The Basics – the Ecliptic, the Equator, and Coordinate Systems
by Dwight Ennis © 1998
The Ecliptic System and the Zodiac
The term ecliptic is mentioned often in astrology. It describes the centerline of that which we call the Zodiac, which extends some eight degrees above and below the ecliptic. In other words, the Zodiac is a belt 16° wide centered on the ecliptic. But what defines the ecliptic?
Simply put, the ecliptic is the plane of the Earth’s orbit around the Sun. A plane is merely a flat two-dimensional surface (it has length and width, but no depth). The Earth’s orbit, viewed from the side (like a circle viewed from the side), is a plane. So the ecliptic is the circle (plane) of the Earth’s orbit extended to infinity.
Since we live on the Earth, and not on the Sun, we seem to see the Sun moving along the ecliptic rather than the Earth. As we pear into the heavens, the Sun appears to follow its endlessly cyclic path through the center of the Zodiac. What’s actually happening is that we’re following our endlessly cyclic path around the Sun, and we’re in the opposite sign (at least, we are from the Sun’s point of view). Nevertheless, we live HERE, on the Earth, so it’s only natural that HERE becomes the center of our point of view, our psyches, and our astrological system.
It takes the Earth about 365¼ days to complete its journey around the Sun – consequently, it takes the Sun 365¼ days to complete its apparent journey around the Earth. This cycle defines our astronomical year. It also comprises our calendar year. Due to that extra ¼ day that it takes the Earth to complete its journey, our calendar of 365 days is corrected once every four years with an extra day added to February (since ¼ day/year X 4 years = 1 day). We call this Leap Year.
Whenever I’ve seen a documentary style show on astrology, a scientist (or scientists), is always interviewed on the subject. Naturally, they always point out that “it can’t possibly work” because it’s a system based upon an Earth-centered solar system, where everything revolves around the Earth. They continue that, although we once believed this to be the case, we now know that the Sun is the center of the solar system, so astrology can’t possibly work because its fundamental assumption is irreparably flawed. I always have to smile when I hear this, since these same scientists use coordinate systems for locating the stars which are also Earth-based, or Earth-centered (usually the Equatorial System which uses Right Ascension and Declination to define coordinates 2 ). Does this mean that the sciences of astronomy and cosmology can’t possibly be right either? Somehow, this question is never asked.
The position of a heavenly body is measured in the Ecliptic System using Celestial Longitude and Celestial Latitude. Celestial Longitude is measured horizontally starting at 0° of Aries 1. Position is measured in degrees, minutes, and seconds of arc in a counter-clockwise direction along the Ecliptic. For example, a planet located at 15° of Taurus could also be said to be located at 45° of Celestial Longitude (the 30° of Aries + the 15° into Taurus).
Having located a body horizontally in Celestial Longitude, we still need to describe its position vertically in order to pinpoint its precise location. In the Ecliptic System, this is done using Celestial Latitude. A body’s position is again expressed in degrees, minutes, and seconds of arc, but this time with reference to its distance above or below the plane of the Ecliptic. This is exactly the same as describing latitude on the surface of the Earth, except we use the Ecliptic in place of the Earth’s Equator. A body above the Ecliptic is expressed as North Celestial Latitude, just as a position on Earth’s surface above the Equator is said to be North latitude. Similarly, a body below the Ecliptic is expressed as South Celestial Latitude.
While Celestial Longitude is used extensively in astrology, Celestial Latitude is not as frequently employed. Instead, astrologers use Declination.
The Equatorial System
Almost everyone has heard of the Equator. It is a circle that bands the sphere of the Earth at it’s thickest point, defining 0° of terrestrial latitude and separating the northern hemisphere from the southern hemisphere. The Equator is perpendicular to the Earth’s Axis of Rotation. The Earth spins on its Axis of Rotation once a day, with this Axis tilted 66½° with respect to the Ecliptic. As a result, the Equator is tilted 23½° with respect to the Ecliptic.
Since the Equator is a circle, it too is a plane which is perpendicular to the Axis of Rotation, and which passes through the center of the Earth. When we extend this plane out into space, it becomes the Celestial Equator, which is, again, tiled with respect to the Ecliptic by an angle of 23½°. So why should we, as astrologers, care?
As was said before, astrologers measure horizontal positions of the planets and other heavenly bodies using Celestial Longitude. However, we don’t measure vertical position in Celestial Latitude. Instead, we use something called Declination. Declination is the distance of a body, in degrees, minutes, and seconds of arc, above or below the Celestial Equator (not above or below the Ecliptic as is the case with Celestial Latitude). The Parallel and Contra-Parallel Aspects are aspects of Declination. In the first case, two or more planets or bodies share the same Declination – i.e. both being at the same degree of Declination either North or South. In the second case, two bodies share the same degree of Declination, but ONE is North and ONE is SOUTH!
The Equator, the Ecliptic, and the Seasons
The tilt of the Earth’s Axis of Rotation is also responsible for our seasons. This is reflected in the yearly cycle of the Sun’s Declination. The Sun is always at 0° of Celestial Latitude, since it is always right on the Ecliptic. However, throughout the yearly cycle as the Earth travels around the Sun, the Earth’s Axis of Rotation will be tilted either towards the Sun, away from the Sun, or somewhere in between. In the illustration below, note that the Celestial Equator intersects the Ecliptic at two points. These points define a line which precisely pass through 0° Aries and 0° Libra. It is precisely this intersection which defines the Vernal Equinox and the Autumnal Equinox, and the first degree of both Aries and Libra (speaking of the Tropical Zodiac).
