Astrologers will often define the ascendant as the point of the ecliptic on the eastern horizon at the time and place of birth. This is a reasonably good definition and is found in reliable textbooks. For example, in The Revised Waite's Compendium of Natal Astrology, the ascendant is defined as the point of the ecliptic which cuts the eastern horizon (Candlish, p. 20). In this textbook it is noted that the ascendant varies according to the time and place (i.e., latitude) of birth.
Robert Hand provides two useful and more precise definitions of the point in his essay on the ascendant, midheaven and vertex in extreme latitudes (for reference see below). Hand's first definition is that the ascendant may be the point of intersection of the rational horizon and ecliptic in the east (i.e., the eastern node). His second definition is that the ascendant may be the ascending node of the ecliptic upon the rational horizon. (Hand, p. 132)
Hand introduces an important refinement in both definitions. He makes it clear that the horizon used by astrologers when identifying the ascendant is the rational horizon rather than the visible or apparent horizon. The visible horizon is the horizon available to the observer at the time and place for which the chart is cast. This is a small circle engirding the place lying parallel to the rational horizon. (Mayo, p. 15-16) The rational horizon is a great circle defined by points at a distance of ninety degrees from the zenith, the point immediately above the observer on the celestial sphere. (Mitton, p. 191)
This definition of the rational horizon uses the concept of zenith distance. The definition of zenith distance is taken from Mitton's Dictionary of Astronomy. Zenith distance is "the angular distance from the zenith to a point on the celestial sphere, measured along a great circle." (Mitton, p. 416) The great circles in the example of the rational horizon will run through the zenith and nadir.
In the following image, the horizontal frame of reference is shown, with the rational horizon being the green plane running through the east, north, west and south points. The red arrows running from the zenith to the rational horizon illustrate the zenith distance of ninety degrees that defines this plane.
Figure 1: The Celestial Sphere (Horizontal Frame of Reference)
In a previous blog entry I used the concept of zenith distance to define the midheaven. The midheaven or MC is the degree of the ecliptic that has attained its minimum zenith distance during a diurnal cycle, irrespective of its direction in relation to an observer or its height in relation to the horizon. (http://www.exeterastrologygroup.org.uk/2015/01/the-definition-of-midheaven_18.html) In this second article, I would like to explore the possibility of defining the ascendant in astrology using the same terms and conditions as that adduced for the midheaven.
In this case, the ascendant ought to be defined without reference to direction (east) or according to above and below (the ascending node option). In this second instance, the ascendant would be defined as the point of intersection between the ecliptic and the rational horizon where the ecliptic moves above the reference plane (the rational horizon).
To define the ascendant using the same terms and conditions as that used for the midheaven, we have to limit ourselves to reference to zenith distance. I wish to suggest that a definition of the ascendant using this concept is that it is the point on the ecliptic that has a zenith distance of ninety degrees but has not yet attained its minimum zenith distance during the diurnal cycle. The two criteria - zenith distance, not yet attained its MZD - are necessary to distinguish the ascendant from the descendant. The latter also has a zenith distance of ninety degrees. The distinguishing feature of the descendant is that it is the point on the ecliptic with this zenith distance that has already attained its minimum zenith distance during the diurnal cycle.
How does this definition work. Firstly, the specification of the zenith distance ensures that the ecliptic degree coincides with a point on rational horizon, i.e., both points have a zenith distance of ninety degrees. This is in fact what the ascendant is - the intersection or coincidence of the two planes: the ecliptic and rational horizon. Secondly, the rising degree - or ascendant - is moving from its intersection with the rational horizon towards its intersection with the meridian when it will attain its minimum zenith distance during the day (it will be at is closest approach to the zenith).
The descendant is the point opposite the ascendant, exactly fulfilling the condition of having a zenith distance of ninety degrees (again coincident with a point on the horizon) but having attained its MZD earlier in the day, i.e., it is now setting.
In this definition of the ascendant, the application of direction (east) or concepts of above and below are not required to make a precise meaning of the term. In fact, the ascendant so defined will always be in the eastern half of the rational horizon and the descendant will always be in the western half of the same plane. However, the application of direction is neither a necessary nor sufficient condition for the definition of the concept. Likewise, the recourse to above and below is not required and can be dispensed with, as we did for the definition of the midheaven.
In the following image, the definition of the ascendant offered above is illustrated. Note that the coincidence of the plane of the ecliptic and the plane of the rational horizon occurs at zenith distance of ninety degrees. The rising degree will move from its position on the horizon to the MC during the course of one diurnal cycle and set later at a zenith distance of ninety degrees.
Figure 2: The Ascendant Definition Illustrated
We now have precise definitions for the angles of the horoscope using the minimum number of concepts. In fact, in both cases, the concept of zenith distance is the only term that needs to be formally employed. Both definitions work at all latitudes and for any time during a diurnal cycle. That is, they are universally applicable, unlike, in particular, alternative definitions of the midheaven that have been offered using the concepts of direction and above/below.
The midheaven is defined as the ecliptic degree that is at its minimum zenith distance during its diurnal cycle. This degree will always be coincident with the meridian of the place but the zenith distance of any particular degree on the MC will vary with the latitude of the place. When a degree is on the midheaven it is at its closest approach to the zenith during its diurnal cycle. The degree may, in some circumstances in the polar circles, be below the horizon.
The ascendant is defined as the ecliptic degree that has a zenith distance of ninety degrees but is yet to attain its minimum zenith distance during its diurnal cycle. It will, in due course, become the midheaven, but will not do so until it coincides with the meridian of the place and attains its MZD. The descendant is the ecliptic degree that has a zenith distance of ninety degrees and has already attained its minimum zenith distance.
The IC (Imum Coeli) is the ecliptic degree that is at is maximum zenith distance during its diurnal cycle. This degree will always be coincident with the meridian of the place but the zenith distance of any particular degree on the IC will vary with the latitude of the place. When a degree is on the IC it is at its furthest distance from the zenith during its diurnal cycle. The degree may, in some circumstances in the polar circles, be above the horizon.
The nonagesimal degree is the point on the ecliptic that is closest to the zenith at the time and place for which the chart is cast - that is, it is the point on the ecliptic that has the minimum zenith distance measured on any great circle running through the zenith and nadir at this time and place.This point is different from the MC; the degree associated with the nonagesimal will have been, or will be, closer to the zenith when it had, or has, its MZD on the meridian. The MC and the nonagesimal will only coincide when the first degree of Aries or the first degree of Libra rises, that is, when the equinoctial axis has a zenith distance of exactly ninety degrees.
In a later blog, I will explore some of the implications of these definitions, particularly for house systems generally employed by astrologers.
Helena Avelar and Luis Ribeiro (2010) On the Heavenly Spheres: A Treatise on Traditional Astrology. AFA Press.
Alan Candlish (1990) The Revised Waite's Compendium of Natal Astrology. Arkana Penguin.
Robert Hand (1982) Essays on Astrology: The Ascendant, Midheaven and Vertex in Extreme Latitudes. Whitford Press.
Jeff Mayo (1976) The Astrologer's Astronomical Handbook. L N Fowler and Co.
Jaqueline Mitton (1993) The Penguin Dictionary of Astronomy. Penguin Books.