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Eclipse Calculator

UPDATE! - there is a newer version of this calculator, plus a lunar eclipse calculator, on the NASA Eclipse Home Page


This calculator is a project of ecliptomaniac Chris O'Byrne. This calculator is in constant development - if you find any bugs, or if you are getting answers that are definitely wrong, then please let me know by email at chris (at) obyrne (dot) com - thank you! This program is distributed under the terms of the GNU General Public License in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

Eclipse:
Latitude: deg, min
Longitude: deg, min
Altitude: metres
Time zone: :

Results :-
Event Date Time Alt Azi P V LC
Start of partial eclipse:
Start of central eclipse:
Mid eclipse:
End of central eclipse:
End of partial eclipse:

Eclipse type: Duration: Coverage:
Magnitude at mid eclipse: Ratio of size of moon/sun:
(Ant)Umbral Depth:

Instructions

  • This calculator uses Javascript to do the calculations for the eclipses, which means that it does not depend on any CGI script or web server to do it's work - all the calculations are done by your computer in your browser. The software to do the calculations is hidden within this web page - this is, essentially, an intelligent web page! To use this calculator while not connected to the Internet, choose "File", "Save as..." from your browser's menu, and save to a location on your hard drive. Then you can load the saved file into your browser at any time by finding the saved file on your hard drive and double-clicking it! Remember, however, to check back here regularly for updates to the calculator!
  • To find the circumstances of an eclipse, select the eclipse and enter your location and time zone information. Then click on the "Click here to do the calculation" button. The results will appear in the boxes below that button. The information given in the first 5 boxes is -
    • The local date and time of the event
    • If the event occurs while the sun is below the horizon, an asterix (*) will appear
    • Alt - The altitude of the sun, in degrees, above the horizon
    • Azi - The azimuth of the sun (0 = due north, 90 = due east etc)
    • P - The angle between the north point on the sun's disk and the contact point with the moon
    • V - The "o'clock" position on the sun's face of the contact point with the moon (eg V=12.0 means that the contact point is in the "12 o'clock" position - ie the top of the sun's disk)
    • LC - the correction (in seconds) that was applied to the time of start or end of total or annular eclipse due to the irregular lunar limb. Such corrections are currently only available (courtesy Fred Espenak, NASA/GSFC) for the 2003 May 31 annular eclipse
    The information in the next 6 boxes is -
    • Eclipse type - the type of the eclipse as seen at your location (taking into account that the sun may not be above the horizon until the eclipse is in progress etc)
    • Duration - the duration of the total/annular eclipse. (This box will display "n/a" if the eclipse is partial only, or "???" if the total/annular eclipse is underway at sunrise or sunset)
    • Coverage - the percentage of the sun's disk covered at mid eclipse. (This box will display "???" if the sun is below the horizon at mid eclipse)
    • Magnitude - the fraction of the sun's diameter covered by the moon at mid eclipse
    • Ratio - the ratio of the apparent size of the moon to that of the sun
    • (Ant)Umbral depth - the percentage of the way from the edge of the total/annular eclipse to the centre line that this location is at

Limitations

  • This calculator does not account for refraction, which makes a difference if the eclipse happens close to sunrise or sunset
  • At present, corrections for the irregular lunar limb are only available for the 2003 May 31 annular eclipse - this can make a difference of a few seconds to the time of the start and end of the total/annular eclipse
  • Also, this calculator uses eclipse elements that I calculated myself, and they appear to differ slightly from elements used by others. (An exception is that the elements for 2003 May 31 annular eclipse were calculated by Fred Espenak, NASA/GSFC).
  • Finally, the earth rotates at a slightly irregular rate, so it's not possible in advance to know precisely where the moon's shadow will fall.

Acknowledgements

  • Jean Meeus - for his excellent books Elements of Solar Eclipses 1951-2200 and Astronomical Algorithms, which provided the algorithms for this page
  • Fred Espenak (aka Mr. Eclipse) and Jay Anderson - who have been serving the eclipse-chasing community for many years with the NASA Eclipse Bulletins. Also, Fred has calculated all the data for the 2003 May 31 annular eclipse that this page depends on.
  • Stephen McCann - who wrote the eclipse duration code
  • Brian Seales - the organiser behind the ecliptomaniacs expeditions, which have provided me with much joy and many precious memories

The original version of this calculator is available on Chris O'Byrne's Eclipse Calculator page.


Copyright © 2003 Ecliptomaniacs.com
Javascript program copyright © 2003 Chris O'Byrne and Stephen McCann