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(currently 26 applets and growing) is available for $20.
   A recurring theme in Giesen's work is the motions of the Sun and Moon. Among his computations are their paths relative to the local horizon, sunrise/sunset times, lunar phases, and the analemma. He frequently turns to the works of Jean Meeus, particularly Astronomical Algorithms, and Oliver Montenbruck and Thomas Pfleger's Astronomy on the Personal Computer as sources of formulas for his astronomical computing projects.

An Applet Sampler
The Sun, Moon and Earth applet is a prime example of the level of interactivity that Java can bring to a Web page. The program's window is divided into two panels: the upper shows a Mercator view of the observer's horizon, while the lower presents the illuminated hemisphere of the Earth on a world- map for the date and time specified. 
   The user can choose his or her desired location in three ways: entering the geographical coordinates of the site, clicking on the world map, or selecting a nearby city from the pop-up gazetteer on the control panel at the top of the window. Here the user can also enter any desired date, time, and time zone from pop-up menus. Every piece of screen estate displays information on the rise/set/transit times for the Sun and Moon, their declinations, Greenwich hour angles, and


Left: The local solar and lunar viewing circumstances are revealed in Juergen Giesen's "Sun, Moon and Earth" applet. Right: Not all of the applet's computational output is revealed by the graphical display _ users can write a comprehensive topocentric ephemeris to the Java console for printing out.

physical appearance. All these data and more can be written to the Java console _ a very useful feature I'd like to see implemented on commercial astronomical software packages.

 If one were to photograph the Sun at local noon every day for a year, its path against the sky would be a drawn-out figure eight, owing to the changing solar declination and the equation of time. Giesen's Analemma

a Mercator view of the local horizon with the positions of the Sun and Moon depicted for various times throughout their periods of visibility. The lower portion of the window shows the world map with the subsolar point and pertinent observational data for the Sun and Moon. Can't take your computer with you? Then use the Java console to print out a precise topocentric ephemeris to bring along.
   Everyone will find a use for Moon Phase. The applet's name says it all _ it computes the date and Universal Time of the four principal phases of the Moon throughout the year.
   Giesen also takes an interest in astrophysics and cosmological models. So, if you feel like controlling the fate of the universe, try his Cosmology applet. You can experiment with the initial state of the Big Bang and modify parameters to see if the universe expands indefinitely or collapses into a Big Crunch. 
  Giesen points out that his astronomical interests are not limited to his "hobby programming," as he calls it. He owns a 4-inch Newtonian reflector with which he keeps a close eye on the Moon and bright planets. He also keeps a record of sunspot activity and tries his hand at astrophotography _ such as capturing the solar eclipse of August 11, 1999. 

ADRIAN ASHFORD is Sky & Telescope's newest associate editor.

Eclipse chasers preparing for Africa this month may want to give the "Solar Eclipse" applet a run. A full topocentric ephemeris con be printed from the Java console.

applies that information to the plotting of a shadow from a gnomon of user defined height. The control panel at the top of the Java window shares much of the functionality of the former example, where the desired location, date, time, and time zone can be selected from pop-up menus. This is a must for the aspiring garden sundial builder.
   Those preparing for a trip to Africa this month for the total solar eclipse on June 21st could do worse than examine Solar Eclipse before departure. The by-now familiar data entry menus appear at the top of the Java window, while the upper portion of the display features

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Sky & Telescope | June 2001   59