A cusp, in architecture, physiology, dentistry ... and astrology, is a point where two things touch, intersect, or fit together -- one against the other. In astrology the point of the "cusp" is more abstract ... referring to the place in the continuum of the Zodiac or in a paper chart where a change occurs between one sign and the next or one house and the next.
In the circular chart of a horoscope -- the document an astrologer uses to read a person's character and the themes and trends of his life -- the cusps between the houses are marked by the dividing lines (like the spokes of a wheel) around the circle. They are assigned a specific degree and minute of arc within the 360° of the circle. (I know. Those of you who snoozed through geometry have no idea what I'm talking about.)
Let's try and put it another way for the non-math majors among us. (Let's also say from the outset it won't be easy, because astrology does necessarily include a certain amount of geometry and math.) All twelve signs of the horoscope are contained in the circle of the chart. Three hundred sixty degrees (in a circle) divided by twelve (the number of signs in the Zodiac) gives thirty degrees per sign.
The house cusps of the horoscope (which is a map of the sky charted at the instant you were born and "superimposed" upon the circle of the Zodiac with its twelve separate signs) will occupy a specific degree and minute of arc ... a certain specific point in space within that circle. (I warned you it wouldn't be easy to explain this.)
In the graphic below, a chart cast for the moment I started writing this article this morning, the cusp of the First House ... also known as the Ascendant ... the far left edge of the horizontal line ... the place where the "9" would be if this were the face of a clock ... is at 16° 26' of Virgo. The Second House cusp (where the circle changes from First House territory to Second House territory -- the place where the "8" would be on a clock face) is 16° 11' of Libra. The Third House cusp -- where the "7" would be on a clock face -- is 15° 31' of Scorpio.
Throughout the day, the chart of the horoscope -- with all its different divisions and "cusps" -- rotates counter-clockwise through the circle of the Zodiac. The degree of the Ascendant changes every four minutes. The Ascendant sign itself changes approximately every 2 hours.
This is why a birth time is essential in casting a valid, personal horoscope. An astrologer trying to read a chart has to have a "starting point" to tell her where the chart "fits" into the sky in order to know where all the other components are in the chart -- and therefore what it is saying about the person's life and character on the piece of paper in front of her.
Since each day contains every single one of the 360 degrees of a circle, in order to give an accurate individual reading, the astrologer simply must have the correct starting point. Otherwise, she has to fall back on a chart cast at sunrise -- which does have validity, as it sets the entire tone and theme of the twenty-four hours to follow -- until the Sun rises at the eastern horizon once more and a new day begins. But the chart, and reading, that results -- while accurate -- is necessarily more general. It is the "one size fits all" equivalent for anyone born in that place on that day.
Once the chart of the horoscope is drawn and the planets are placed in the position of the sky they occupied at the time the horoscope was cast, they will necessarily be in one of the twelve houses. Or so it would seem. In actual fact, a planet in astrology can occupy two places at the same time in terms of the influence its metaphors have in a person's life.
Keep in mind again, for our purposes at this point, planets now placed in the chart, fixed in time by a birth or a new beginning or an event, progress in a counter-clockwise direction. A planet situated very close to one of the cusps in a chart (one of the dividing lines that mark the divisions of the twelve houses) while it is physically located in one house will actually show its strongest effects in a person's life and personality in the areas of concern ruled by the house it has yet to enter.
In fact, a planet located on a cusp can actually seem more intense in its meaning and influence than a planet located well inside the house itself. Being inside the preceding house, however, it will also show lots of effects on the concerns found there. As you can see, a planet on a cusp will be quite complicated and much more inclusive in its metaphors (and meaning) than one just rocking through a house, tending to simple business in that one area of life.
Take a look at the graphic again. Notice how the Saturn - Pluto opposition falls nicely along the 4th House / 10th House axis in this chart. Pluto is at 12° 33' of Sagittarius, the 4th House cusp is at 15° 0' of Sagittarius; Saturn is at 14° 19' of Gemini, the 10th House cusp is at 15° 0' of Gemini. As each of these move on -- with some time out for retrograde motion along the way -- they will eventually cross those dividing lines and transit through the house they are approaching.
Each planet's influence in the previous house is still present ... but its influence in the following house is getting underway, and its eventual presence there will usher in significant early experiences that will leave their mark on whatever is being described by this chart. (In this case, this article.)
What determines whether a placement is on a cusp?
Most astrologers agree that a placement within 5 degrees of a cusp line places any planet within that space "on the cusp" of that house ... with planets still placed in the preceding house (as in the graphic) having the most power -- the most divided attention and influence in the way their metaphors describe the experiences, abilities, challenges, and concerns of that area of a person's life.
They will show themselves in the way the person handles matters of the house they occupy and their authority shows up emphatically in the concerns of the house they have yet to enter.
Thus, for example, a person with a Sun technically in the Third House ... but on the cusp of the Fourth House will still be a highly intellectual spirit, restless, inquisitive, and very interested in communication and learning, but he will also be emphatically involved with family issues and arrangements in his private life, concerned about his home conditions, exceptionally sensitive to matters affecting his security and other psychological needs, and very much in need of a "home base" -- such as a physical house that he owns ... which are not normally pressing issues for Third House Sun folks.
Now, having explained this concept (perhaps ad nauseam), let me move on to say that most non-astrologers' familiarity with the concept of being "born on the cusp" concerns birthdays that occur near the day the signs of the Zodiac change ... usually around the 21st of any month.
Many of the principles I've just described apply to this situation ... and some others besides. But let me note that (given the explanation above) a person can be born with his Sun (or any planet for that matter) on the cusp of a house in his chart no matter what day he was born, but depending entirely on the minute of a given day he was born.
However, being "born on the cusp" to most people means a birthday (usually their own) falls on or about the 21st of the month. This is approximately the day the Sun changes from one sign to another in the Zodiac, although calendars and other human structures being what they are, even that causes confusion. Sometimes the date is the 19th; sometimes the 23rd. People don't understand the discrepancies and may wonder for years which sign they actually are.
(c) 2003 Rebecca Brents, All rights reserved.
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Born on the Cusp in Astrology -- The Technical Explanation ---> A cusp, in architecture, physiology, dentistry ... and astrology, is a point where two things touch, intersect, or fit together -- one against the other. In astrology the point of the "cusp" is more abstract ... referring to the place in the continuum of the Zodiac or in a paper chart where a change occurs between one sign and the next or one house and the next.
Were You Born on the Cusp? -- Examples all around the Zodiac ---> The self-sacrificing, mystical, artistic, and usually peace-loving Pisces-to-Aries person will sometimes be surprisingly temperamental, impatient, aggressive, and daring, with an unmistakable warrior energy camouflaged, as a rule, by the normally passive, spiritual qualities of her Pisces sign.
What Does It Mean to be 'Born on the Cusp?' ---> Being "born on the cusp" to most people means a birthday (usually their own) falls on or about the 21st of the month. This is approximately the day the Sun changes from one sign to another in the Zodiac, although calendars and other human structures being what they are, even that causes confusion. Sometimes the date is the 19th; sometimes the 23rd. People don't understand the discrepancies and may wonder for years which sign they actually are.