GEMini Atari/GEMini_Atari_CD-ROM_Walnut_Creek_December_1993.iso/files/astronmy/stargide/starprg3.doc

Text File | 1985-11-19 | 22KB | 448 lines

                THE AMATEUR ASTRONOMER'S GUIDE TO THE NIGHT SKY.



   Written by M.Kudlowski, December 1989, for the Atari ST 520.


   1. Introduction.


   2. Running the program.


   3. The Constellation Chart option.


   4. The Calendar Chart option.








































1. INTRODUCTION.

     This program is a guide to the stars and constellations visible in
     the night sky from almost anywhere on earth. It features the following:-

          1) 1160 stars down to magnitude 4.75, including variable stars
             whose maxima exceed magnitude 4.75, and double or multiple
             stars whose combined magnitude exceeds 4.75.

          2) 370 selected stars which can be individually identified on the
             Constellation Chart option. All stars of magnitude 3.5 and
             brighter are included,together with all double and variable
             stars on the main file.

          3) 267 deep-sky objects which can be optionally plotted on the
             Constellation Chart option. The limiting magnitude for these
             objects is about 9 for clusters and gaseous nebulae, 10 for
             external galaxies and 11 for planetary nebulae.
             Most of these objects can be seen through a moderate telescope
             of about 6 inches aperture, but the external galaxies and
             planetary nebulae are mostly faint and elusive, and the gaseous
             nebulae generally require a very transparent night for viewing.





2. RUNNING THE PROGRAM.

          Note: This program can only be run in medium resolution.
          To run the program, insert the disk in drive A and load
          the program "STARPRG3.PRG". The data files take about 40
          seconds to load, and while they are loading, a screen of
          explanatory text is shown. When the files have loaded,
          a menu screen appears with 3 boxed options:-

          1) Constellation Chart Option.
          2) Calendar Chart Option.
          3) Quit program.

          The Constellation Chart option draws a map of the constellation
          as chosen by the mouse. There is also an option to include deep-
          sky objects as well as including or excluding stars belonging to
          adjacent constellations. The Own Choice allows you to define your
          own region of sky to be displayed with an option of six scales.

          The Calendar Chart option draws maps of the night sky as viewed
          from any latitude,and at any date or time. Directional maps are
          drawn for north,east,west,south and overhead aspects in addition
          to a circular whole-sky plot.

          The Quit option is the most drastic and should only be used
          if you want to leave the program. For the other two options
          you should click the left mouse button in the required box to
          the left of your choice.






3.CONSTELLATION CHART OPTION.

     3.1. THE CONSTELLATIONS.

          There are 88 constellations in the sky and they vary in size from
          sprawling groups like Hydra and Eridanus, which cover a quarter of
          the entire sky, down to tiny constellations such as Crux and
          Delphinus. To act as a guide to a constellation's size, each chart
          includes a grid of celestial co-ordinates known as Right Ascension
          and Declination.
          The oldest constellations date back to Egyptian and Greek times, and
          were catalogued by Ptolemy in his 'Almagest'. These are often termed
          'original' constellations, and number 48.
          Most of them are named after famous people and creatures in Greek
          and other mythologies. Examples are Orion, Pegasus, Leo and Scorpius.
          The far southern constellations, however, were unknown because they
          could never rise above Greek or Egyptian horizons, and so it was
          up to 17th- and 18th-century astronomers to add new groups in those
          hitherto uncharted regions. A few minor groups were also added in
          the northern heavens in faint regions unclaimed by the original
          groups. It must be frankly said that some of these modern groups
          were a waste of time - Leo Minor, Horologium and Caelum are notorious
          examples. The constellation of Mensa is not even included here since
          its brightest star is fainter than magnitude 4.75.
          Also, Argo Navis,one of the 48 originals, was so large that it had to
          divided up into three parts for convenience.

