True Sidereal (MTZ midpoint)
Last updated: 2026‑02‑28
Purpose of this page: explain the constellation‑aware method used by True Sidereal, how MTZ midpoint boundaries are derived, where it excels, and what to watch out for. For a system comparison, see Tropical vs Sidereal; for a short overview hub, see Zodiac Systems.
What is True Sidereal?
True Sidereal is a constellation‑aware approach to the zodiac. Instead of dividing the ecliptic into twelve equal 30° signs, it builds sign boundaries from the midpoints between adjacent constellations along the ecliptic. This produces non‑uniform sign widths that echo sky geometry and includes Ophiuchus because the ecliptic crosses it.
Think of it as a “planetarium‑style” frame: you’re seeing where the ecliptic passes through the constellations and using those spans to define zodiac segments.
How MTZ midpoint boundaries are built
- Map ecliptic spans: Determine where the ecliptic intersects each zodiacal constellation (including Ophiuchus).
- Find adjacency: For each neighboring pair, compute the midpoint in ecliptic longitude.
- Place boundaries: Use those midpoints as boundaries to form a closed ring of signs.
- Result: A full zodiac of non‑uniform widths that better mirrors the sky’s geometry than equal‑30° systems.
Implementation details (epoch, exact spans) can vary by project. AstroClock’s implementation is labeled True Sidereal — MTZ midpoint so you know exactly which method the app applies.
Constellation spans & Ophiuchus
Because constellations do not occupy equal arcs along the ecliptic, True Sidereal yields noticeably different sign widths. Some spans (e.g., Virgo) are long, others short. Ophiuchus appears as an additional constellation because the ecliptic passes through it; in a midpoint model, it becomes a genuine sign segment with its own boundaries.
This is not a simple “offset” like ayanāṃśa. It’s a different way of carving the sky itself.
Scope & caveats (astronomy vs practice)
- Inspired by sky geometry: MTZ midpoint is a constellation‑aware model; it is not a verbatim reproduction of every official constellation boundary detail.
- Unequal signs ≠ classical assumptions: Techniques that assume equal 30° signs (traditional dignities, decans) won’t map 1:1.
- Interpretive shift: Sign meanings may evolve when spans differ; treat this as a distinct interpretive framework rather than a minor tweak.
Houses & aspects compatibility
- Aspects by angle: Unchanged. Conjunction (0°), sextile (60°), square (90°), trine (120°), opposition (180°) still apply.
- Houses: House systems (e.g., Whole Sign/Equal/Placidus) still work; however, sign‑based techniques behave differently with non‑uniform widths.
- Boundaries matter: For placements near sign edges, high‑precision ephemeris is essential for clear, repeatable results.
Using True Sidereal in AstroClock
- Open Settings → Zodiac system.
- Select True Sidereal — MTZ midpoint.
- Placements update in real time (no server calls).
AstroClock computes planetary positions via the Swiss Ephemeris running locally in WebAssembly (WASM), then applies MTZ midpoint boundaries. After first load, it works offline.
Want context across systems? See Zodiac Systems (overview) or compare frames in Tropical vs Sidereal (decision guide).
Related pages
True Sidereal — FAQ
How are MTZ midpoint boundaries actually built?
Find each constellation’s ecliptic span, then set boundaries at midpoints between neighbors. The result is a complete ring of non‑uniform sign widths.
Is this identical to official IAU constellation borders?
No. It’s a constellation‑aware model aligned to ecliptic spans, not a verbatim copy of every IAU border nuance.
Why do sign widths vary so much?
Because the ecliptic crosses constellations that don’t occupy equal 30° arcs; Virgo is long, others are short, and Ophiuchus adds another segment.
Do aspects change when I switch to True Sidereal?
Aspects by angle remain unchanged; sign‑based interpretation changes because sign widths are non‑uniform.
Which ayanāṃśa applies to True Sidereal?
True Sidereal derives sign boundaries from constellation geometry rather than a single offset; reference choices still matter in practical implementations.
How does AstroClock compute True Sidereal positions?
AstroClock uses the Swiss Ephemeris for planetary longitudes and applies MTZ midpoint boundaries locally in WASM, enabling fast updates and offline use.