Celestia Forums

I am working on a script to help explain celestial navigation and would like to be able to show a line from the centre of Earth to a navigational star. The point on the Earth's surface through which the line passes would be the stars geographic position. I will also need similar lines from the Earth's centre to the elevated celestial pole and one passing through the observer's assumed position and projecting out into space.

Is this possible?

I have attempted this by editing the asterisms.dat file, but it does not seem possible to use an orbiting body (ie Earth) as an end point for a line.

Any suggestions are eagerly awaited.

Thanks,
ChrisA

The way I'd do it would be to create a long, skinny model mesh to be the pointer, use OrbitFrame to place one end at the center of the earth (or perhaps at the observer's position) and use BodyFrame to point its axis toward the appropriate object. I did something similar in my Hale Telescope Addon so the telescope would point toward the desired target.

Thanks for the advice Selden.
I have created a long thin model and attached it to the Earth as suggested. I have also had a look at your very impressive Hale telescope add on, I can see that there is lots to learn from that. Still a long way to go, but at least it is a start. Unfortunately time has not allowed me to progress any further just yet. I will post an update when I make some progress (or hit a problem).
Thanks again,
ChrisA

ChrisA,

You're very welcome.
I'n looking forward to seeing your results.

Keep in mind, however, that the Earth is not a sphere, but is a somewhat flattened oblate spheroid - and that geographic latitude, as is used in celestial navigation, is not measured as the central angle at the Earth's center. It is measured from the line that is normal to the Earth's surface at the location in question. The Earth's central angle at the location in question is called the geocentric latitude, and is not used in celestial navigation. Sextants measure geographic latitude. Geocentric latitude never equals geographic latitude - except at the poles and equator. Elsewhere, the geocentric latitude is always less than the geographic latitude.