Cartes du Ciel is free software released under the terms of the
GNU General Public License
From the menu: Setup → Observatory Settings
This dialog box can be reached from the menu by: Setup → Observatory, by the icon in the bar at the left hand side, or by Setup → All configuration options → All configuration options.
The Observatory settings dialog box has two tabs:
For a proper display of objects on the chart, don't forget to check your date / time settings as wel.
It is important to select the correct time zone (7) for your observatory because SkyCharts needs it to calculate the UT from your Daylight Savings setting. This is very important to do proper ephemeris calculations in order to display the right chart.
It is recommended to use the country time zone as it correct for DST for any epoch where the rules are know.
If you use the special GMT time zone, beware the hour offset sign is opposite of what you expect, zones west of GMT have a positive sign. See http://en.wikipedia.org/wiki/Tz_database#Area
When your chart is set to use the Alt-Az coordinate system, you can display your local horizon as a line or as an area. To read more about changing the coordinate system, click here.
You can load a local horizon file by setting the path.
You can write a file with a simple ASCII-editor like Wordpad or Vi to define your local horizon. As an example, you can open the file [installation directory]/data/horizon/horizon_Geneve.txt. As you can see from the file, the horizon is defined by a serie of records. Every line contains a pair of two values. The first value is the 'azimuth', the second is the 'altitude'. The units are degrees, where azimuth 0° is North and 90° is the Zenith. A dot (.) can be used as a decimal separator. You can put comment in your horizon file by lines that start with a mesh (#) character.
The other possibilities will be obvious from their description.
* Maybe you want to display objects below the horizon line.
* If your site is high on the moutain, maybe you want to simulate the horizon as a depressed line.
Above all, you can specify temperature and air pressure. This allows SkyChart to calculate and compensate for atmospheric refraction.