Geocentric and Heliocentric Views: Changing the Centre in Hermes

The direction to a planet from Earth’s centre differs from the direction to it from the Sun’s centre. Keeping the date fixed while changing the origin can change the planet’s longitude, distance and angular relationships with other planets.

Geocentric calculations use Earth as their centre; heliocentric calculations use the Sun. In Hermes’s heliocentric calculation, Earth replaces the Sun in the calculated body list and the Moon is calculated separately. Lunar nodes, Lilith and Fortune are excluded from that path. Houses remain a local terrestrial reference, rather than a new Sun-centred horizon.

This guide covers two entries in the coordinate/view menu:

Centre, zodiac and coordinate plane are separate choices

A longitude alone does not completely describe a position. Where is the origin, which plane is used, and where does the angular scale begin?

Selecting a sidereal zodiac does not make an Earth-centred calculation Sun-centred. Nor does anchoring the Sun at 0° Aries produce heliocentric positions: that operation subtracts one common angle from existing longitudes.

The NASA/JPL Horizons manual’s definition of a centre distinguishes the coordinate origin from a surface observing site. “Geo” refers to Earth’s centre, rather than the birth location on its surface. Entering a birthplace does not automatically make the planetary calculation topocentric.

The Topocentric / Polich–Page option in a house-system menu also concerns house division. It is a separate choice from calculating planetary positions relative to a surface observer.

Can one longitude offset convert the whole chart to heliocentric?

No. Changing the centre concerns position vectors. At the same instant, expressed in the same axes, let r(P/S) be a planet’s vector relative to the Sun and r(E/S) Earth’s vector relative to the Sun:

r(P/E) = r(P/S) − r(E/S)

Subtract the vectors first, then derive the longitude angle from the new vector. Bodies at different distances and directions need not undergo the same angular change.

For geometry only, place the Sun at (0, 0) and Earth at (1, 0) in a two-dimensional plane. Keep the axes fixed and use arbitrary length units:

P’s direction changes by 45°, while Q’s does not change. Applying P’s offset to Q’s longitude would not reproduce the centre transformation.

This plane example is not an ephemeris calculation. It omits light-time, aberration and related corrections. Comparing real software outputs requires matching the reference frame and position corrections as well as the time.

What does replacing the Sun with Earth mean?

In Sun-centred coordinates, the vector from the origin to the Sun itself has zero length. Its direction is undefined; that does not place the physical Sun at 0° Aries. The same distinction applies to Earth at a geocentric origin.

The Swiss Ephemeris heliocentric documentation, §5.3, says that calculations for the Sun, lunar nodes and lunar apogees can return zero coordinates without an error. Such a software return should not be interpreted as an actual 0° Aries position.

Hermes handles the distinction in its body list: it replaces the Sun entry with Earth, and removes the North Node and Lilith. Earth’s position relative to the Sun is then calculated.

Match bodies by name when comparing tables. A Sun row in the geocentric table and an Earth row in the heliocentric table do not represent the same body merely because they occupy the same position in a list. Record the name, origin and longitude together.

How does Hermes handle the Moon and special points?

Keeping the Moon does not mean copying its geocentric longitude. A Sun-centred lunar position is calculated separately. Earth and the Moon are distinct bodies, rather than a single row or point.

Distinguish the absence of a derived point such as Fortune from an ephemeris failure. Hermes does not carry its geocentric Sun–Moon–Ascendant construction into the heliocentric list. Do not fill the absence with 0° Aries.

What limits apply to retrograde markers and houses?

Geocentric retrograde motion concerns changes in position measured from Earth. Heliocentric motion uses another origin. Hermes suppressing the retrograde field in heliocentric records is also a product display rule; it does not establish that no possible body can have decreasing coordinates in any Sun-centred reference.

Houses require similar care. A terrestrial horizon depends on Earth’s rotation and the supplied surface location. Making the Sun the origin for planetary positions does not define a new local horizon on the Sun for that birthplace.

Hermes retains the house and Ascendant/Midheaven reference, while producing a flag to hide house numbers. The presence of a house ring does not establish that a heliocentric planet can inherit the same interpretation as its geocentric natal-house placement. The planetary longitudes and house ring have different references.

Record two questions separately in your first comparison: “What is the planet’s Sun-centred longitude?” and “Which local calculation produced the house lines?” This keeps the geometric result distinct from an interpretive choice.

What happens with sidereal selection and other transformations?

With the Normal zodiac transformation, Hermes has a sidereal calculation branch for both geo and helio. It retains the heliocentric flag in that branch; selecting a sidereal reference does not move the planetary origin back to Earth.

Zodiac transformations such as D12, antiscia, draconic and planetary anchors are applied in this path only to Normal geocentric longitude views. When requested with helio, the engine resets the transformation to Normal and produces a warning. Selecting two settings does not establish that both have been applied to the result. Read the output reference and any warning.

Do not assume that changing the display of an already calculated geocentric pane can supply heliocentric positions. Hermes rejects helio requests from a derived pane. Build the first comparison as two fresh calculations from identical source data.

Compare the calculations at the same time

Use the proposed synthetic data 20 March 2000, 12:00:00 UTC, 41° north, 29° east; Gregorian calendar. Convert the time appropriately if the form expects local time. Use this as an exercise and record your own calculated result.

1. Create the geocentric source. Begin with the tropical zodiac, Placidus, Normal cusp setting, Normal coordinates and Normal zodiac transformation. Record the other settings.
2. Create the heliocentric calculation from identical inputs. Change only the coordinate/view option in the first trial. Keep UTC and the calendar fixed.
3. Match planets by name. Compare common bodies such as Mercury, Venus and Mars. Do not look for one offset shared by all of them.
4. Examine Earth, the Sun and Moon separately. Record which body each row represents and its origin. Distinguish an absent point, a zero return and a calculation error.
5. Check nodes, Lilith and Fortune. Even if enabled in the source, record what the heliocentric result actually supplies.
6. Inspect houses, retrograde markers and warnings. Record the house ring, numbers and Ascendant/Midheaven references. Run a sidereal trial later as a separate calculation.

For numerical validation against JPL or another ephemeris, also match geometric versus apparent positions, time scale and reference epoch.

For saving preferences and distinguishing them from new-chart defaults, see the Hermes Chart Settings guide. Make one statement clear in the first research note: “This position was calculated for this body, from this origin, using this zodiac reference.”