What is Imum Coeli in Western astrology?

Definition

Imum Coeli is the lower-meridian point of a Western astrological chart, the ecliptic point where the ecliptic crosses the meridian in the north, exactly opposite the Midheaven, and it usually marks the cusp of the fourth house.

In plain words

The Imum Coeli, often abbreviated IC, is one of the four primary angles of a Western astrological chart. Astronomically, it is the point where the ecliptic crosses the meridian in the north, exactly opposite the Midheaven, as described in Wikipedia's Imum coeli article. In a chart it usually marks the cusp of the fourth house, though Wikipedia's article notes that this is reversed in the Southern Hemisphere.

Also asked as

• What is the Imum Coeli astronomically?
• How is the Imum Coeli calculated?
• Does the Imum Coeli require an exact birth time?
• What is opposite the Imum Coeli?

History

It describes astrology's broader historical development, from Mesopotamian celestial omens through Hellenistic astrology to later Sanskrit and Chinese traditions, so the IC's history must be read within that wider frame. Wikipedia's Imum coeli article notes that Hellenistic astrologers considered the IC to be the home of the underworld, or Hades, because it was the most distant point from the visible part of the horoscope. That placement shows how an angle's meaning was tied to its position in the chart rather than to a fixed object in the sky.²³

Quick timeline

1. Early record — It describes astrology's broader historical development, from Mesopotamian celestial omens through Hellenistic astrology to later Sanskrit and Chinese traditions, so the IC's history must be read within that wider frame. Wikipedia's Imum coeli article notes that Hellenistic astrologers considered the IC to be the home of the underworld, or Hades, because it was the most distant point from the visible part of the horoscope. That placement shows how an angle's meaning was tied to its position in the chart rather than to a fixed object in the sky.²³
2. Classical framework — In Christian Astrology, William Lilly associates the fourth house with fathers, lands, houses, and inheritances. In nativities or questions, he states that the fourth house represents fathers, and that the Sun by day and Saturn by night share in that signification. Within his framework, the IC therefore carries a fatherly meaning tied to the fourth house. Readers should treat this as one historical school's reading of the IC, not a measured fact of the sky, and should note that the reading belongs to Lilly's interpretive tradition rather than to the calculated position of the angle itself.⁵
3. Later transmission — According to Wikipedia's Imum coeli article, modern astrologers may use the IC as a significator for the father, or for both parents, and there is no consensus on which parent it best represents. Readers can treat this open question as an invitation to compare sources and to state which convention a chart uses.³
4. Modern usage — The Wikipedia Horoscope article lists the Imum Coeli, also called the IC or lower-heaven, as one of four primary angles of the horoscope, placed in the Fourth House.⁴

What classical and modern sources say

Classical view

In Christian Astrology, William Lilly associates the fourth house with fathers, lands, houses, and inheritances. In nativities or questions, he states that the fourth house represents fathers, and that the Sun by day and Saturn by night share in that signification. Within his framework, the IC therefore carries a fatherly meaning tied to the fourth house. Readers should treat this as one historical school's reading of the IC, not a measured fact of the sky, and should note that the reading belongs to Lilly's interpretive tradition rather than to the calculated position of the angle itself.⁵

Modern view

According to Wikipedia's Imum coeli article, modern astrologers may use the IC as a significator for the father, or for both parents, and there is no consensus on which parent it best represents. Readers can treat this open question as an invitation to compare sources and to state which convention a chart uses.³

The Wikipedia Horoscope article lists the Imum Coeli, also called the IC or lower-heaven, as one of four primary angles of the horoscope, placed in the Fourth House.⁴

Astronomy and calculation

Astronomical definition

The Imum Coeli is the ecliptic point on the lower meridian, exactly opposite the Midheaven. Lilly's Christian Astrology does not supply this definition itself, so the calculated point is described here as a geometric position in the chart.⁵