     3.2. DRAWING THE CONSTELLATION CHART.

          An explanatory screen will be displayed on entering the menu. On
          pressing the left mouse button, an index screen of constellation names
          will be displayed. To plot the chart, simply press the left mouse
          button either in the box to the left of the constellation name or
          on the name itself, and then press the left mouse over the text marked
          'Ready', which will appear in the lower right of the screen.
          The program then plots the stars making up the constellation, gives
          the name of the constellation in Latin and English together with
          its Latin genitive, and displays brief textual notes about the
          constellation. The standard international abbreviated constellation
          name is displayed in the upper right corner of the map.
          Culmination dates and times are also shown on the left map margin
          to inform when the constellation is at its best viewing position,
          subject to its declination and the viewer's latitude. Thus, from
          Northern Europe, at a latitude of 50 degrees north, all stars
          south of declination -40 degrees are permanently out of sight.
          Because the constellations are so variable in size, a coordinate
          grid is included on the map for reference.

          Pressing the left mouse button again will link up the constellation
          figure to form a distinctive shape. From this point on, you can
          identify individual stars by pressing the left mouse button above
          them. If the star is one of those individually listed, then
          details about the star will be printed on the screen. These include
          its Greek or other letter, its (often-barbarous) Arabic proper name,
          its visual magnitude, its spectral type, its distance in light-years,
          its absolute magnitude and its celestial coordinates.

          The coordinates of Right Ascension and Declination correspond to
          latitude and longitude on the earth. The earth's axis is aligned
          with the celestial poles, whose declinations are 90 degrees north
          and south. The plane of the earth's equator, if extended, aligns
          itself with the celestial equator, declination zero degrees.
          The declination of a star is sufficient to determine if it is visible
          from a given latitude. For example, the latitude of Manchester is
          53 degrees north. Subtracting 53 from 90 gives 37, which means that
          any star south of declination 37 degrees south can never rise.
          In addition, any star north of declination 37 degrees north will never
          set. All stars with intermediate declinations will spend a greater or
          lesser period below the horizon.
          Right Ascension is generally measured in units of time, and the zero
          point of reference is the Vernal Equinox. On March 21, when the sun
          is on the celestial equator, its right ascension is zero hours.
          Culmination of a star occurs when it is at its highest position in
          the sky. The Right Ascension of a star is the time it culminates after
          the vernal equinox has done so. This gives an idea of the best times
          for viewing a satr as follows. A rough way of estimating it is to
          halve the right ascension in hours and reckon that many months from
          October to give its culmination date at 10 p.m. Thus a star with
          Right Ascension of 14 hours is best placed for evening viewing 7
          months after October, in May.

          The Greek lettering system originated with Bayer in the early 17th
          century, though other astronomers used Latin letters or numbers.
          The visual magnitude system is a throwback to ancient astronomers'
          conventions. The brightest stars were classed as being of the first
          magnitude, slightly fainter ones were classed as second magnitude,
          until stars which were on the limit of naked-eye visibility were
          classed as sixth magnitude. Nowadays, there is an exact ratio of
          100 between the brightness of stars of 1st and 6th magnitudes.
          The magnitude scale has also been extended to include zero and
          negative magnitudes for the brightest stars, and below 6th magnitude
          for stars visible only through telescopes.
          The visual magnitude of a star bears no relation to its true
          luminosity, for the simple reason that the stars are all situated
          at such varying distances from us. If all the stars were placed at
          a standard distance of 10 parsecs or 32.6 light-years from us, they
          would have different magnitudes, and the term given for this is
          absolute magnitude. The spectral type of a star is generally
          related to its surface temperature, and also the colour of its light.
          Type O and B stars are the hottest and bluish-white in colour. Type
          A stars are white and cooler. Type F stars are cooler still and
          white or yellowish. Type G stars, of which our Sun is an example,
          are yellow in colour. Type K stars are cooler with an orange hue
          and type M stars are distinctly reddish.
          If a star is double, then the magnitudes of its components, their
          separation in seconds of arc, and their colours (if any) will be
          stated. Variable stars will have their period, class, and magnitude
          ranges quoted.

          You can continue searching for stars within the constellation for as
          long as you wish. To return to the main menu, click the right mouse
          button.

