Archive: excel.zip Length Date Time Name -------- ---- ---- ---- 125952 09-16-04 21:58 starorbs.xls 19968 09-16-04 21:58 dsc.xls -------- ------- 145920 2 files
Update of 16Sep04: I've fixed a couple of my Excel spreadsheets, replacing the B1950.0 obliquity of the ecliptic with the correct value for J2000.0. The effects are trivial for any purpose I can think of, but it's nice to have things *right*.
The DSC.XLS Excel spreadsheet allows rapid initial orientation of billboard objects and 3DS models, so that the billboard image or model is set up aimed precisely towards the Sun, with the top of the original image pointing towards celestial north.
The input data required are the right ascension and declination of the billboard object - these values can be entered in decimal degrees, or degrees, minutes and seconds. Right ascension can also be entered as decimal hours, or hours, minutes and seconds.
The spreadsheet then calculates the necessary Axis and Angle commands to orientate your billboard correctly - just copy the two lines from the spreadsheet and paste directly into your dsc file.
Note that pasting into WordPad causes the cell formatting codes to be included, not just the Axis and Angle text. Pasting into Notepad works as desired: only the words Axis, Angle and their numeric values are included.
The spreadsheet works correctly both with Microsoft Excel and when imported into Open Office.
Grant July 2003
![]() Example dsc.xls spreadsheet |
Archive: excel.zip Length Date Time Name -------- ---- ---- ---- 125952 09-16-04 21:58 starorbs.xls 19968 09-16-04 21:58 dsc.xls -------- ------- 145920 2 files
The most commonly provided orbital elements for binary star orbits are plane-of-sky elements - useful for the Earth-based observer, but incompatible with Celestia's ecliptic-based coordinates. The plane-of-sky elements are:
Such elements are available from a number of on-line sources, including the Sixth Catalog of Orbits of Visual Binary Stars at http://ad.usno.navy.mil/wds/orb6.html.
Only the period and the eccentricity are of use to Celestia unprocessed. The position of the ascending node must be converted from the plane of the sky to the ecliptic plane, and the argument of the periastron must then be recalculated from that new ascending node - to make the conversion, we must know the right ascension and declination of the primary star. The real semimajor axis of the binary orbit, in astronomical units, must be derived by performing some trigonometry on the angular separation of the stars and the distance of the primary from the Sun. Finally, the mean anomaly must be derived by calculating the elapsed time between the date of periastron and Celestia's default epoch, and dividing that by the orbital period.
This spreadsheet therefore accepts as input the right ascension, declination and distance of the primary star (in various commonly used units), and the plane-of-sky elements of the binary orbit. As output, it generates an elliptical orbit definition ready to copy-and-paste into a Celestia ssc file.
Grant Hutchison, July 2003
![]() Example starorbs.xls spreadsheet |
A common standard for orbit definitions of Earth satellites is the NORAD two-line element set (TLE) which is not supported by Celestia (yet, as of version 1.3.1pre6).
This spreadsheet accepts as input a standard TLE pasted from a text source, and outputs the Celestia orbit definition for an Earth satellite, ready to copy-and-paste into a Celestia ssc file.
Note that this utility simply translates the "mean" orbital elements provided in the TLE directly into the corresponding Celestia orbital elements, without further processing. It therefore generates a representative orbit for the designated satellite, but not one which can be used for satellite tracking.
(I've put "mean" in scare quotes simply because the TLEs don't provide a conventional arithmetic mean, but a "mean" which arises by the suppression of periodic terms.)
TLEs for many satellites are available from a number of on-line sources, including http://celestrak.com/NORAD/elements/ and http://spaceflight.nasa.gov/realdata/elements/index.html.
As well as building the elliptical orbit definition, the spreadsheet supplies a default name which is the alphanumeric International Designator for the object, retrieved from the TLE - but you'll probably wish to replace this with something more memorable. The spreadsheet also generates an Obliquity, EquatorAscendingNode and RotationOffset, which will rotate your satellite model so that its principal axes align with its orbit. In many cases, you'll need to make a final adjustment to its orientation, using the Orientation command, with or without an additional RotationOffset of 90, 180 or 270 degrees - this is necessary if the principal axes of your model are not aligned with Celestia's principal axes.
Grant Hutchison, July 2003 & January, 2010.
![]() Example tle.xls spreadsheet |
Herewith a spreadsheet to automate the calculations necessary to place a panorama cube on the surface of a (spherical) planet or moon. Some care needs to be taken when translating feature coordinates for use in Celestia.
Celestia uses the 'right-hand rule' to establish the north pole of a body - a body appears to rotate in a counterclockwise direction if you look down upon it from above the Celestia north pole. This differs from the definition used by the USGS and IAU, the usual sources for feature coordinates - these organizations designate whichever pole lies to the north of the ecliptic plane as the north pole. The two definitions are identical for objects that rotate in the same direction as the Earth, but are opposite for bodies that rotate in a retrograde direction. So for retrograde rotators you should reverse the USGS/IAU coordinates before using them in Celestia: north becomes south, south becomes north and east becomes west.
The USGS/IAU system measures longitude 360 degrees eastward for all retrograde rotators, and 360 degrees westward for almost all direct rotators. The Earth and Moon are exceptions - longitude is measured both west and east of the prime meridian, through 180 degrees. On Mars, two coordinates systems are in use: planetographic latitudes are quoted with west longitudes, while planetocentric latitudes are given with east longitudes. The planetocentric/east longitude system is the appropriate system to use in Celestia.
So, to use USGS/IAU coordinates in this spreadsheet (and in Celestia): For direct rotators: North latitudes are positive, south latitudes negative, east longitudes positive and west longitudes negative. For retrograde rotators: Longitude will be given in degrees east. Since all coordinates must be reversed to match the Celestia system, north latitudes should be made negative, south latitudes positive, and east longitudes negative.
Grant Hutchison, January, 2004
![]() Example panorama.xls spreadsheet |
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