Celestia Forums

Hi there!

Is there a way to set fixed markers in the sky?

I'm using Celestia to identify the stars as they appear on the sky. So I've positioned myself on the Earth's surface, at the appropriate latitude and longitude, everything's working fine.

Unfortunately, as I live in a big city, all I can see is a small slice of the sky between skyscrapers. It's hard to correlate it with the full sky view seen in Celestia.

Is there a way to easily demarcate the edges of the visible part of the sky? For instance, I know that at X hours the sun 'sets' behind the Xth floor of the skyscraper across the street, so I'd like to put a marker there. Or at Y hours the moon is directly behind an antenna on the top of another building, so another marker would go there etc. Of course the markers would have to stay there as the sky 'rotates' behind them.

Is such a thing possible without having to go into the complicated business of building a 3D model of the skyscrapers around me? Simple fixed markers would suffice.

Sorry if this is an obvious question, but I've looked into the manual and searched the forums, to no avail.

Thanks in advance & congrats on a great program!

Tom B

tom-b wrote:Hi there!

Is there a way to set fixed markers in the sky?

I'm using Celestia to identify the stars as they appear on the sky. So I've positioned myself on the Earth's surface, at the appropriate latitude and longitude, everything's working fine.

Unfortunately, as I live in a big city, all I can see is a small slice of the sky between skyscrapers. It's hard to correlate it with the full sky view seen in Celestia.

Is there a way to easily demarcate the edges of the visible part of the sky? For instance, I know that at X hours the sun 'sets' behind the Xth floor of the skyscraper across the street, so I'd like to put a marker there. Or at Y hours the moon is directly behind an antenna on the top of another building, so another marker would go there etc. Of course the markers would have to stay there as the sky 'rotates' behind them.

Is such a thing possible without having to go into the complicated business of building a 3D model of the skyscrapers around me? Simple fixed markers would suffice.

Sorry if this is an obvious question, but I've looked into the manual and searched the forums, to no avail.

Thanks in advance & congrats on a great program!

Tom B

You want to implement an arbitrary horizont.

There are better programs for your purpose:

Stellarium: Here you can even compose a 360 degree mosaic image with your camera and have a perfectly realistic horizon with all your skyscrapers

It also uses very nice OpenGl graphics and is ideal for amateur astronomy.

XEphem: Here you may also input an arbitrary horizon, but Stellarium is more fancy and uses more modern graphics...Xephem is presumably more accurate as a 2D astro-mechanical ephemeris tool and correspondingly widespread.

Bye Fridger

Simple fixed markers would suffice

If you still want to use Celestia for this, it could be done by:

- create an ssc file, and:
- use a simple 3d model (or just define a tiny sphere) for each spot you want marked.
- place the markers using the longlat definition. You need to know the exact latitudes and longitudes for each marker, of course. Or do some experimenting...

-rthorvald

You don't need to define a model to mark a location. Use a declaration like the ones in solsys_locs.ssc:


(The arguments are longitude and latitude in degrees, plus altitude in km.)

I'm not sure if there's actually a location type of building, but it works. You can use this technique to mark the corners of the building. Defining the locations of the corners is left as an excercise for the student

A script containing Mark commands can turn them on:



It's a lot of typing, though, and I suspect placing cubes of the appropriate sizes in a 3D model would work better.

Hi Fridger, rthorvald, selden, thanks for answering!

Fridger: I downloaded & took a look at Stellarium, its 360 photo horizon approach is nice looking, but doesn't seem very practical or accurate - might work for objects low on the horizon, but there's no mention on how you would align large objects very close to you, that blot out 1/3rd of the sky or more - other than distorting them in photoshop and kinda hoping that they match up by trial and error.

XEphem, on the other hand, seems to have exactly what I want (see here:)
http://www.clearskyinstitute.com/xephem ... ocId669677

Unfortunately, all they have available for free is the source code, and transforming it into a working application requires alchemy that's out of my league. Also, having to install a separate window manager and the arcane-looking interface (I'm on Mac OS X) sorta put me off it.

I also had a look at Starry Night Pro - they have the manual online:
http://www.starrynight.com/support/manuals.html
It uses both approaches for the horizon, photo 360 and point-and-click. What's not clear from the manual is how high can horizon objects go and whether you draw the horizon directly on the sky view, or in a separate window, again having to go by trial and error (duh!) Also, it's commercial only, so I'd have to locate a "liberated" copy to check it out.

rthorvald and selden: any tips on defining lat/lon/altitude of each object, other than pure trial and error? I'd like to align them precisely to the sky, in a Stonehenge-like way.

Maybe there's someone somewhere working on an add-on to expand Celestia's planetarium functionality? This doesn't seem too complicated, and uses elements that are already there (markers, labels), just click to set a marker and it stays there (that is, it follows Earth's rotation)... cardinal points (N-S-W-E) would also be nice!

Otherwise, I'll have to go by the extreme oldskool route and draw on my screen with a magic marker, and never change angle/FOV again in my life

OK guys, thanks for the input, if you got anything else lemme know!

Tom B!

tom-b wrote:Hi Fridger, rthorvald, selden, thanks for answering!

XEphem, on the other hand, seems to have exactly what I want (see here:)
http://www.clearskyinstitute.com/xephem ... ocId669677

Unfortunately, all they have available for free is the source code, and transforming it into a working application requires alchemy that's out of my league. Also, having to install a separate window manager and the arcane-looking interface (I'm on Mac OS X) sorta put me off it.
...

Tom,

OK, XEphem is clearly /perfect/ since I collaborated for 11 years (!) with XEphem's author Elwood Downey. The horizon demo in XEphem is actually from my backyard

Let me emphasize that XEphem 3.6.4 builds "out of the box" both for native Linux and Cygwin Linux (installed "on top of" Win XP). In this forum there are quite a few people who compile even Celestia under Mac OS. What's so hard? Almost everything may be learned, notably with some helpful hands?

Bye Fridger

Whoa Fridger, that's impressive!

Thanks for the encouragement, I'm sure that everything is learnable if you put your mind to it. It's just a matter of personal priorities you know... my time on this planet is limited, and there are many other personal projects i'm already not doing, so learning how to compile binaries, however fascinating, would not be the first thing on my list...

For me this starwatching thing is suposed to be something simple and fun to do in my spare moments... that's why I'm looking for plain, straightforward, ready-to-use tools.

Anyway, with the thick smog & light pollution around here, the only things visible are the planets, and maybe Arcturus and Spica... but it was thrilling to observe Venus, Mercury and Saturn clumped together right after sunset today, and knowing that the big fat one hovering overhead right now is Jupiter.

Tom!