and the keys that invoke them. Esc.. This will return you to the Editing Screen. While at the editing screen. You may add objects or modify existing ones. The objects are listed at the top of the screen. Those that are underlined are objects that have already been defined. Underneath is a list of options and their corresponding keys. Amending Objects ---------------- Hold down SHIFT and press a key A-L for the object you want to edit. You will then be asked to enter new values for the object's mass, initial position, initial velocity and the object's name. Press RETURN if you wish to leave a quantity unchanged. You may use scientific notation for very large numbers (eg 1.3E+13). NB `SI' units are used throughout (ie, metres, metres per second, Joules, etc). To delete an object, just follow the above procedure, except type `0' when asked for the object's mass. If you wish to switch on object off, so that it does not appear in the simulation, then hold down Ctrl and press the appropriate key A-L. Repeating this switches the object back on again. You may also alter the `timestep'. This is the length of time that elapses between calculating the positions of the objects. The default is 1/2 day. Reducing the timestep will cause the simulation to run more slowly, but it will be more accurate. The amount of time that has passed since the simulation began is shown (in `Earth Years' and `Earth days') in the top-left corner of the screen. You may also alter the `mapscale'. This is the distance (in metres) that is to be represented by a the screen's width. Pressing S will enable you to change the size that objects are drawn on the screen: you can have 1, 2 or 3 pixel objects. For Hi-Res screen modes, it is advisable to have an object size of at least 2. NB: before typing in the new object size, you must delete the old size first by pressing `Del'. Pressing U will enable you to specify whether you want the objects to leave a trace on the screen. Other Editing Screen Options ---------------------------- SPACE will start the simulation from Year 0. RETURN will continue the simulation from where it was before you pressed `Esc'. Pressing SHIFT and R will run the program from the beginning. Use this option if you wish to restore the default solar system, or choose a new screen mode. Press SHIFT and ? to exit from the program. You can also save your edits, and load them at a later date, using the Save System and Load System options. Note that only the initial values of the system are loaded/saved. There is a file in the gravity drawer called gravity.cfg1. This file contains information on an imaginary solar system. There is a sun and three planets in identical orbits to Mercury, Venus, and Earth. However, there is also a star heading towards the solar system which will cause disruption to the usual orbits of the planets. The star has the same mass as the sun. By adjusting the trajectory and mass of the incoming star, different effects can be achieved. Versions ญญญ----ญ V1.2. First released version. V1.3. Facility to highlight objects included, plus a couple of other minor improvements. V1.4. Ability to choose screen mode, Potential Energy and Total Energy calculations. Objects highlighted with a box, rather than a line. V1.5. Maximum of 12 rather than 6 objects. Object Names. Ability to choose three object sizes. Error in calculation of Total Potential Energy in V1.4 fixed (It was twice as big as it should have been). Gravity V1.5 is public domain and may be freely distributed. Gravity V1.5 was written if GFA BASIC. No responsibility for the accuracy (or inaccuracy) of this program is accepted by the author. Author ญญญญญญ Steven Pugh 10 Wellington Close Sandhurst Green Sundorne Shrewsbury Shropshire England SY1 4SP