Solar System Live Details

How did you do that in HTML?

The HTML documents form only the skeleton of Solar System Live. They assemble a request which is sent, using the CGI mechanism, to a server application written in C. The server, given the viewing parameters you selected in the HTML file, creates a GIF image of the requested view, then returns an HTML document which references the GIF file as an in-line image. Fields in the form of this result document are initialised to the parameters passed in with the request. To better understand what's going on, view the source of the result of a query. Note that since the source is custom-generated for each query, it will be different every time.

Okay, I just looked at the source. What's with this cache directory?

Ahhhh.... That's interesting. Since we want to return the result of the query as an in-line image, it must reference an external GIF file. (If the query server returned the GIF file directly, we wouldn't be able to embed it in the form that allows subsequent queries. Furthermore, Mosaic would spawn an external viewer for the image rather than show it on the page.) So, having to write a separate GIF file raises the issue of where it goes and how to keep the system from being clogged with leftover images. My answer to this problem is to create GIF files in the cache directory with unique names generated based on the process ID of the server program. As each file is written, an at job is queued to delete it five minutes later. This way the image files are automatically cleaned up.

How do you calculate the positions of the planets?

The heliocentric co-ordinates of Mercury through Neptune are calculated using the algorithms given in Jean Meeus' Astronomical Algorithms, based upon the VSOP87 planetary theory of Bretagnon and Francou, The position of Pluto is calculated, for dates between 1885 and 2099, using the method given in Chapter 36 of Astronomical Algorithms. No analytical theory of the orbit of Pluto exists at present, because Pluto has been observed over an insufficient portion of its orbit since its discovery in 1930 to construct one.

How do you calculate the orbits of comets and asteroids?

The specified orbital elements are used in conjunction with the general solutions for elliptic and parabolic motion given in Chapters 32 and 33 of Astronomical Algorithms. The orbit is plotted by calculating positions before and after the specified epoch until the orbit either closes (if elliptical) or reaches a preset distance from the Sun (if parabolic). The time step is calculated based on the rate of motion of the object, smaller near perihelion, larger at aphelion.

How do you draw the orbits in the image?

Once the orbit has been calculated, in heliocentric co-ordinates, a viewing transform is established, taking into account the heliocentric viewpoint and stereoscopic viewing options. The transform projects the three-dimensional orbit into the pixel space of the bitmap. A 64-sided polygon is drawn to approximate the elliptical orbit, using the Postage Stamp Rasteriser available by FTP from this site.

What do the numbers in the "orbital elements" box mean?

Please see the help for orbital elements. If you check the "Echo elements" box, the elements will be listed, individually labeled, below the text box.

Pluto disappeared! What's going on?

As mentioned above, Pluto's orbit is not known with sufficient accuracy to permit analytic extrapolation beyond the period from 1885 through 2099, and I'm not about to perform a numerical integration of solar system motion, now matter how nicely you ask. So, if you specify a date outside that interval, Pluto isn't plotted.

Wasn't it a lot of work to build this program?

Actually, only a couple of days; the documentation and on-line help took much longer than the program itself. Like its sibling Earth Viewer, much of Solar System Live is based on code lifted directly from Home Planet and, in addition, I was able to stand on the shoulders of others who developed freely available code or algorithms the program employs; if I've failed to acknowledge any contributor, please accept my apologies and let me know of any lacunµ so I can properly cite their contributions. Home Planet was developed as a title for the Autodesk Science Series, to follow Cellular Automata Lab and James Gleick's CHAOS: The Software. When new management arrived at Autodesk, one of their first actions was to terminate the Science Series, along with a number of other products, some profitable, which did not "meet the business model". I obtained the rights to Home Planet and decided to place it in the public domain. Since then, in my spare time, I've been mining the man-year I invested in its development to build public domain Windows and Web tools such as Earth Viewer and the Sky screen saver for Windows. This isn't the last; stay tuned.

My Whatever browser doesn't work with Solar System Live.

We're a long web away from total compatibility among browsers. I've tested Solar System Live with NCSA Mosaic Version 2.4 and Netscape 1.0N and had no problems. Your browser must support forms and in-line graphics. I use a few of the HTML extensions in Netscape to make things a little prettier and faster for users of that browser, but they appear, as designed, to cause no problems to other browsers which simply ignore them per the HTML spec.

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