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Event Tool Manual

Usage - Browser notes - Source - Copyright notice - History - Glossary of astronomical terms

Event Tool searches for various solar, lunar and planetary events. The following events can be detected by Event Tool: Solstices, equinoxes, peri/aphelion, lunar phases, lunar perigee/apogee, inferior/superior conjunctions of Mercury and Venus, solar conjunctions/oppositions of outer planets, greatest elongations of Mercury and Venus, mutual conjunctions involving two planets or the Moon and a planet.


Brief instructions:
To use Ephemeris Tool set your location and time zone, specify start date and desired time span. Mark the desired events and click 'Search'.

The search can take a considerable time depending on the speed of your computer, the browser used and the requested time span. Don't panic if it looks like your browser hangs. It is just thinking! Newer versions of Mozilla (and Firefox) may occasionally show an alert box warning that "A script is causing mozilla to run slow" and offering you to abort it. Just click "Cancel" to continue running the script.

For help concerning the Location and Date and Time interfaces, please see the help manual for Ephemeris Tool. Event Tool shares the observatory cookie with other tools in the Astrotools suite


Check the checkboxes for all desired events before clicking Search. The search may take some time as mentioned above. The results will be presented in a new window.

The 'Seasons' and 'Moon phases' buttons are shortcuts for searching for astronomical seasons and lunar phases independent of the checked events.

See the glossary if you need explanations for the terms used


The program achieves sufficient accuracy for normal observational purposes. Moon phases and conjunctions with planets are accurate to a few minutes from the correct values. The uncertainties on conjunction times involving the slower Sun and planets are greater. The positions used for calculating events have an accuracy of about 2 arcminutes and the uncertainty is related to the time it takes the planet or Sun to move this angle. The times for peri/aphelions and greatest elongations can be off by a few hours from the values calculated using more precise methods.

Event Tool uses the following theories for calculating planetary and lunar positions: Planetary calculations are based on the formulae in Paul Schlyter's article "Computing planetary positions". He claims a precision of about 2 arcminutes on positions. Lunar calculations use a truncated version of the method presented in "Astronomical Algorithms", 2nd edition by Jean Meeus. Lunar positions should be good to about 1 arcminute. "Astronomical Algorithms" is a strongly recommended book for anybody interested in performing astronomical calculations. It has helped me with various formulae for translating between coordinate systems, calculating rise and set times and much more. Please note that the theories only provide the specified precision between 1000 AD and 3000 AD or so. Outside this time interval errors will grow quickly.

Browser notes

Event Tool is demanding on the javascript interpreter of the browser and requires CSS2 support. Older browsers may crash or report errors. Most recent browsers should be able to run Event Tool. Recommended browsers are Mozilla 1.7, Firefox and Internet Explorer 6 or better. The script has been tested on various computers with the following browsers running under Windows 98 SE, ME, 2000 and XP:
Mozilla 1.7.3, Firefox 1.0 and Internet Explorer 6

Speed: The speed of Event Tool depends on the computer, browser and search time span. Time spans of more than a couple of months may require a considerable time to execute (and possibly alert boxes in some browsers as mentioned above).

Copyright Notice

Astrotools and all its subcomponents like Event Tool have been released under the GNU General Public Licence (GNU GPL).

    Copyright (C) 2002-2005  Ole Nielsen

    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program; if not, write to the Free Software
    Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA

Please see gpl.txt for the full text of the GNU GPL.

Please also visit the Javascript Ephemeris web site of Peter Hayes. His site and javascript code has been the original inspiration for my application. Although the majority of my code is new or completely rewritten, some code fragments can still be traced back to his original code. Any code provided by Peter Hayes is copyright © Peter Hayes 1999-2001.


You can run Astrotools off-line from your own hard disk. Download to your computer and unzip the archive to a folder of your choice. Open "astrotools2.html" in your browser to start AstroTools (In Windows Explorer you can also double click the file to open it in your default browser). Don't forget to bookmark this file in your browser for later use.

You may redistribute Astrotools under the terms of the GNU GPL (see above). You must include all files including the text of the GNU GPL (gpl.txt).

Home page: Ole's Astronomy Site

If you came here via a search engine: AstroTools