At the Vernal Equinox, which is the beginning of spring, and also defines 0° Aries (at least in the Tropical Zodiac), the Earth’s Axis of Rotation is perpendicular to the Sun, and perpendicular to the Line of Intersection between the Ecliptic and the Celestial Equator. The days and nights all over the globe are of equal length (the word equinox is derived from the medieval Latin aequus, meaning equal, and nox meaning night).The Sun’s Declination is 0°, since the Sun is also aligned with the Line of Intersection.
Three months (and 90° of orbit), later, we have the Summer Solstice. At this point, the Earth’s Axis of Rotation is tilted towards the Sun, and is aligned with the Line of Intersection. Since the Northern Hemisphere is tilted towards the Sun, this is the longest day of the year. The Sun’s rays strike us at less of an angle, temperatures are consequently warmer, and we have Summer in the Northern Hemisphere, while the Southern Hemisphere, experiencing the exact opposite orientation, has its Winter. Again, due to the Northern Hemisphere being tilted as much towards the Sun as it’s going to, the Sun reaches maximum Northern Declination in the sky – exactly 23½° North Declination. It is this which defines the Tropic of Cancer, another belt which girdles the Earth exactly 23½° above the Equator. Also note that the Arctic Circle, which lies at 66½° terrestrial latitude, just touches, or is tangent to, the terminator (the line separating Earth’s day-side from its night-side). It is this point of tangency which in fact defines the Arctic and Antarctic Circles. On the Arctic Circle at the Summer Solstice, the Sun will never completely set, and will just touch the horizon before beginning to climb back into the sky again. At and above this latitude, the Sun never sets in the summer. The opposite is of course true at or below the Antarctic Circle, where 24 hours of night reigns.
After another three months (and another 90° of orbit), we have the Autumnal Equinox. Again, the Earth’s Axis of Rotation is perpendicular to the Sun, and perpendicular to the Line of Intersection. The days and nights are again of equal length, and the Sun’s Declination is again 0°.
The last of the seasons is Winter, which happens three months (and 90° of orbit), after the Autumnal Equinox. At this point in its orbit, the Earth’s Axis of Rotation is tilted as far away from the Sun as it will get (so far as the Northern Hemisphere is concerned). This is the shortest day of the year. The Sun’s rays strike us at their greatest oblique angle, temperatures are consequently colder, and we have Winter in the Northern Hemisphere. Again, the Southern Hemisphere experiences the exact opposite orientation, and has its Summer. The Sun reaches maximum Southern Declination in the sky – exactly 23½° South Declination. It is this which defines the Tropic of Capricorn, a third belt which circles the Earth exactly 23½° below the Equator. The Antarctic circle and latitudes below are bathed in 24 hours of daylight, while the Arctic Circle and points above are having 24 hours of night.
Long and Short Days
While not necessarily of use to the astrologer, it is nonetheless interesting to look at the reason for the varying length of the day throughout the year. This can be most clearly illustrated with another illustration.
For simplicity, the above picture assumes one is located exactly on the Tropic of Cancer at 23½° north latitude. As can be seen, the angle of the Earth’s surface contained within the daylight side varies with the time of year from 201.8° at the Summer Solstice to 158.2° at the Winter Solstice. Since the Earth rotates one full revolution, or 360° every 24 hours, we can calculate that it rotates 360/24=15° each hour. So, at the Summer Solstice, the longest day of the year, we have 201.8° of daylight, or 201.8°/15° per hour = 13.45 hours = 13 hours 27 minutes of daylight.
At the Winter Solstice, the shortest day of the year, the angle of daylight is only 158.2°. In this case, we have 158.2°/15° per hour = 10.55 hours = 10 hours 33 minutes of daylight.
Lastly, we know the Equinox is supposed to contain equal hours of day and night, or 12 hours each. We see above that the angle of daylight is 180° – exactly half of 360°. We can see right away that the length of daylight equals the length of night, but if we want to do the proof, we have 180°/15° per hour = 12 hours of daylight.
1 0° of Aries is defined in the Tropical Zodiac by the point of the Vernal Equinox, or that point where the Sun has 0° of Declination as it moves north after the Winter Solstice. The Sidereal Zodiac, however, is a system based upon the constellations themselves, and defines 0° Aries by a different system. Due to the Procession of the Equinoxes, these two zodiacs have, at this point in time, diverged almost a full sign, so that what is in Aries in the Tropical Zodiac is in Pisces in the Sidereal Zodiac. The astronomical phenomenon responsible for the Procession of the Equinoxes, and for the Great Ages, of which the “Age of Aquarius” is one, will be the subject of a future article.
2 While we, as astrologers, use Celestial Longitude divided into the Zodiac to define horizontal coordinates, astronomers generally use Right Ascension. As with Celestial Latitude, measurement begins with 0° corresponding to the point of the Vernal Equinox. However, where Celestial Latitude measures counter-clockwise, Right Ascension measures clockwise from this point, with the measurement being expressed either in degrees, minutes, and seconds of arc, or in hours, minutes, and seconds of time. In any event, it is still a system which used the Earth as its center of reference. Why that’s logical and okay for astronomical measurement, but irrational and not okay for astrological measurement is beyond me!
About the author:
Dwight Ennis has studied astrology for more than 20 years. He was one of the founding members of the South Bay Astrological Society, and was its first President, serving two terms. His primary astrological focus is on the blending of astrological symbolism with Jungian Depth Psychology.