Mean node and true node: Rahu and Ketu calculation
The mean node is the average position of the Moon's ascending node, and the true node is its actual position at a given moment. The difference is periodic and can reach about 1.5 degrees.
What it is
The mean node moves steadily at about 19 degrees a year backward. The true node adds short periodic corrections from the Sun and Moon's gravitational pull on the Moon's orbit.
How it is worked out or read
This site uses the mean node, using Meeus equation 47.7. The true node would need a lunar theory that includes these corrections, which the site does not yet implement.
Where it appears on this site
The method is stated on the method page. The birth chart says Rahu is the mean node.
Why the nodes wobble
The Moon's orbit is tilted to the ecliptic, and its intersections with the ecliptic, the nodes, move backward over time. The mean node follows a smooth path, while the true node is pushed back and forth by the Sun's gravity and by the Moon's own irregular motion. The difference between the two is periodic, and it reaches its largest value of about one and a half degrees at certain points in the cycle, so the true node can sit noticeably ahead of or behind the mean node.
Which node a school uses
Many traditional Jyotish texts use the mean node, because it can be computed with simple formulas. Modern software often uses the true node, because it reflects the actual orbit. The choice changes Rahu's sign at the boundaries, so readers comparing two calculators should check which node each one uses before comparing the results. This site uses the mean node, and the method page gives the formula.
The effect on a chart
A difference of one and a half degrees changes the sign of Rahu only when Rahu is within that distance of a boundary, and it changes its nakshatra in the same way. For most charts the choice does not alter the main reading, but for a chart where Rahu is close to a boundary, the reader should note the choice and compare both results before relying on the sign.
How large the difference is
The true node swings back and forth around the mean node by up to about one and a half degrees, with its main cycle repeating roughly every 173 days. For short periods the true node can even move forward. Most of the time the two are within a degree of each other, so in most charts Rahu and Ketu fall in the same sign and nakshatra under either method.
When it matters
It matters when the node is near a sign or nakshatra boundary, or when a reader uses fine divisions such as the navamsha. It also affects dasha calculations only indirectly, since the Vimshottari sequence starts from the Moon, not the nodes. A reader comparing charts should check which node each source uses before treating a difference in Rahu's position as significant.