Data Sources
How we calculate lunar data and Void-of-Course periods.
Methodology
This page explains the astrology rules we use to label the Moon’s sign changes, Void-of-Course windows, and the four major lunar phases.
- Moon sign ingresses: computed from the Moon's ecliptic longitude crossing 30° boundaries.
- Void-of-Course (default): the window from the Moon’s last exact major aspect (0°, 60°, 90°, 120°, 180°) to Sun/
Mercury/ Venus/ Mars/ Jupiter/ Saturn/ Uranus/ Neptune/ Pluto until it enters the next sign. This is the rule most modern apps use—but it’s only the default. See Choosing a definition below. - Major lunar phases: the New, First Quarter, Full, and Last Quarter Moon correspond to the Sun–Moon angle reaching 0°, 90°, 180°, and 270°.
Choosing a Void-of-Course definition
Astrologers have never fully agreed on what makes the Moon “void.” Rather than quietly pick one rule, this calendar lets you choose—the default above is what most modern apps use, and each alternative is a real historical position. Change it from the definition bar on the calendar.
- Definition: Modern (last exact aspect until the next sign—a 9th-century rule from Abū Maʿshar, popularized in the 1970s); Lilly’s orb-based rule(William Lilly, 1647, as read by Sue Ward—voids become rare and can begin mid-sign); or Hellenistic (no exact aspect within the Moon’s next 30°, ignoring sign boundaries—Antiochus & Porphyry, revived by Chris Brennan; only a few voids a year).
- Planets: all ten (Sun–Pluto, the default) or the traditional seven visible planets, as horary astrologers use—with optional lunar nodes or Chiron.
- Aspects: the five Ptolemaic aspects (the near-universal practice) or, optionally, the minor aspects.
- Lilly’s exception: optionally mark voids in Taurus, Cancer, Sagittarius and Pisces as “soft,” where the Moon still “performs somewhat” (Lilly, after Bonatti).
These options change how frequent and how long voids appear—by as much as two orders of magnitude between the modern and Hellenistic rules. Calendar-feed subscriptions currently use the default (modern) definition.
Where the data comes from
The calendar times on this site are calculated from an astronomy ephemeris (a standard reference used by many astrology tools), then saved as fixed timestamps. In other words: the site isn’t “guessing” in real time—it’s displaying precomputed lunar timings.
- VoC intervals are derived from the Moon’s aspects to the planets—per whichever definition you’ve selected—through the relevant sign ingress.
- Moon sign ingresses are derived from the Moon entering a new zodiac sign.
- Phase times are derived from the Sun–Moon angle reaching the quarter points (New/First Quarter/Full/Last Quarter).
These timings are published in UTC and shown in your selected timezone on the site. When you pick a non-default definition, your calendar is recomputed in your browser from the same precomputed aspect data—no live server calls, and nothing leaves your device.