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 NIGHTFALL(1)                                                   NIGHTFALL(1)
 Nightfall manual                                           Nightfall manual

                              28 December 1999

      nightfall - binary star astronomy

      nightfall -h
      nightfall -U [more options] [mass_ratio inclination primary_size
      secondary_size primary_temperature secondary_temperature]
      nightfall [-G] [-A] [more options] mass_ratio inclination primary_size
      secondary_size primary_temperature secondary_temperature
      nightfall [-G | -U] [-A] [more options] -C file

      nightfall is an interactive astronomy program for fun, education and
      science.  It can generate animated views of eclipsing (or non-
      eclipsing) binary stars, compute light curves and radial velocity
      curves, and determine best-fit models for observational data.

      In the simplest case, nightfall computes the light curve for a binary
      star system with some given mass_ratio (mass of secondary star/mass of
      primary star), inclination (0 = plane-on view, 90 = edge-on view of
      the orbital plane), stellar sizes primary_size,secondary_size
      (dimensionless, in the range 0 - 1.3), and stellar temperatures
      primary_temperature,secondary_temperature (in Kelvin), and writes the
      light curve to a file NightfallCurve.dat.

      nightfall is able to show many non-trivial, and sometimes spectacular,
      physical effecs in binary stars, as it uses a detailed physical model
      rather than simply assuming the stars to be spherical.

      The full documentation  for nightfall is distributed only in DVI and
      HTML format, as it is quite big,  and  thus  not very well suited to
      the 'man' page format. It includes some discussion of binary stars (at
      a popular science level, hopefully) that you may find helpful in
      understanding what the program does.

      -h   Print a usage message on standard output and exit successfully.

      -U   Run in interactive mode.

      -C file
           Use binary star parameters from a configuration file instead of
           giving them on the command line. Sample configuration files are
           in share/nightfall/cfg.

           Plot a graph of the lightcurve after its computation (P|S - zoom
           on primary/secondary eclipse, 1|2 - plot 1/2 orbits).

                                    - 1 -      Formatted:  November 28, 2021

 NIGHTFALL(1)                                                   NIGHTFALL(1)
 Nightfall manual                                           Nightfall manual

                              28 December 1999

      -A   Generate an animated view of the binary star.

           Visualize the geometry of the binary star system (v - view of the
           stars, i - image of the potential, c - contour plot  of the
           potential, a - all of them).

      -H   Send plot to a postscript file. If the postscript file exists, it
           will be overwritten.

           Select the filter/bandpass for which the lightcurve should be
           plotted.  U-K range from ultraviolett to infrared, best match to
           the human eye is V.  u-y are narrow-band filters.  1|2 will
           select radial velocity curves instead of a light curve.

      -fP/-fS value
           By default, nightfall assumes synchroneous  rotation, which means
           that the stars are rotating with the orbital period, and thus
           show each other always the same 'side'. With this option, you can
           set the ratio of stellar rotation frequency to orbital frequency
           to some value different from one, seperately for the primary P
           and the secondary S.

      -e eccentricity periastron_length
           By default, nightfall assumes the orbit to be circular. With this
           option, you can set the eccentricity of the orbit (0 is circular,
           maximum is less than 1), and the periastron length, which is the
           point of closest approach of the two stars in their orbit (0 -
           360 degree).

      -sP/-sS longitude latitude radius dimfactor
           Place a spot on the primary (P) or secondary (S).  The spot
           parameters are the location of the spot (longitude,latitude), its
           radius, and the dimfactor by which the temperature is reduced (or
           increased) within the spot area.

      -tP/-tM/-tD value
           Set the absolute value for the orbital period P (in days), the
           total mass M (in units of solar masses), and/or the orbital
           separation D (in solar radii) of the system. Any two of these are
           independent, the third is then calculated from Kepler's laws
           (i.e., you should set only two of these).

      -I file
           Read in observational data from a data file. Sample data files
           are in share/nightfall/data.


                                    - 2 -      Formatted:  November 28, 2021

 NIGHTFALL(1)                                                   NIGHTFALL(1)
 Nightfall manual                                           Nightfall manual

                              28 December 1999

           Diagnostic output (v - verbose, w - warnings, b - status

      The definition of primary/secondary is inverse to the usual convention
      in astronomy.

      Obviously, the size of a star in a binary system is limited by the
      orbital separation of the two stars. Instead of having to calculate
      the maximum useful stellar size herself, the user simply gives the
      desired stellar size as a fraction (0.001-1.3) of the maximum polar
      radius of the star (which is calculated by the program). In the output
      file NightfallCurve.dat, you will then find the 'real' size of the

      If no absolute values for total mass and orbital period/separation are
      given, the program will use some default values (mass = two solar
      masses, orbital separation = distance earth-sun).  In this case,
      sizes/masses/velocities given in absolute units (e.g. kg, m, m/s,
      solar masses/radii) are fictuous only - they would be valid only for a
      system with the assumed default values of total mass and orbital

      The newest version of nightfall can be found on, and on the homepage

      To subscribe to the nightfall mailing list, send mail to with a body of subscribe nightfall-l.

      Rainer Wichmann (

      If you find a bug in nightfall, please send electronic mail to  Please include your operating system and
      its revision, the version of nightfall, what C compiler you used to
      compile it, and the output from 'configure'.

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