     3.3. CONSTELLATION CHART SUB-OPTIONS.

          There are several extra options available with this part of the
          program. After clicking the left mouse button over the constellation
          name, you will see the words 'No deep-sky' displayed on the right-hand
          side of the screen.
          This means that a default map plot will not include any deep-sky
          objects. Clicking the left mouse over the text alters it to 'Deep
          sky' and clicking the left mouse over the 'Ready' text will result
          in deep-sky objects being plotted on the map in addition.
          Just as with the star search, clicking the left mouse over the symbol
          reveals explanatory text about the object, and its operation is
          identical to that of the star search.
          The main catalogue of deep-sky objects is the New General Catalogue
          or NGC, published in 1888, together with a supplementary Index
          Catalogue (IC or I.). However, many brighter examples are also known
          by Messier (M.) numbers, after a catalogue compiled by the French
          astronomer Messier at the end of the 18th century.

          There are, in the main, 5 types of deep-sky object:-

               1) Galactic or open clusters, containing up to 300 or so stars.
                  These are among the finest deep-sky objects visible with
                  modest equipment. Examples include the Pleiades, the Double
                  Cluster in Perseus, and the Jewel Box in Crux. In several
                  cases, stars of different colours may be seen.

               2) Globular clusters, containing thousands of stars in a
                  globular formation, with the highest concentration in the
                  centre. A moderate telescope will resolve the brighter ones
                  into stars. Examples include Omega Centauri and the Hercules
                  Cluster, M.13.

               3) Gaseous nebulae, which consist of interstellar gas and dust,
                  and are made visible by the light of nearby stars. The finest
                  example of such an object is the Sword of Orion, M.42, which
                  is visible with the naked eye. Other examples appear clearly
                  on long-exposure photographs, but are difficult to see because
                  of their large areas and low surface brightness.

               4) Planetary nebulae, which are stars surrounded by a shell of
                  tenuous gas. Their resemblance to faint planetary disks led
                  to their nickname, although they are neither planets nor
                  nebulae. Most of them are faint, however, with magnitudes
                  below 9. Brighter examples include the Ring Nebula in Lyra,
                  M.57, and the Dumbbell Nebula in Vulpecula, M.27.

               5) External galaxies, which for a long time were erroneously
                  known as 'spiral nebulae'. The Magellanic Clouds in the far
                  southern sky are satellites to our own Milky Way galaxy. The
                  only other galaxy visible with the naked eye is the Great
                  Spiral in Andromeda, M.31.
                  Most external galaxies are faint, however, and it takes a
                  powerful instrument beyond the scope of most amateurs to
                  reveal any structure, which may be spiral, elliptical or
                  irregular.

          Again, after clicking the left mouse button on the constellation,
          you will notice the text 'All stars' in the top right-hand corner of
          the screen. This means that the default map includes any stars
          which belong to neighbouring constellations. If the left mouse button
          is clicked above this word, the text changes to 'Exclusive', which
          prevents stars and deep-sky objects belonging to neighbouring groups
          from being plotted on clicking the left mouse button over the 'Ready'
          text. This has the virtue of making the plot slightly faster.

          In the lower right-hand corner of the constellation list there is
          a box labelled 'Own Choice'. This allows you to plot any section of
          the sky you like. if you press the left mouse button over this box,
          the top right section will display a panel showing Right Ascension
          and Declination coordinates, and a map scale factor showing the
          extent of Declination covered.

          To advance the Right Ascension, click the left mouse button over the
          'hours' value to advance the hours, and over the 'minutes' value to
          advance the minutes in 5-minute steps. Using the right mouse button
          sets the Right Ascension back.
          To advance the Declination northward, click the left mouse button
          over the value in the box. You can advance in steps of 10 degrees if
          you click over the 'tens' digit, otherwise you advance in steps of 1
          Using the right mouse button in this way will advance the declination
          southward.
          To change the scale of the map, click the left mouse button over the
          Scale box until the desired scale appears.
          When you are satisfied with the coordinates and scale, click the left
          mouse button over the 'O.K.' box, click the left mouse over the 'Deep-
          Sky' text if desired, and finally click the left mouse over the text
          marked 'Ready'.







