REALISTIC ASTROGRAPHY (version 2.0)
for Traveller and other scifi RPGs
by Constantine Thomas


Last Update: 2rd April 2010.
MAJOR UPDATE. I've updated the J2000.0 Equatorial to Galactic Conversion Spreadsheet (found in Section 1) so that it also lets you convert from XYZ to Galactic to J2000.0 Equatorial! Thanks to Veeger, PraedSt, and Grant Hutchison on BAUTforum for all their help with this one! :)


SECTION 0: INTRODUCTION

The aim of this project is to map the stars around Sol in a manner that preserves their direction and distance relative to eachother as much as possible. The existing canonical GDW maps of the Sol subsector and its environs are extremely distorted compared to reality - direction is not preserved, and distances from any given star become increasingly distorted beyond J2 range. The inaccuracies of the GDW map are largely due to stellar positions being distorted by "flattening" of a 3D space into a single 2D map, inaccurate stellar location data to start with. and possibly some other unknown factors - it also obviously omits any low mass stars discovered in astrometric searches between 1977 and 2003. Either way, the positions of the stars on the GDW Sol subsector map bears little resemblance to the real spatial distribution of stars around Sol.

In Section 3 I present a series of stacked 2D maps of a 3D volume around Sol using the latest available astrometric data of the stars in the vicinity of Sol. This avoids any "flattening" issues since each layer will preserve the vertical position of the maps, and the data will be much more accurate since it is based on HIPPARCOS and other catalogues. The resulting maps are very different to those presented in Traveller canon - thus they are entirely non-canonical - but are much more physically accurate. As a result, you will not find any attempt to link any star positions on the canonical GDW map with this realistic map.

This dataset is aimed at anyone interested in astronomy or who is interested in adding nearby stars to their own RPG or sci-fi/Traveller universes - the raw data in particular is easily adaptable to any sci-fi setting or RPG. Every effort has been made to make the maps as simple to interpret as possible, though jump routes have not been calculated.


SECTION 1: DATA PROCESSING

Data processing: The original dataset used for this project was the RECONS list of the 100 nearest star systems. This represents the state of our knowledge of the space around Sol as of 1/1/2003 - more recently discovered stars have not been added, but might be added at some point later on if/when their positions are confirmed.

+ + + Ver 2.0: The HIPPARCOS catalogue provided the co-ordinates for more distant stars beyond the RECONS distance limit (6.86 parsecs from Sol). These stars are shown on the Solomani Rim sector map in red, and on the layered maps in black to distinguish them from the RECONS data. Only a very small number of the known distant stars are shown here (including all the ones listed in SolStation Nearby Stars list) - according to HIPPARCOS there are actually over 3,480 stars within an 11 parsec radius of Sol! + + +

The J2000.0 Right Ascension and Declination (equatorial co-ordinates) for each star were taken from this table and converted to corresponding alpha/delta (decimal degree) values and then into galactic co-ordinates - trigonometric parallax values for the stars were also taken from the RECONS table and converted from milliarcseconds to lightyears and parsecs (the correct distances are assumed to be in the centre of the error bars). The galactic co-ordinates and parallaxes were then converted into orthogonal X/Y/Z co-ordinates so that they could be placed on the 2D layers.

Co-ordinate Converter: A spreadsheet is available that converts any star locations into galactic co-ordinates if their J2000.00 RA/Dec and parallax values are known:

Download: J2000.0 equatorial co-ordinates to galactic co-ordinates conversion spreadsheet (Excel 97-2003, 34 KB)
UPDATED 2nd April 2010: The conversion spreadsheet now also lets you convert from XYZ (or galactic) to J2000.0 equatorial co-ordinates!

This conversion is also accurate for J1991.25 equatorial co-ordinates from the Hipparcos database - no adjustments to the formulae are necessary. The spreadsheet requires the Right Ascension and Declination (in hr/min/sec and deg/min/sec respectively, both in J2000.00 epoch) and the J2000.00 trigonometric parallax values (in arcseconds).

The spreadsheet will not produce accurate results for data from earlier epochs (e.g. 1950.00) - use only HIPPARCOS or later data here! This is due to inaccuracies in the older data, along with precession of the star co-ordinates since that epoch.

Other data (e.g. star type, mass etc) are taken from the RECONS list for now. This will be updated from other sources later.

Thanks to Veeger, PraedSt, and Grant Hutchison from BAUTforum, and Spaceman Spiff and Selden from the Celestia forums for all their help with figuring out the conversions! :)

Other star catalogue links: The following catalogues were used to check common star names and spectral types. Many of these can be used to find the RA/Dec of stars beyond the range of the RECONS list. Catalogue numbers can be found at Solstation and at Alcyone BSC (or by looking in Celestia). The links are listed in order of complexity - the top links are very understandable, the bottom links are much more technical.

