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Experience gained from building an astronomical VRML world The following experience has been gained:

• General results:
• Movement of stars, the Sun and the Moon (and if needed other planets) is possible in a VRML world.
• Buildings/monuments can be incorporated in the world.
• Time steps implemented are: ±1-60 [min], ±1-24 [hr], solar and lunar periods. Automatic time increase/decrease can be activated.
• Use of Java, AWT (Abstract Windowing Toolkit) and EAI (External Authoring Interface) provides possibilities to have input fields for time and date. This seems not to work with Internet Explorer (6.0), Netscape seems not to work properly over the Internet (to be investigated).
An alternative (plug-in depending?) could be using only JavaScript or using blaxxun.
• Shadow casting is nowadays also possible inside a VRML world.
• The accuracy of a VRML world is bigger than 5' for alignments smaller than 2500 m.
• The accuracy needed for the astronomical calculations can be reached (see also below for this).
• Curvature of the earth can be included in the VRML world itself (changing height of DTM according to curvature), this is not done in the example. Compensation for curvature is needed when the world is bigger than 9 km in width or length (~radius_earth*5'/180*p=6371*0.0015).
  This can be compensate in some instances, see next bullet.
  
• Terrestrial refraction is not implemented in VRML plg-ins. I don't know if other 3D technologies (can) do this?
The need for it is though only important for world sizes bigger than 110 km (altitude_change [']=0.044*size [km], size [km]=altititude_change/0.044=5/0.044= 115 [km]; using Thom [1971], page 32).
If the world is e.g. only distant terrain, one could lower the distance object's height with an approximation of the terrestrial refraction (which incl. curvature of earth).
• VRML plug-in compatibily with this world:
• At this moment the Cortona, BS Contact VRML 6.2 and blaxxun Contact 5.104 VRML plug-ins have comparable results.
  
• When using an VRML plug-in, the depiction of the world could depend on the type of renderer (like software, OpenGL, DirextX). The software renderer certainly works, so if you are missing e.g. the Sun try to use the software rendering or try also the other hardware renderers. The software renderer is normally slower.
• I use for the extinction angle a transparent imagine (2-component), it works properly with at least Cortona 4.1. Looks to have something to do with browser decision what to draw and distance between nodes. This is solved since the new b version of Cortona (4.1): right clicking on Cortona window: 'Preferences...', 'Renderer', 'Advanced-Extended Z-buffer' and 'Oke'.
• blaxxun Contact (5.104) had problems when 'Energy saving' options are enabled.


If this is seen try to disable the Windows 'Energy saving': 'Start', 'Settings', 'Control Panel',  'Display', 'Screen Saver', 'Settings...', 'System standby: Never', 'Turn off monitor: Never', 'Turn off hard disks: Never', 'OK', 'OK'
• Some inconsitencies with plug-ins (bold ones are being studied together with the software producer):

(using Explorer 6.0.2600 and Netscape 7.1)	Cosmo Player Windows 2.1.1	blaxxun Contact Windows 5.104	Cortona Windows 4.2 (Rel. 93)	BS Contact VRML 6.200
PointSet node	no	yes	yes	yes
'Z-buffer depth' option hiding objects	no yes	no no	yes no	yes (in OpenGL) no
EAI	no	yes	yes	?
hitTexCoord_changed	no	yes	yes	yes
utf char. set  (e.g o (Deg) symbol)	yes	yes	yes	yes
ambientIntensity	no	yes	yes	yes
shadows group	no	yes	no	yes
weird pixels on screen	no	no	no	no
Stereoscopic view	no	yes	yes	yes
Cache empty option	?	yes	yes (uses browser cache)	yes
Rendering properly	?	OpenGL: yes (Z-depth: 16) Software: yes	OpenGL: yes NVIDIA, no: Intel 855 DirectX9: yes Software: yes	DirectX7: yes DirectX9: yes OpenGL: yes (Z-depth: 16) Software: yes
Caching behavoirs	?	don't understand	IE: oke Netscape: need to empty cache always	don't understand

• Features that can relatively easy be implemented:
• DE404/ELP2000-85 needs to be implemented for the Sun/Moon paths to be accurate from 3000 BCE onwards. C++ programs available, just have to code them into JavaScript.
• An interface must be made to calculate time/date when the Moon or Sun arrives in a defined sky window.
• The changing outline of the Moon and Sun due to phase (using basic formula presented here) and/or astronomical refraction has to be incorporated.
• In case shadow casting is important:
• The blaxxun Contact VRML plug-in does do shadow casting, see this example. Here one can see the sun light in a prototype Maeshowe.
• Another alternative is a ray tracing program, like POV-Ray. It is not easy to convert VRML, using Script nodes and EXTERNPROTO, to POV-Ray, so in that case VRML is perhaps not useable for this and one has to directly write it in POV-Ray (and forget the web accessibility).
• In case stars are needed for the study:
• The rotation of the stars is not yet implemented fully.
• Astronomical refraction for the stars needs to be incorporated. This can be implemented in two ways:
• by treating every star as a separate object. This needs to be implemented and checked if it is not using up too much computer cycles.
The question is if this will change much due to the fact that star are not visible at low altitudes (extinction angle):
• from 1-2° (extinction angle for most stars) the astronomical refraction is in the order of 18-24'.
• the accuracy of a VRML world is bigger than 5' for alignments smaller than 2500 m.
So comparing the above; the astronomical refraction is important and needs to be incorporated for the stars.
• An alternative is to use a VRML node that can simulate refraction. I have not yet encountered an implementation of this.
• Proper motion of stars is not incorporated in the VRML world, could be done with a JavaScript as part of the VRML world (can then also implement astronomical refraction at the same time).

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