Definition of constellation boundaries by the IAU
The celestial sphere is conventionally divided into 88 regions, called constellations, whose boundaries have been standardized in 1930 on behalf of the International Astronomical Union by the Belgian astronomer Eugène Delporte.
The edges of each constellation boundary are segments of meridians of right ascension and parallels of declination in the equatorial coordinate system relative to the equinox of B1875.0. Delporte's book Délimitation Scientifique des Constellations (Cambridge University Press, 1930) provides coordinates for the edges of every constellation boundary. It is still the reference source for these coordinates.
There are interesting sites covering the topic of constellation boundaries, for instance Ian Ridpath's and Andrew James's (which contains an English translation of the introduction to Delporte's book). Felice Stoppa's site, mainly in Italian, has reproductions of Delporte's book, including the pages that specify the coordinates of the constellation boundaries.
Available constellation boundary data
The constellation boundary data is available through the VizieR service in catalogue VI/49. The file of interest is bound_18.dat, which seems to be the closest version of Delporte's original data available in electronic form.
However, the data it contains differs from the original in several respects.
 The coordinates have been converted from sexagesimal to decimal format and rounded, so that many of the values are only approximations of the original ones. In a couple of instances, the value was truncated rather than rounded (declination value of +86.16666 instead of +86.16667).

In order to facilitate plotting (as stated in
the documentation), some vertices
were added to the original set:
1.00000 +88.0000 CEP 0.00000 90.0000 OCT 24.00000 90.0000 OCT 12.00000 90.0000 OCT
Also, the right ascension of24.00000 75.0000 OCT
was set to 24 instead of 0.
Catalogue VI/49 is adequate for practical purposes, but it seems unsatisfactory that such standard data shouldn't be available in a form closer to the original.
Data available on this page
This is why I have compiled the constellation boundary data directly from Delporte's book and made it available on this page, along with various potentially useful derived data sets. The following files are available.
 FileName Lrecl Records Explanations  ReadMe 80 File descriptions bound_ed.dat 23 1562 Boundary data from E. Delporte's book verts_18.txt 38 694 Vertices (B1875, sex) edges_18.txt 57 781 Edges (B1875, sex) bound_verts_18.txt 38 1562 Boundary vertices (B1875, ccw, sex) bound_edges_18.txt 57 1562 Boundary edges (B1875, ccw, sex) bound_18.txt 23 1562 Boundaries (B1875, ccw, dec) bound_in_18.txt 23 13238 Interpolated boundaries (B1875, ccw, dec) bound_in_20.txt 23 13238 Interpolated boundaries (J2000, ccw, dec) lines_18.txt 26 1194 Merged edges (B1875, decimal) lines_in_18.txt 26 7121 Interpolated merged edges (B1875, dec) lines_in_20.txt 26 7121 Interpolated merged edges (J2000, dec) centers_18.txt 32 89 Constellation centers (B1875, dec) centers_20.txt 32 89 Constellation centers (J2000, dec) 
The file descriptions below (and gathered in the ReadMe file) follow the conventions for catalogue descriptions of the VizieR service.
Original boundary data
The original constellation boundary data compiled from Delporte's book is contained in the file bound_ed.dat. The format of this file is similar to that of bound_18.dat. Each constellation boundary is represented by a sequence of vertices, one vertex per line keyed by the constellation abbreviation, according to the following format.
 Bytes Format Units Label Explanations  1 2 I2 h RAh Right ascension B1875 (hours) 3 A1  D1 [:] delimiter 4 5 I2 min RAm Right ascension B1875 (minutes) 6 A1  D2 [:] delimiter 7 8 I2 s RAs Right ascension B1875 (seconds) 10 A1  DE Declination B1875 (sign) 11 12 I2 deg DEd Declination B1875 (degrees) 13 A1  D3 [:] delimiter 14 15 I2 arcmin DEm Declination B1875 (minutes) 16 A1  D4 [:] delimiter 17 18 I2 arcsec DEs Declination B1875 (seconds) 20 23 A4  Con Constellation abbreviation 
The last field contains the abbreviation of the constellation corresponding to the current boundary. All the constellation abbreviations consist of 3 letters, or 3 letters followed by a digit for SER1 and SER2, the two parts of Serpens, treated as distinct constellations in most places on this page.
Delporte's book describes boundary edges rather than vertices (see this example), and therefore contains much redundancy. The vertices description is more compact and lends itself better to automatic processing. In bound_ed.dat, the vertices are enumerated in the order in which they appear in the book.