  • Celestia - The best star and solar system viewer around, with all the stars from the Hipparcos database. Invaluable for reality checking this project.
  • RECONS Top 100 Near Star List - More up to date than HIPPARCOS, but only for the nearest 100 star systems to Sol.
  • Solstation database - excellent, very detailed and understandable nearby star/planet database. Provides catalogue numbers.
  • Internet Stellar Database - very useful database of nearby stars.
  • Alcyone Bright Star Catalogue - useful info on many bright stars. Provides catalogue numbers too.
  • ARI Near Star Database - lists all Gliese, GJ, LHS, and other stars.
  • SIMBAD query form - finds details on stars if catalogue numbers are known.
  • Advanced HIPPARCOS search form (click the I/239/hip_main button to access the catalogue) - this is a more advanced engine that allows the user to customise the search through the HIPPARCOS dataset and returns multiple results.


    SECTION 2: MAPPING METHODOLOGY

    Mapping Technique: Each hex is assumed to represent a volume of 1 cubic parsec (1 x 1 x 1 parsec across). Distances are measured from the centre of each hex - if a star is located between 0.5 and 1.5 pc of a point, it is placed 1 hex away from it. If it is located 1.51 to 2.50 pc from that point, it would be placed two hexes away. And so on. This applies in all three orthogonal co-ordinate directions (X/Y/Z). The origin of the co-ordinate system is the centre of the yellow hex on the map (the location/projection of So onto that layerl). The +X axis points directly toward the Galactic Core, and galactic longitude is measured anti-clockwise from this axis, so the +Y axis points to the left. The map itself is viewed from the Galactic North Pole, which is directly "above" Sol. The viewer is therefore looking down on the map from the +Z direction - negative Z points into the screen.

    Why Hexes?

    One might wonder, since I treat every hex as a cube, why I'm using hexes instead of squares here. This is because because hexes are:

    1. the system that Traveller uses.
    2. more useful because there are six directions from a given hex where distances to the centres of the hex can be measured in integer units of parsecs, rather than four directions that one would have with a square system.
    3. the distance between the centres of adjacent hexes in any direction is always 1 pc. The distance between adjacent squares is either 1 pc (if you go orthogonally) or 1.4 pc (if you go diagonally).
    Hexes are slightly more complex to use though, since the orthogonal Y distances in pc derived from the astronomical data don't translate directly to hex columns - because of the arrangement of the hex columns, the distance between the centres of adjacent columns on the hex grid actually represents 0.866 pc, not 1.0 pc as they would on a non-staggered square grid. So each orthogonal Y distance (in parsecs) must be divided by 0.866 to convert it to a number of hex columns that one must count across to place the star.

    It doesn't actually make much practical difference if I used square hexes though, since it doesn't really solve any of the inaccuracies on the map (see below).

    Map Inaccuracies: Although the actual star co-ordinates are very accurate, the accuracy of their positions as represented on the map are only accurate to about +/- 0.5 pc. The reason for this is the integer nature of the hexes - it is impossible to show the positions of stars that are actually located near opposite edges of a given hex. This applies in the vertical axis too - it is impossible to distinguish between a star that is near the top of the layer and near the bottom of the layer. The hex map can only really show that the star is located somewhere within a given cubic parcec of space (and a cubic parsec represents a huge volume of space - about 35 cubic lightyears!). Sometimes this may lead to stars being further apart or closer together than they are in reality.

    There are 28 RECONS systems on the hex map that are actually a parsec further or closer to Sol on the hexagonal map than they are in reality. This is largely due to rounding errors in the translation from decimal orthogonal X/Y/Z co-ordinates to a map divided into integer units (if a square grid was used instead, the map would have 25 such 'inaccurate' systems, so it wouldn't really make any difference to change the mapping system). The only way to correct this would be to add more resolution to the map, but this would take a long time and would also cause the map to be too cumbersome to be usable. I've therefore not attempted to fix these errors on the map. It may sound bad that over a quarter of the systems are off by a parsec on the map, but the practical difference isn't actually that earth-shattering - the important thing is that the stars are in the right places relative to eachother and approximately at the correct distances. Besides, the raw numbers are provided and those are fully accurate - so if GMs want to experiment with alternative mapping methods that may be more accurate then they're free to do so.

    Also note that the star SO025300.5+165258 (Teegarden's Star) is not included on these maps - partly because it was discovered after 1/1/2003, and partly because its distance has not been firmly nailed down yet. It should be located on the -1pc or -2 pc layers either 1 or 2 hexes to the rimward/spinward of Sol, depending on how far it actually is.


    SECTION 3: NEAR STAR MAPS

    In these maps, Sol is located in the centre of the 0 pc layer map - the yellow central hex in each the layers represents the vertical projection of Sol's location onto that layer. All co-ordinates are measured from the centre of the Sol hex. The centre of each layer is 1.0 from the adjacent layer above or below it - each layer is 1 pc thick. Traveller subsector boundaries/names are shown for reference in orange. A smaller version of the +0 pc map is shown on the left.