Calculation

Compute the Midheaven for the UTC instant and location, then add 180 degrees modulo 360. Because the IC sits opposite the Midheaven, this step gives the angle that a chart-making program would display for the lower meridian.⁵

Time sensitivity

Imum Coeli depends on observation time and location; an inaccurate birth time can change the calculated angle. Checking the time and place used for a chart therefore matters before comparing any reading of the IC.⁵

Layer boundary

The ecliptic point is calculable, while meanings assigned to it belong to interpretation. Keeping these two layers separate lets a first-time reader see which parts of a chart come from arithmetic and which come from tradition.⁵

The astronomy

The Imum Coeli is the point where the ecliptic crosses the meridian in the north, exactly opposite the Midheaven, as described in Wikipedia's article on the Imum coeli. This geometric definition names a point on the sky's apparent path, not a symbolic quality.³

Wikipedia's Imum coeli article states that the IC marks the fourth house cusp in most house systems, though this is reversed in the Southern Hemisphere. A reader working in the southern hemisphere should therefore check which house system and convention a chart program applies.³

Where astrologers differ

The Wikipedia Horoscope article ranks the Imum Coeli fourth in power among the four primary angles, behind the Ascendant, Midheaven, and Descendant. That ranking places the IC at the lowest level of strength in this scheme, so a reader comparing angles should note its position in the order. The IC is also the point opposing the Midheaven, which is the cusp of the 4th house and the northernmost point of the chart. Its meaning is tied to the fourth house, so readers can compare it with the Midheaven, which sits opposite it on the chart.

Then and now

Then

In Christian Astrology, William Lilly associates the fourth house with fathers, lands, houses, and inheritances. In nativities or questions, he states that the fourth house represents fathers, and that the Sun by day and Saturn by night share in that signification. Within his framework, the IC therefore carries a fatherly meaning tied to the fourth house. Readers should treat this as one historical school's reading of the IC, not a measured fact of the sky, and should note that the reading belongs to Lilly's interpretive tradition rather than to the calculated position of the angle itself.⁵

Now

According to Wikipedia's Imum coeli article, modern astrologers may use the IC as a significator for the father, or for both parents, and there is no consensus on which parent it best represents. Readers can treat this open question as an invitation to compare sources and to state which convention a chart uses.³

Worked out from the evidence

Reasoned from the cited facts, not stated outright by any one source.

Because the IC is the fourth-house cusp in most house systems and sits opposite the Midheaven, a reader who knows where the Midheaven falls can locate the IC without a separate table of angles.

Because the IC depends on observation time and location and the fourth house is tied to fathers in Lilly's reading, a chart with an inaccurate birth time can shift both the calculated angle and the house it is read through.

Not yet confirmed

• Wikipedia's Imum coeli article says there is no consensus on which parent the IC best represents, so it is unclear which convention a given modern chart should follow.
• Wikipedia states the IC marks the fourth-house cusp in most house systems but reverses in the Southern Hemisphere, and the sources do not settle which house system a reader in the south should adopt.

Follow the parts of Imum Coeli

Continue through Midheaven, Ascendant.

Questions people also ask

What is the Imum Coeli astronomically?

Wikipedia's Imum coeli article describes it as the point where the ecliptic crosses the meridian in the north, exactly opposite the Midheaven. This is a geometric point on the sky's apparent path, not a symbolic quality.³

How is the Imum Coeli calculated?

Compute the Midheaven for the UTC instant and location, then add 180 degrees modulo 360. Because the IC sits opposite the Midheaven, this step gives the angle a chart-making program would display for the lower meridian.⁵

What is opposite the Imum Coeli?

The Midheaven is opposite the Imum Coeli, according to Wikipedia's Horoscope article. The IC is the cusp of the 4th house and the northernmost point of the chart.⁴

Method and review

Every statement about Imum Coeli was checked against the wording of its 5 sources (Ptolemy (LacusCurtius), Encyclopaedia Britannica, Wikipedia and Lilly (Skyscript)), and its positions and boundaries were calculated with Astronomy Engine. How the Machine works.