Firstly the nodes (Rahu and Ketu) are not “real” bodies in space. They are locations at which the path of the moon crosses the path of the sun. When the Moon and Sun are present at the same time at either of these two locations, we get an eclipse.
In between eclipses we infer the location of the eclipse points – the “nodes” – the intersections of the paths of sun and moon. We infer it mathematically by a ratio of distance between one eclipse and the next. In very ancient times we did this using a mean average ratio. But this did not always predict the next eclipse with perfect accuracy, it would often be inaccurate by several days or more.
Further investigation revealed a more complex formula to understand where the eclipse points really were. They do not move at a constant, standard speed, but rather have unsteady motion, sometimes halting sometimes briefly reversing direction – much like the other normal planets. Using this more modern formula we can always predict eclipses with perfect accuracy.
The old method of calculating the position of Rahu and Ketu (the eclipse poiints) is called the “mean node” method. The new method is called the “true node” method. In some charts there will be very little difference or no difference at all in the position of the nodes regardless of what method you use to calculate them. In other charts there may be a few degrees of difference, depending how close the birth time is to an unusual motion of the nodes, a change in their direction.
– Vic DiCara