Calculating the Circumstances of a Solar Eclispe

I have written a Javascript program to calculate the circumstances of a solar eclipse using methods described in the Surya Siddhanta  (Warning; there are some descrepancies between the values calculated by these ancient methods and the values calculated by more modern methods.) Learn more about the Surya Siddhanta; a Brief History of the Surya Siddhanta and a Description of the Solar Eclipse Calculation . 

Starting with the Date of a Sun Moon Conjunction

To run the Surya Siddhanta program you need to first find a day on which there is a conjuction between the Sun and the Moon, i.e. a New Moon date. The first set of data boxes will help you find such a date. Type in a year, month and day close to when you want to find a conjunction. Then click the "Do it!" button. The display will give you the month and day of a New Moon close to the day you typed in 

Input a date to calculate a New Moon. 

Year = Month= Day = 

New Moon: 
Year = Month = Day= 
Approximate Time (U.T) = hours minutes

(This calculation is performed using an algrorithm found in Astronomical Algorithms by Jean Meeus ,2nd edition, December 1998,Willmann-Bell, ISBN: 0943396611, and is not a part of the Surya Siddhanta calculation. It is here only to help you get started in finding a date for the conjunction of the Sun and Moon. Had you been using the ancient Hindu lunar calendar you would not need this help.) 

Calculating the Circumstances of the Conjunction

Now you can input the date for the New Moon in the calculation below and click the next "Do it!" button. 

Year MonthDay

The circumstances of the eclipse (if there is one) as calculated by the Surya Siddhanta algorithm are given below. If the Gamma is greater than one or less than minus one there is no solar eclipse on the Earth.

Julian date = 
Hindu day count = 
Time of day of Conjunction =  (U.T.) 

True Longitude of Moon at conjuction =  degrees 
Eclipe magnitude = 
Gamma = 
Longitude distance; Moon to ascending node =  degrees 

Did you miss the Conjunction Date?

If the date you entered is not a New Moon date  you will get "NA" for output data. You can get some idea of how far you missed a New Moon date by knowing that for the beginning of the day that you entered the Moon was behind the Sun by longitude of  = degrees. 

Finding an Eclipse Date?

There can be an eclipse of the Sun only if the longitude distance from the Moon to ascending node of the Moon is close to zero degrees, close to 360 degrees, or close 180 degrees. This longitude distance is calculated and displayed above. If you have not found an eclipse date you can now adjust the date of conjunction to make the longitude distance to the node approach zero, 360, or 180 degrees. To do this find a conjunction date which is a few months later than your first try. Does this make the longitude distance to the ascending node closer to one of the desired values? Try another conjunction date and see if you are closer to one of the desired values for the longitude distance to the ascending node. Eventually you will find the date of a Solar eclipse. 

You can see and have a copy of the Javascript program which runs the Surya Siddhanta calculations on this page (stripped down version). When it is running it looks like this. 

I have also written a Brief History of the Surya Siddhanta and a Description of the Solar Eclipse Calculation. 

You can compare these calculations with modern calculations by NASA's Fred Espenak linked from his Eclipse Home Page.

If you really need to know when a particular New Moon occurs I have a Javascript New Moon Calculator  that may help you.