Solar System Live Details
- How did you do that in
- The HTML documents form only the skeleton of Solar System
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.
- 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'
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
- 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 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 siblings
and Your Sky,
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
lacunŠ 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
Web and Windows tools such as
Your Sky, and the
Sky screen savers for Windows.
by John Walker