    Galactic directions are shown in bold text on the four edges of each layer. The axes run directly through the central yellow hex - coreward (galactic longitude 0°) is directly to the centre of the top edge of the map from the centre of this hex , rimward (galactic longitude 180°) is directly to the centre of the bottom edge of the map from the centre of this hex, spinward (galactic longitude 90°) is directly to the centre of the left edge of the map from the centre of this hex, and trailing (galactic longitude 270°) is directly to the centre of the right edge of the map from the centre of this hex.

    + + + Ver 2.0: The maps now span multiple subsectors and are no longer 7pc-radius around Sol. Hex numbers have been added in orange text, and the subsector boundaries have now been corrected since they were inaccurate in the original versions. A selection of HIPPARCOS stars that are (a) listed at SolStation and (b) within +/- 6 pc of Sol are now also shown on the layered maps in black text. These lie beyond the RECONS stars, but within the mapping area. These are not the only other stars in this volume of space (there are thousands more!). + + +

    The maps should be stacked as shown below, with the +6 pc layer at the "top", going down through the 0 pc layer to the -6 pc layer at the "bottom". Each map is an approximately 100 KB in size.

    To download the maps, right-click on the links below and select "Save Target As..." (they can be also viewed in a web browser by left-clicking the links).

    Full star data: This excel file contains the raw star data on which these maps are based.
    -Sheet 1: raw RECONS Star List (includes all data)
    -Sheet 2: RECONS Star list adapted for mapping (includes X/Y/Z co-ordinates in ly and pc)
    -Sheets 3-15: Separate star lists divided according to vertical layer. Distances are given in lightyears and parsecs. To find the Y-distance in hex columns, divide the Y-distance in parsecs (shown in blue) by 0.866.

    Download: Near Star List Spreadsheet - raw and processed data (Excel 97, 261KB).

    Download: CHView file showing the RECONS dataset. CHView, 12KB).

    + + + Ver 2.0: For now, only the RECONS Excel data and CHView files will be made available - the HIPPARCOS add-on files may be added at a later date. Full information on the additional (non-RECONS) stars can be found at SolStation. + + +


    SECTION 4: JUMP DRIVES

    Jump drives and distances: Each jump increment in this 3D mapping system represents the middle of the range of distances that the ship can jump - unlike Traveller canon, it does not directly represent the maximum distance that the ship can jump. This is illustrated on the table below:

    Jump
    increment    		 Hex
    distance    		 Corresponding
    distance
    J0* 0 0.00 - 0.50 pc
    J1 1 0.50 - 1.50 pc
    J2 2 1.51 - 2.50 pc
    J3 3 2.51 - 3.50 pc
    J4 4 3.51 - 4.50 pc
    J5 5 4.51 - 5.50 pc
    J6 6 5.51 - 6.50 pc

    *: J0 drives are usually primitive affairs that can only microjump within a system. Their maximum range is about 100,000 AU (0.63 ly).

    Note that it does not cost any more or less fuel - or take any more or less time - to jump to anywhere in the 1pc range of the destination hex. A jump to a system 4.6 pc away will take the same time and use the same fuel as a jump to a system 5.4 pc away.

    Example: A system that is three hexes away is actually between 2.51 and 3.50 parsecs away - this distance range requires a J3 (or greater) drive to reach in a single jump.


    Vertical distances: The table below shows the J-drive required to make a jump that has both vertical and horizontal components. To determine this, first count the number of layers between the two stars - this is the "Down" number. The "Across" number is the number of hexes between the two stars as seen on a single layer - simply count the hexes that lie on a line between one star and the projection of the other onto that layer (or the star itself, if it is on the same layer). Simply cross-reference the numbers on the table to see the jump drive required to jump that distance. Non-bold numbers represent jumps that are too long-range to be made in a single J6 jump.

    0 1 2 3 4 5 6 <-- Across
    Down
    0 0 1 2 3 4 5 6
    1 1 1 2 3 4 5 6
    2 2 2 3 4 4 5 6
    3 3 3 4 4 5 6 7
    4 4 4 4 5 6 6 7
    5 5 5 5 6 6 7 8
    6 6 6 6 7 7 8 8

    Example: A J4 ship wanted to jump between star A and star B, where B was 3 hexes across and 4 layers up relative to A. Cross-referencing the table, we find that it wouldn't be able to make the jump because the distance between the two stars is 5 hexes. If it had a J5 or J6 drive, it could make the jump.

    (this table was made by calculating the distances between the hexes, and then rounding to the nearest whole number. Exact distances in parsecs between the hexes can be found here).


    SECTION 5: STILL TO DO...

  • I may describe some of these systems in detail, with links to relevant websites about the systems if any planets are known to exist around those stars. I may even make some conjectural systems around them too.

  • I also plan to examine the Charted Space map and locate bright stars relative to Terra (the positions of the named stars on the Charted Space map also do not tally with reality).

  • A Solomani Rim sector map showing some of the major stars is also on the way.

    Stay tuned for more!


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    FrameSet URL: http://www.evildrganymede.net/rpg/world/mapping.htm