True Node and Mean Node: Two Ways to Calculate the Lunar Nodes

You open the same chart twice. Its date, time, location and zodiac agree, but the Lunar Node has a different degree. If one calculation uses True Node and the other uses Mean Node, examine that choice first.

True Node is calculated from the Moon’s instantaneous orbital plane; Mean Node is obtained from a mean orbital model. North and South identify the opposite ends of the nodal axis. True/Mean and North/South answer different questions.

The Lunar Node control in Hermes offers True Node and Mean Node. By the end of this guide, you will be able to compare them with the same data and record the differences in degree, sign and aspect separation.

How can one degree change an aspect assessment?

The values in this section were chosen for teaching. They are not ephemeris measurements or Hermes results.

Assume these two North Node positions:

True minus Mean is 1°10′. Both North Nodes remain in Taurus, but keeping the same sign label does not preserve every degree relationship.

Now hold an illustrative planet at 16°00′ Taurus:

• Its separation from the True position is 0°40′.
• Its separation from the Mean position is 1°50′.

If your study uses a conjunction threshold of 1°00′, the first falls inside it and the second outside. Two choices explain the result: the node calculation and the orb threshold. Record both.

This arithmetic does not validate an interpretation. It demonstrates how different calculations can produce different aspect selections under the same threshold.

Avoid a 359° discrepancy at the Aries boundary

For another teaching example, assume True North at 0°20′ Aries and Mean North at 29°40′ Pisces. The short separation around the circle is 0°40′.

Raw subtraction gives −359°20′. Adding a complete circle produces +0°40′. The sign labels differ, but the points are close together.

Define the difference in your worksheet as True − Mean, expressed as the shortest signed arc between −180° and +180°. This prevents a crossing of 0° from appearing to be a very large separation.

What does a Lunar Node represent?

The plane of the Moon’s orbit is inclined to the plane of Earth’s orbit around the Sun. The line where the two planes intersect defines two opposite nodal points on the celestial sphere.

• North or ascending node: The direction where the Moon crosses the ecliptic from south to north.
• South or descending node: The opposite direction, where it crosses from north to south.

North and south here refer to the ecliptic plane. Do not interpret them directly as locations in Earth’s Northern and Southern Hemispheres. The nodes are calculated geometrical points rather than planets or additional moons. NASA defines the ascending and descending nodes in its discussion of the Moon’s orbit and eclipses, §4.4.

For a given model, add 180° to the North Node’s longitude and normalise it to the circle to obtain the South Node. For example, a North Node at 17° Aries has its South Node at 17° Libra.

How do True and Mean calculate the same axis differently?

The Moon does not follow one unchanging ellipse. The Sun’s gravitational influence, in particular, changes the orbital parameters. An instantaneous orbit and a model of mean motion therefore need not give the same nodal longitude.

True Node — the osculating node: Calculated from the intersection of the Moon’s orbital plane at the selected instant with the ecliptic. “Osculating” refers here to an instantaneous orbital description representing the Moon’s position and motion. Do not treat this value as a longitude fixed in advance for the next actual node crossing.

Mean Node: Calculated from a model of average motion that smooths short-period fluctuations. Selecting Mean does not ask the program to take a simple arithmetic average of the previous day’s True Node positions.

Swiss Ephemeris §2.2.1–2.2.2 describes the mean model and instantaneous node separately. “True” names the calculation definition; it does not measure whether an astrological interpretation is more accurate, effective or meaningful.

Writing four positions in a table does not create four separate nodal axes. Each of the two models provides a North/South pair.

Is the difference the same on every date?

Do not use a difference observed on one date as a fixed correction for another chart. Short-period changes in the lunar orbit make the relationship between the two calculations vary over time.

The nodes’ overall motion is separate from that short-period difference. NASA’s account describes the ascending node moving retrograde around the ecliptic in approximately 18.6 years relative to the stars. Investigating the True–Mean difference means comparing two model positions for the same instant. These studies examine different timescales.

Rather than writing “about one degree apart,” record the date, both full longitudes and the direction of the difference. The following example shows why those details matter.

Compare both node calculations in Hermes

Use the synthetic sample 20 March 2000, 12:00:00 UTC, 41° north, 29° east; Gregorian calendar. If the form requires local time, verify the time-zone conversion for that date.

1. Fix the shared settings. Use Tropical and Placidus. Keep the cusp setting, zodiac transformation and coordinate view at Normal. Record the application version.
2. Make the True Node record. Write down the full North Node longitude. Include the South Node and speed/direction if displayed, along with its house and any aspect you are examining.
3. Change only the Lunar Node control to Mean Node. Record the second set for the same UTC and other settings. Check that the Sun, Moon and ASC positions have remained the same as controls for the comparison.
4. Calculate the difference around the circle. Convert signs and degrees into 0°–360° longitudes. Compare the North Node positions and check that each South Node is 180° opposite its corresponding North Node.
5. Use the same orb for an aspect comparison. Record both the node method and the threshold. Return to your starting selection and compare it with the first record.

The Hermes Chart Settings guide explains the controls and purposes of saved preferences.

How can you investigate the changing difference over time?

After completing the first pair, repeat it on 21 March, 27 March and 20 April 2000, keeping 12:00 UTC and the location fixed. Compare True and Mean with each other on each date.

Do not mix the dates: the difference between True on 20 March and Mean on 21 March is not a comparison of two methods for the same instant. Four sample dates also cannot establish the largest or smallest difference for every year.

Rather than relying only on a retrograde indicator, record both longitudes and the actual interval between dates. A node is derived from orbital geometry; a speed or direction label describes the changing coordinate of that point.

Which other choices can interact with the node reference?

Node-based houses: With an option such as “North Node in the first house,” explained in our point-based houses guide, the nodal longitude anchors the houses. Changing the node calculation can therefore affect the house origins too. Beginning with Placidus and Normal settings keeps that additional effect out of your first True–Mean comparison.

Draconic transformation: Hermes offers separate “Draconic · Mean Node” and “Draconic · True Node” transformations. Do not assume that the Lunar Node control automatically governs every draconic calculation path. Record the transformation name and node reference separately. Keep the transformation at Normal for your first comparison.

Sidereal comparisons: For a sidereal study, keep the same ayanamsha in both trials. Changing the zodiac and the node calculation together makes the source of a difference harder to identify.

Which calculation should you use for your own study?

If you are reproducing a book or lesson, first establish whether it uses True or Mean. If the source does not specify, record the missing information. “North Node” alone does not identify the calculation method.

For your own research, you can choose one method consistently, then compare it with the other by changing only that preference. Preserve the method used in earlier notes when you switch. You will then be able to explain why two records of the same named node carry different degrees.

A useful record includes the date and UTC, zodiac, ayanamsha if sidereal, True/Mean selection, North Node longitude and aspect threshold. Establish a calculation you can reproduce before interpreting it.