Local Circumstances Calculator (v1.0.4)
- To find the local circumstances of an eclipse, first select the eclipse and then enter your location and time zone information in the Geographic Coordinates section. Then click on the "Calculate Eclipse Circumstances" button. The predicted eclipse circumstances will appear in the table below that button.
To start with other default geographic or eclipse parameters you can append a query string to this web page URL. The parameters can be specified in any order or can be omitted. It should look something like :
- Eclipse is a text string specifying the eclipse’s date in the following format YYYYMMDD,
- Latitude is a number in decimal notation and is positive in the Northern hemisphere,
- Lng for Longitude is a number in decimal notation and is positive if East of Greenwich,
- Altitude is a number,
- Time Zone is a text string in the following format SHHMM,
- DST for Daylight Savings Time is a number (0 for Regular Time, else 1).
- The times and local circumstances of the eclipse from your location are given in the Predicted Eclipse Local Circumstances section. The information given in the first five lines is -
The information in the bottom three lines is -
- Eclipse event,
- The local date and time of the event (if the event occurs while the sun is below the horizon, an asterisk (*) will appear after the hour),
- 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).
This differs from the definition of V in the "Annular and Total Solar Eclipses of 2003" (NASA TP 2002-211618) bulletin, where V is measured counter-clockwise in degrees,
- LC - The calculator is able to determine a correction factor to the time of the start and end of the eclipse due to the fact that the Moon’s limb is not smooth. When it is, then the correction (in seconds) it has applied is given. When it isn’t, then you can use a lunar limb correction diagram, available for each eclipse, that will indicate the correction is seconds. For second contact start by reading the angle "P" value and then look at the corresponding correction on the diagram. Do the same at third contact.
- 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 eclipse (this box will display "n/a" if the eclipse is partial only, or "???" if the eclipse is underway at sunrise or sunset),
- Obscuration - The percentage of the Sun’s disk surface 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,
- Umbral/Antumbral depth - The percentage of the way from the edge of the eclipse to the centre line that this location is at.
- This calculator does not account for atmospheric refraction, which makes a difference if the eclipse occurs close to sunrise or sunset (a good example of this was the midnight sun eclipse in Antarctica on November 23, 2003),
- Corrections for the irregular lunar limb are only available when you are online, i.e. connected to the Internet. This can produce a few seconds time difference on the start and end of totality or annularity,
- It is not possible to predict the exact value of ΔT in advance, although the extrapolated value should be good to better than 0.5 seconds.
The code contained that does the calculations has been released under the terms of the GPL Version 2.
Last page update on May 3, 2004.