These programs are intended to assist observational astronomers.
Starchart produces astronomical charts from a variety of databases, producing output for, in rough order of quality, PostScript, X11, sunview, Atari St, IBM PC, X10, tektronix, unix plot, HP laserjet, and tty displays, on Unix, vms and other systems. It is straightforward to write drivers for other output devices. Observe is a multifunction program for use in planning an observing session. It calculates the positions of planets and their satellites, asteroids and comets, calculates rising and setting times, and much more.
This software may be redistributed freely, not sold.
No representation is made about the suitability of this software for any purpose. It is provided "as is" without express or implied warranty, to the extent permitted by applicable law.
The author disclaims all warranties with regard to this software to the extent permitted by applicable law, including all implied warranties of merchantability and fitness. In no event shall the author be liable for any special, indirect or consequential damages or any damages whatsoever resulting from loss of use, data or profits, whether in an action of contract, negligence or other tortious action, arising out of or in connection with the use or performance of this software.
Several programs are included in this package, all intended to aid
observational astronomers. The observe program calculates positions
of moving objects and helps in planning an observing session. There are
several starchart programs for preparing astronomical charts; there is a
separate program for each output device. There are also several support
programs which operate on data files for these programs.
Starchart programs are quite general star map drawing programs and have many potential uses, but their unique utility is in preparing custom charts for particular observing projects, finding charts, and a pages that can be put in observing notebook and annotated at will.
The observe program provides many facilities of use in planning
an observing session. It can generate ephemerides of planets, minor
planets, and comets, and puts the coordinates in files to be used with
starchart programs. It calculates the approximate altitude and azimuth
of obects at sun rise and set and morning and evening twilights. It can
calculate positions of the major satellites of Jupiter and Saturn. Of
importance to observers is the generation of the timetable of events for
a night.
Other programs are provided to facilitate the use of these programs.
There is a sky. There are things in the sky. The starchart programs
draw maps of things in the sky. The observe program helps you plan
to look at things in the sky.
The things in the sky include stars, planets, nebulae, clusters of stars, and galaxies. For thousands of years, people have grouped the stars in the sky into patterns, and constellations. More recently, the sky has been divided into areas based on these constellations. For hundreds of years, astronomers have used a latitude-longitude grid for defining the locations of celestial objects. The longitude is usually referred to as right ascension or RA, the latitude is the declination or DEC. Through the year, the sun follows a path in the sky, called the Ecliptic, which is the plane of the earth's orbit. Other planets in the solar system are roughly in this plane.
The locations of stars, nebulae, clusters, and galaxies have been tabulated for hundreds of years. These programs use computerized forms of such databases.
You should be familiar with the method of specifying a location on the surface of the earth: two coordinates are used: latitude north of the equator and meridian of longitude east of Greenwich. A similar system is used to specify the locations of points in the sky. the meridians are called right ascension, and the latitude is called declination. RA is measured from the point at which the sun crosses the equator in March. It is measured in hours, with 15 degrees = 1 hour.
There are two other coordinate systems commonly used in addition to RA--dec. These apply only to a particular observation location and time. The first of these is Hour angle -- declination. It is similar to R.A.--declination, but instead of R.A., the angle is measured west from the line from north to south passing directly overhead. The second is altitude -- azimuth (alt--az): altitude in degrees above the horizon and azimuth in degrees east from north.
The coordinates of a "fixed" object are actually only approximately constant. Two factors change the coordinates in R.A. and declination.
First, stars (and other galactic objects) are not fixed in space. The sun and stars all move. Only the closer stars move significantly, but closer stars are also brighter, so this is an important effect. The epoch is the time for which the coordinates are valid including the effects of this proper motion.
The second and more important factor is that the R.A. and dec. coordinate system changes: the north pole and thus the equator change due to precession. R.A. changes as the equator moves and changes the point at which the sun crosses the equator. The equinox date, or equator and equinox is the time at which the coordinate system is valid. Equinox 2000.0 and 1950.0 are commonly used coordinate systems. The equator and equinox of the date is also occasionally important.