     4. THE CALENDAR CHART OPTION.

     4.1. INTRODUCTION.

          Because of the earth's rotation about its axis, and its revolution
          around the sun, the night sky varies hour by hour, day by day, during
          the year as seen from any inhabited latitude.
          The stars appear to rise and set about 4 minutes earlier each day,
          and the time-interval between two consecutive risings of the same star
          is called a sidereal day, 23 hours 56.1 minutes long. Therefore the
          night sky at 10 p.m. on January 15 is identical to that on January 16
          at 9.56 p.m. on January 16, 9.52 p.m on January 17, and so on.
          This difference amounts to two hours per calendar month.

          Because of the earth's revolution about the sun, many stars remain
          out of sight for varying periods of the year as they are above the
          horizon only during the daytime. Thus, in  December, Orion is high in
          the south at midnight, but in June, with the Sun in Gemini, the
          Hunter rises and sets in daylight, and hence is invisible.




     4.2. SETTING THE VIEWING PARAMETERS.

          The help screen for the Calendar Chart option includes the default
          input parameters, with date set to January 15, time set to 21.30
          hours, and latitude set to 52 degrees north.
          Clicking the left mouse over the month, date, viewing hours and
          viewing minutes sets them forward. Clicking the right mouse over
          the above fields will set them back. Similarly, clicking the left
          mouse above the latitude sets it northward, and clicking the right
          mouse above the latitude sets it southward.
          The poles and equator are, however, not included in the latitude
          ranges. This is solely as a protection against program errors.
          When setting the time, it must be remembered that the program works
          on the local standard time of the viewing location. If any measures
          such as British Summer Time are in force, then you must remember to
          deduct one hour, or whatever it may be, from your civil clock time.
          Also, if you live near the border of a time zone, even your civil
          standard time might be about an hour fast or slow of the star time.
          On setting the parameters, click the left mouse over the word 'READY'
          in the lower right corner of the screen to process.
          There is no need to include the year in the input parameters because
          the stars return to virtually the same positions on the same date
          and time each year.
          The program takes about 25 seconds to process, giving a display of
          how many stars it has processed. When ready, click the right mouse.






     4.3. PLOTTING THE CALENDAR CHARTS.

          A box with 8 cells will be displayed in the top left corner of the
          screen. The cell options are North, East, South, West, Overhead, Full,
          Change and Quit. All these options are activated by clicking the
          left mouse button in the appropriate cell.

          If the input date and time correspond to darkness, then the stars
          will be plotted on a black background ; if they correspond to twilight
          the background will appear dark blue ; and if daylight, black stars
          will be plotted on a light blue background.
          The charts will also include altitude and azimuth co-ordinates for
          directional maps and azimuth co-ordinates for the overhead and full
          sky maps. Altitude is the elevation, in degrees, above the horizon,
          with the overhead point, or zenith, having an altitude of 90
          degrees. Azimuth is the compass bearing in degrees, with north
          at 000, east at 090, south at 180 and west at 270.



          North - displays the northern sky, altitude 0 - 70 degrees,
                                             azimuth 300-060 degrees.

          East  - displays the eastern sky,  altitude 0 - 70 degrees,
                                             azimuth 030-150 degrees.

          South - displays the southern sky, altitude 0 - 70 degrees,
                                             azimuth 120-240 degrees.

          West  - displays the western sky,  altitude 0 - 70 degrees,
                                             azimuth 210-330 degrees.

          Overhead - displays the portion of sky at over 45 degrees altitude.

          Full Sky - displays the entire night sky.

          Note that the orientation of the last two charts depends on the
          latitude chosen. If it is north of the equator then North will be
          at the top of the chart ; if it is south then the chart will be
          the 'other way up'.

          Change -  reverts to the main menu.

          Quit   -  leaves the program. Only use if you really want to quit!

          When the chart has been plotted, click the right mouse button to
          reveal the constellation patterns. The standard international
          abbreviated name will also appear on all charts (except the Whole
          Sky chart, for clarity).
          After linking up the stars, you can continue with the left mouse
          to display other views or return to the menu.