For verification, among other things, I converted the VizieR file bound_18.dat to sexagesimal format (rounding to the nearest second), removed the additional vertices mentioned above and normalized right ascensions. The resulting file is identical to bound_ed.dat except for the position of the vertex 00:00:00 75:00:00 OCT which, in the book, appears first instead of last in the constellation Octans.
Derived data
The constellation boundary edges in Delporte's data set have been designed so that every edge is shared by exactly two constellations. Say that three points P, Q, R lie on the same segment of meridian of parallel, with Q between P and R, and that PR borders constellation C on one side, while PQ and QR border different constellations D and E on the other side. Even though the edge PR could be used in C's boundary, the data set will enumerate PQ and QR separately in C's boundary in order to satisfy this requirement.
A desirable feature of the constellation boundaries data set would be that all the boundaries be enumerated in the same direction, say counterclockwise (looking at the celestial sphere from inside), so that every edge would appear twice, once in each direction. This was apparently not a concern of the author. The following constellations are enumerated clockwise: ANT, APS, CAE, CAP, CAR, CEN, CHA, CIR, CRT, CRU, CRV, DOR, GRU, IND, LEP, LIB, LUP, MUS, NOR, PAV, PHE, PIC, PSA, PUP, RET, SCT, SGR, TEL, TUC, UMI, VEL, VOL. The rest of the datasets described on this page consistently use the counterclockwise orientation when applicable.
Vertices in sexagesimal B1875.0 coordinates
The file verts_18.txt enumerates the boundary vertices in the order of their appearance in Delporte's book and assigns them a unique key. Each line describes one vertex, according to the following format.
 Bytes Format Units Label Explanations  1 3 A3  Key Vertex key 5 6 I2 h RAh Right ascension B1875 (hours) 7 A1  D1 [:] delimiter 8 9 I2 min RAm Right ascension B1875 (minutes) 10 A1  D2 [:] delimiter 11 12 I2 s RAs Right ascension B1875 (seconds) 14 A1  DE Declination B1875 (sign) 15 16 I2 deg DEd Declination B1875 (degrees) 17 A1  D3 [:] delimiter 18 19 I2 arcmin DEm Declination B1875 (minutes) 20 A1  D4 [:] delimiter 21 22 I2 arcsec DEs Declination B1875 (seconds) 24 38 A15  Cons Constellation abbreviations 
The last field contains the spacedelimited abbreviations of the constellations whose boundaries contain the vertex. A vertex can belong to two, three, or four constellations.
Edges in sexagesimal B1875.0 coordinates
The file edges_18.txt enumerates the boundary edges. Each edge appears only once, oriented in the direction of increasing right ascensions (for parallels) or declinations (for meridians). Each line describes one edge, according to the following format.
 Bytes Format Units Label Explanations  1 3 A3  Key1 Key of 1st vertex 4 A1  D1 [:] delimiter 5 7 A3  Key2 Key of 2nd vertex 9 A1  Type Edge type: [M]eridian or [P]arallel 10 A1  Dir Edge direction: + increasing or  decreasing 12 13 I2 h RA1_h Right ascension B1875 (hours) of 1st vertex 14 A1  D2 [:] delimiter 15 16 I2 min RA1_m Right ascension B1875 (minutes) of 1st vertex 17 A1  D3 [:] delimiter 18 19 I2 s RA1_s Right ascension B1875 (seconds) of 1st vertex 21 A1  DE1 Declination B1875 (sign) of 1st vertex 22 23 I2 deg DE1_d Declination B1875 (degrees) of 1st vertex 24 A1  D4 [:] delimiter 25 26 I2 arcmin DE1_m Declination B1875 (minutes) of 1st vertex 27 A1  D5 [:] delimiter 28 29 I2 arcsec DE1_s Declination B1875 (seconds) of 1st vertex 31 32 I2 h RA2_h Right ascension B1875 (hours) of 2nd vertex 33 A1  D6 [:] delimiter 34 35 I2 min RA2_m Right ascension B1875 (minutes) of 2nd vertex 36 A1  D7 [:] delimiter 37 38 I2 s RA2_s Right ascension B1875 (seconds) of 2nd vertex 40 A1  DE2 Declination B1875 (sign) of 2nd vertex 41 42 I2 deg DE2_d Declination B1875 (degrees) of 2nd vertex 43 A1  D8 [:] delimiter 44 45 I2 arcmin DE2_m Declination B1875 (minutes) of 2nd vertex 46 A1  D9 [:] delimiter 47 48 I2 arcsec DE2_s Declination B1875 (seconds) of 2nd vertex 50 57 A8  Cons Constellation abbreviations 
All the edges in this dataset have the + direction, meaning that their end points are listed in the order of increasing coordinates (right ascensions for parallels, declinations for meridians).
