IAU vs Midpoint Boundaries

The two most common true sidereal boundaries

21 July 2024

Different from mainstream astrology, true sidereal astrology uses the actual size of the constellations as they are seen visibly in the sky. Two prominent methods for determining the size of the constellations are the IAU and Midpoint systems. Each with their own methods for determining where each constellation begins and ends. This article delves into the specifics of these systems, explaining why their boundaries differ and what this means for true sidereal astrology.

Origins and Definitions

The International Astronomical Union (IAU) and the Midpoint system approach the zodiac from distinctly different perspectives.

The IAU, established in 1919, is an organization of professional astronomers. In 1930, the IAU formalized the constellation boundaries, aligning them with the star patterns used mostly by the Ancient Greeks. The boundaries were determined by drawing boundary boxes around all 88 constellations in the sky. This method for determining the boundaries was intended for astronomical observation, without reference to the ecliptic - the apparent path of the Sun (and other planets) across the sky.

Astrology is primarily concerned with the zodiac constellations along the ecliptic in order to determine what constellations the planets are in. Thus, astrology is all about referencing the ecliptic in order to determine the correct placement of the planets. It's crucial to point out that the IAU boundaries were never intended to be used with astrology and thus do not reference the ecliptic.

*Above is the yellow IAU boundary box of Leo. Notice how the boundary box does not reference the red ecliptic line.

The Midpoint system, on the other hand, was designed specifically for astrology. It divides the constellations by their halfway points between one another along the ecliptic. It uses the last and first stars of the constellations to determine the midpoints. For example, the boundary between Cancer and Leo is the midpoint between the last star of Cancer and the first star of Leo along the ecliptic. This creates a more astrologically relevant zodiac for determining what constellations the Sun and other planets are in.

*Above is an example of using the Midpoint system to determine the zodiac constellation boundaries. The yellow line in the center is the ecliptic. Notice the zodiac boundaries are the midpoints between the first and last stars of the zodiac constellations.

Zodiac Boundaries and Dates

Below are the differences in Sun sign dates between systems. Also included are the mainstream Western dates (tropical) and commonly used mainstream sidereal dates (Lahiri and Fagan-Bradley). Notice the difference in dates between the IAU and Midpoint systems.

Astrological Significance

The IAU boundaries serve a crucial role in astronomy, offering a standardized reference for scientific study. However, for astrology, which relies on the accurate placement of the planets in the sky, the IAU’s fixed boundaries can be limiting. The lack of alignment with the ecliptic path may result in interpretations that do not fully capture the correct placement of the planets.

In contrast, the Midpoint system’s ecliptic-centric approach ensures that each sign covers it's visual location along the ecliptic. This method respects the visual sizes of the constellations while providing accurate placement of the planets in the signs. For astrologers, this system offers a clearer, more precise reflection of the planetary placements, enhancing the depth and accuracy of astrological readings.

Practical Implications for Astrologers

For those practicing astronomical astrology, understanding the differences between the IAU and Midpoint boundaries is significant. The choice of system can influence the interpretation of birth charts and the timing of astrological events. While the IAU’s boundaries may align with the traditional constellations, they may not reflect the proper placement of the planets along the ecliptic.

In summary, the IAU and Midpoint boundaries offer different perspectives on the zodiac. The IAU’s fixed boundaries serve astronomical needs, while the Midpoint system aligns with the ecliptic for astrological accuracy. For those seeking a zodiac that reflects the true positions of the stars, the Midpoint system offers a dynamic and precise alternative. To calculate your birth chart using the Midpoint method, use our true sidereal chart calculator.