The starchart programs use data in equinox 2000.0 coordinates.
Precession and epoch were discussed above. In addition there are smaller effects which must be considered for precise astrometry. These include effects of motion about the earth-moon barycenter, light travel time, nutation, and aberration of starlight.
Magnitudes are defined with respect to standards.
Objects emit photons. The number of photons per second observed from an object under given conditions and equipment is the intensity of light. A brighter object emits more photons. The difference in magnitudes between two objects is -2.5 times the log (base 10) of the ratio of intensities, i.e.
Magnitudes are measured in different ways. The most important are visual, photographic, and photometric. Visual and photographic magnitudes are measured using those methods to estimate relative magnitudes. Photometry is the counting of photons received from objects. Standard filters are generally used. The most common set is UBVRI, Ultraviolet to Infrared. The V filter approximates the response of the eye, while the B filter is approximately the response of photographic film.
For a magnitude number to be truely meaningful, the system used must be specified. Generally V or visual may be assumed.
For many objects (V-B), that is the magnitude measured photometrically with the V filter minus the B magnitude, indicates the color of the object.
Time is a very complex subject.
Time systems include UT (= UT1), UT0, TA1, and UTC. These times may differ by a second or so. They are based on the rotation of the earth. As the earth slows, and since the day is not exactly 86400 seconds long, leap seconds are occasionally inserted. They are all approximately the time at 0 degrees longitude. Most astronomical times are quoted in UT (universal time). The differences are rarely critical for amateurs.
Another significant time system TDT (formerly ET), and TBT. TDT or Terrestrial Dynamical Time is based on the orbits of the planets, as is TBT or Terrestrial Barycentric Time (based on the center of motion of the earth-moon system). These times currently differ from UT by about a minute. TDT is the time which should be used for planetary calculations.
Time zones relate local time to the time at 0 longitude. Be aware that there are some fractional time zones in the world. Daylight savings time (or "summer" time) is an additional complication. You should learn how your time zone is related to the time at 0 longitude (UT or GMT). EST is 5 hours behind, EDT is 4 hours behind.
These times are all related to the position of the sun: the sun should be overhead at about noon local standard time. A different time is sidereal time, based on the positions of the stars overhead. Two important sidereal times are GST or Greenwich Sidereal time, and LST or local sidereal time.
Calendars are confused and confusing. To avoid confusion between the many calendars in use historically, JD Julian date is used. The JD 0 is a day more than 4000 years BC (BCE).
Refraction affects alt-az coordinates: light from objects are bent by the atmosphere, making them appear higher in the sky than they would if there were no atmosphere. The error can reach 34 minutes of arc at the horizon. Extinction, absorption and scattering make objects fainter the more atmosphere the light from them must pass through (that is, the closer to the horizon they are). Other effects of the atmosphere are seeing (the effect which produces twinkling of stars), scintillation, airglow, and of course light pollution.
To avoid frustration and ensure meeting goals you should plan your observing session in advance. More serious the goal the more carefully you should plan. At least, having a plan may help you avoid wondering what to do next on a clear night.
A timeline of events ensures that an object will be observable, and helps schedule a night to observe all objects when they are well placed. The events are sun and moon rise and set, astronomical twilight, and the rising, setting and transit times of objects. In addition, objects should be observed when possible when they are above 30 degrees above the horizon, or at least above 20 degrees. These times should also be noted in the time line.
Charts are used in identifying planets, asteroids, and comets, and finding objects. For very faint objects, a photographic atlas of the area should be xeroxed.
A notebook is an important part of observing. It can be of scientific notebook quality, or a simple note of what objects you observed and how they looked. Starchart programs may be used to produce finder charts which you can annotate and keep in a notebook.
Use grep, awk and shell scripts. Unix tools can be very helpful in many situations.