Reference · Astronomy
What is the Imum Coeli?
Kevin Blaxton — software builder; astrology student for 40+ years · figures computed by the OrbHeaven engine
Published · Last revised
The Imum Coeli — IC, Latin for “the bottom of the sky” — is the degree of the ecliptic exactly opposite the Midheaven, where the zodiac meets the lower meridian beneath the horizon. In the quadrant systems OrbHeaven uses it is the cusp of the fourth house, and the tradition reads it as home, roots and private life.
What is the Imum Coeli?
It is the lower of the two points where the ecliptic crosses the meridian — the north–south circle through the zenith. The upper crossing is the Midheaven; the IC is the one below, exactly opposite, and usually beneath the horizon (though at high latitudes it can be above it). It is always 180° from the Midheaven, the same degree of the opposite sign.
How it is found
Directly from the Midheaven: the IC is the Midheaven plus 180°, so it needs the same accurate birth time and place. In OrbHeaven’s example chart, whose Midheaven is about 10° Capricorn, the IC is the opposite point, about 10° Cancer — the fourth-house cusp. Like the Midheaven, it depends on the local sidereal time, not just the date.
The example chart marks all four angles; the IC sits opposite the Midheaven on the meridian axis.
The example chart, calculated
Calculated with astronomy-engine 2.1.19 · tropical zodiac · mean lunar node · placidus houses
What it is read as
Interpretation, labelled as such: the IC is read as the private foundation of the chart — home, family, roots, ancestry and the inner base a person returns to — the counterpart to the public Midheaven at the top of the sky. The fourth house it heads is traditionally the house of home and origins.
A short history
The meridian axis — Midheaven above, Imum Coeli below — is one of the four classical angles of the horoscope, marked since Hellenistic astrology, where the lower culmination carried the sense of the hidden base of the chart. The name Imum Coeli (“lowest of the heavens”) is the Latin of that tradition.
Common misconceptions
- Is the IC always below the horizon?
- Usually, as the lower meridian crossing — but at high latitudes the geometry can place it above the horizon. It is defined by the meridian, not by being underground.
- Is the IC a separate calculation from the Midheaven?
- No. It is the point opposite the Midheaven — the Midheaven plus 180°. The two are found together and move together.
References & further reading
- [1] Meeus, J., Astronomical Algorithms, 2nd ed. (Willmann-Bell, 1998) — the meridian and the Midheaven–IC axis.
- [2] Holden, J. H., A History of Horoscopic Astrology — the angles and the fourth place in tradition.
- [3] OrbHeaven engine (astronomy-engine) — the example chart’s angles (IC opposite the Midheaven); covered by the chart tests.
Suggested citation
Blaxton, K. (2026). What is the Imum Coeli. OrbHeaven Reference. https://orbheaven.com/reference/the-imum-coeliRevision history
- First build (concise). Reused engine example chart (IC opposite the Midheaven, ~10° Cancer); geometry and sidereal-time dependence, home/roots reading kept separate; cross-links The Midheaven, The Ascendant and The Twelve Houses. No photo, no interactive.
Related glossary entries
About a minute. Birth time optional, and no account until after you have seen it.
More in the reference library
What is the Midheaven (MC) in astrology?
The Midheaven is where the ecliptic crosses the upper meridian — not the zenith, and not the ecliptic’s highest point. This covers the celestial-sphere geometry, an engine-computed example, its relationship to the house systems, and the traditional reading.
What is the Ascendant, and why does your birth time decide it?
The rising sign is the degree of the zodiac climbing over the eastern horizon at your exact birth moment and place — explained with the celestial-sphere geometry, an engine-computed interactive, and why signs rise unevenly.
What are the twelve houses in astrology?
The houses are the twelve areas of life a chart is read through, drawn from the horizon at your birth. This covers the astronomical geometry, the twelve house topics, and how placement depends on birth time, place and the house system.