Mean and true positions in Jyotish
A mean position is the average of a body's motion, ignoring periodic variations. A true position includes the corrections from the body's actual orbit. Both appear in Jyotish history and in the tools' methods.
What it is
The Rahu node is a mean node in the tools, while the Sun, planets and Moon are true positions from the library's model. The difference for Rahu can be up to about 1.5 degrees.
How it is worked out or read
The distinction matters for the node and for older texts that used mean models. A chart from a mean-based method can differ from a chart from a true-based method, and the method must be stated.
Where it appears on this site
The node is discussed in the mean and true nodes article, and the method is on the method page.
Two kinds of position
A mean position is where a body would be if it moved at a perfectly steady rate around its orbit. A true position is where it actually is, after accounting for the shape of its orbit and the disturbances that act on it. The difference can be large: for the Sun it reaches nearly two degrees, for the Moon over six degrees, and for the planets, seen from the moving Earth, it can amount to many degrees and includes their retrograde loops.
How the siddhantas handled it
The classical Indian method computed the mean position first, the madhyama graha, by multiplying the days elapsed by a fixed daily motion. Then it applied corrections to reach the true position, the sphuta graha. The manda correction accounted for the elliptical shape of the orbit, and for the five planets the shighra correction accounted for the effect of viewing them from the Earth rather than the Sun. These were worked out with geometric models of epicycles, and the results were impressively close for their time.
What Jyotish uses
For the Sun, Moon and five planets, Jyotish uses true positions: the sign, nakshatra and degree in a chart are true values. The panchang's tithi, nakshatra and yoga are also computed from true positions, which is why their lengths vary. Using mean positions instead would place the Moon's nakshatra changes many hours away from their actual times.
The nodes: a special case
Rahu and Ketu are the one case where both mean and true positions are in common use. The mean node moves steadily backwards at about three minutes of arc a day. The true node oscillates around the mean position by up to about a degree and a half and occasionally moves forward for short periods. Many traditional practitioners use the mean node, and so does this site; others prefer the true node. The difference can move Rahu across a sign or nakshatra boundary in some charts, so it is worth knowing which one a given chart uses.
A practical note
When comparing a chart with one from another source, the mean or true node is a common reason for a small difference in Rahu and Ketu, while the other planets agree. The article on mean and true nodes discusses the choice in more detail.