The last field contains the spacedelimited abbreviations of the two constellations adjacent to the edge. The constellation for which the edge appears counterclockwise figures first.
Boundary vertices counterclockwise in sexagesimal B1875.0 coordinates
Every vertex can belong to two, three, or four constellations. The file bound_verts_18.txt enumerates the boundary vertices, just like bound_ed.dat, but all the boundaries are enumerated counterclockwise, the vertex keys are specified, and all the constellations to which the vertex belongs are given. Each boundary is represented by a sequence of vertices, one vertex per line, according to the same format as verts_18.txt.
In the last field, the constellation corresponding to the current boundary appears first.
Boundary edges counterclockwise in sexagesimal B1875.0 coordinates
The file bound_edges_18.txt enumerates the boundary edges, much like Delporte's book, but all the boundaries are enumerated counterclockwise. Each boundary is represented by a sequence of edges, one edge per line, according to the same format as edges_18.txt.
In the last field, the constellation corresponding to the current boundary, and for which the edge appears counterclockwise, figures first.
Boundaries counterclockwise in decimal B1875.0 coordinates
The file bound_18.txt is the counterclockwise and decimal version of bound_ed.dat. The right ascensions and declinations of the boundary vertices are respectively given in decimal hours and decimal degrees, more convenient than sexagesimal values for practical purposes. Its format is as follows.
 Bytes Format Units Label Explanations  1 8 F8.5 h RAhr Right ascension B1875 (decimal hours) 10 A1  DE Declination B1875 (sign) 11 18 F8.5 deg DEdeg Declination B1875 (decimal degrees) 20 23 A4  Con Constellation abbreviation 
Interpolated boundaries counterclockwise in decimal B1875.0 coordinates
Even though the edges of the constellation boundaries align with meridians and parallels of the B1875.0 equatorial coordinate system, they can take the form of complex curves in an arbitrary projection. It is probably more convenient to draw them as sufficiently smooth polygons. In the file bound_in_18.txt, the boundary edges have been interpolated by adding points with whole degree declinations (along meridians) or whole degree right ascensions (along parallels) between vertices. This granularity seems sufficient for many applications. The imprecision becomes apparent only in regions of extreme distortion in some projections. The format of bound_in_18.txt is identical to the format of bound_18.txt.
Interpolated boundaries counterclockwise in decimal J2000.0 coordinates
In order to draw constellation boundaries in the current J2000.0 equatorial system, the coordinate transformation corresponding to the precession of the equinoxes between B1875.0 and J2000.0 needs to be applied. In that coordinate system, the constellation boundaries are not aligned with meridians and parallels any more, and an interpolated data set is even more necessary. The file bound_in_20.txt contains the same points as bound_in_18.txt, but represented in the J2000.0 coordinate system. The transformation has been effected according to the Explanatory Supplement to the Astronomical Almanac (P. K. Seidelman, Ed., Sausalito: University Science Books, 1992), pp. 103104, and using the Besselian to Julian epoch conversion as implemented by the SOFA software library. The format of bound_in_20.txt is identical to that of bound_in_18.txt and bound_18.txt, except that the coordinates are relative to the J2000.0 equatorial system.
 Bytes Format Units Label Explanations  1 8 F8.5 h RAhr Right ascension J2000 (decimal hours) 10 A1  DE Declination J2000 (sign) 11 18 F8.5 deg DEdeg Declination J2000 (decimal degrees) 20 23 A4  Con Constellation abbreviation 
Merged edges in decimal B1875.0 coordinates
The data sets bound_18.txt and its variants are appropriate when drawing the boundary of a single constellation. However, when drawing all the constellations boundaries, they cause every edge to be drawn twice (once in each direction, since all the boundaries are oriented counterclockwise). This can be undesirable when using a dotted or dashed line pattern, or for efficiency reasons.
In such a situation, a data set such as a decimal version of edges_18.txt, where every edge is mentioned only once, is preferable. Better yet, contiguous edges along the same direction can be merged into a single segment, providing an additional optimization. The data set lines_18.txt contains the segments obtained by merging contiguous edges as much as possible. Each segment is represented by two vertices, one per line, tagged with a unique key for the segment, made from the vertex keys assigned by verts_18.txt. The format is as follows.
