Semi-Arc

Definition

A semi-arc is half of a celestial body’s apparent daily path either above or below the local horizon. The diurnal semi-arc covers the interval between the horizon and the upper meridian, while the nocturnal semi-arc concerns the corresponding passage involving the lower meridian. Its length varies with geographic latitude and the body’s declination, so it is not always a uniform 90 degrees.

Astrological Meaning

In classical and traditional astrology, semi-arcs help describe a planet’s mundane position: where it stands in its daily rotation relative to the horizon and meridian. They are especially important in some methods of primary directions and in house systems that divide a planet’s diurnal or nocturnal motion proportionally. The calculation therefore differs from simply reading zodiacal longitude, which measures position along the ecliptic. Semi-arc techniques add a location-specific, rotational dimension to chart interpretation.

Why It Matters

Understanding semi-arcs clarifies how older astrologers connected celestial motion with earthly place and time. The concept can help practitioners evaluate traditional house calculations, angularity, and directional techniques more accurately. Because different schools use semi-arcs in different ways, the chosen method, latitude, chart data, and distinction between diurnal and nocturnal motion should always be stated clearly.

Related Encyclopedia Entries

• Traditional Astrology
• Astrological Terminology
• Natal Chart