 Bytes Format Units Label Explanations  1 8 F8.5 h RAhr Right ascension B1875 (decimal hours) 10 A1  DE Declination B1875 (sign) 11 18 F8.5 deg DEdeg Declination B1875 (decimal degrees) 20 26 A7  Key Segment key 
Interpolated merged edges in decimal B1875.0 coordinates
In order to draw the merged edges in an arbitrary projection, interpolated segments are useful. The dataset lines_in_18.txt provides this data. All the interpolated points along a segment are tagged with the segment's key. The format of this file is identical the format of lines_18.txt.
Interpolated merged edges in decimal J2000.0 coordinates
The file lines_in_20.txt is the J2000.0 version of lines_in_18.txt, and its format is similar, as follows.
 Bytes Format Units Label Explanations  1 8 F8.5 h RAhr Right ascension J2000 (decimal hours) 10 A1  DE Declination J2000 (sign) 11 18 F8.5 deg DEdeg Declination J2000 (decimal degrees) 20 26 A7  Key Segment key 
Constellation centers in decimal B1875.0 coordinates
The file centers_18.txt contains the coordinates of constellation center points in B1875.0 coordinates, as well as the constellation areas in square degrees and the constellation ranks in order of decreasing area. The centers are useful when drawing maps. They can serve as default positions for constellation names, for instance.
The constellation areas were computed by subdividing the constellation into coordinate rectangles and adding the rectangle areas. The area of a coordinate rectangle between right ascensions α_{0}, α_{1} and declinations δ_{0}, δ_{1} on the celestial sphere of radius 1, obtained by integrating the element of surface dS = cos δ dα dδ over the rectangle, is given by
S = (α_{1}  α_{0}) (sin δ_{1}  sin δ_{0}).
The constellation centers are defined as their barycenters on the sphere. Their coordinates are given by the integrals of α dS = α cos δ dα dδ and δ dS = δ cos δ dα dδ over the constellation, divided by the area of the constellation. They can be obtained as a weighted averages of the barycenters of the coordinate rectangles mentioned above. The barycenter of the coordinate rectangle between right ascensions α_{0}, α_{1} and declinations δ_{0}, δ_{1} is given by
α_{c} = (α_{0} + α_{1}) / 2
δ_{c} =
(δ_{1} sin δ_{1}  δ_{0} sin δ_{0} + cos
δ_{1}  cos δ_{0}) / (sin δ_{1}  sin δ_{0}).
The ranks represent the positions of the constellations in their order of decreasing area. To compute the rank, the two parts of Serpens have been merged, their individual areas added, and the rank of Serpens assigned to both SER1 and SER2.
Reassuringly, the areas and ranks computed here agree with those found on Ian Ridpath's site (here and here). The format of centers_18.txt is as follows.
 Bytes Format Units Label Explanations  1 8 F8.5 h RAhr Right ascension B1875 (decimal hours) of center 10 A1  DE Declination B1875 (sign) of center 11 18 F8.5 deg DEdeg Declination B1875 (decimal degrees) of center 20 26 F7.2 deg2 Area Constellation area (square degrees) 28 29 I2  Rank Constellation rank in area 31 34 A4  Con Constellation abbreviation 
Constellation centers in decimal J2000.0 coordinates
The file centers_20.txt is the J2000.0 version of centers_18.txt and follows a similar format.
 Bytes Format Units Label Explanations  1 8 F8.5 h RAhr Right ascension J2000 (decimal hours) of center 10 A1  DE Declination J2000 (sign) of center 11 18 F8.5 deg DEdeg Declination J2000 (decimal degrees) of center 20 26 F7.2 deg2 Area Constellation area (square degrees) 28 29 I2  Rank Constellation rank in area 31 34 A4  Con Constellation abbreviation 
Maps
The celestial maps gathered here illustrate the constellation boundaries. The constellation boundaries were drawn using the lines_in_18.txt or lines_in_20.txt data sets. Individual constellations were traced using the bound_in_20.txt data set for coloring.
I hope that this page will be useful or interesting to someone. Don't hesitate to drop me a note for comments, questions, or suggestions for improvements. While I have taken great care to ensure the accuracy of the data, errors are always possible. I would be grateful, if you find any, that you let me know about it.