We recently had a question come up because of ambiguous wording in someone’s book about local and relocational astrology. The question was “Do local aspects change when you move?”. The answer to the question depends on the coordinate system you are using to measure your aspects.
In ordinary (ecliptic coordinate) relocational astrology used by most astrologers, you calculate new house cusps for the new location, but the longitudes of the planets don’t change. Except when you are using parallax-corrected positions for the Moon, all geocentric planetary longitudes normally used in astrology are positions as viewed from the center of the earth. Except for the Moon, the difference between a central view and a surface view is small enough to be unnoticeable and therefore not worth the extra calculations required to correct from a central view to a surface view. Thus, as long as you are talking about the same instant in time (UT unchanged), geocentric longitudes are the same no matter where you are on the earth. The framework of houses moves with respect to the fixed planets, but all ecliptic aspects between planets are unchanged in a relocation chart. Since you do get new angles (MC, IC, Ascendant, Descendant, East Point, West Point, Vertex, Antivertex), all aspects involving angles will be changed in a relocation chart.
In the technique being called “local space”, which gives planetary positions in a framework of altitude and azimuth, the answer is quite different. Altazimuth coordinates are a local framework, intimately tied to the location of the observer on the surface of the earth. If you move to a new location on the earth, you are defining a new set of altazimuth coordinates for everything in the sky. Any planetary positions given in altazimuth will change when you move, and aspects between the altazimuth positions will change.
Altazimuth coordinates are also called horizontal coordinates because they are based on a horizon. The rational horizon is a great circle—an idealized flat horizon through the center of the earth parallel to the bumpy visible horizon. The horizon divides the sky into top (visible) and bottom (invisible) halves. The meridian is a great circle perpendicular to the horizon, going through the north and south points of the horizon and the zenith (the point straight overhead) and the nadir (the point straight underfoot). The meridian divides the sky into east and west halves. The Prime Vertical is the third great circle in the horizontal system, perpendicular to both the horizon and the meridian, going through the east and west points of the horizon and the zenith and nadir. The Prime Vertical divides the sky into north and south halves. Measurement around the horizon is called azimuth. There are several different starting points used in measuring azimuth, with the two forms most used in astrology starting at the north point and going clockwise or starting at the east point and going counterclockwise to mimic signs and houses. Measurement up and down from the horizon is called altitude.
The coordinate system most used in astrology is tied to the ecliptic, a great circle defined by the Sun’s apparent path in the sky. The ecliptic is at an angle of about 23 1/2 degrees to the celestial equator, which is a projection into the sky of the earth’s equator. (The intersections of the ecliptic and the equator are called tropical zero Aries and zero Libra.) The angle between the three great circles of the horizontal system and the equator (and the ecliptic) depends on the latitude of the observer. In the northern hemisphere, the equator is in the southern half of the sky, down from the prime vertical toward the horizon by the angle of the latitude. In the southern hemisphere, the equator is down the angle of the latitude from the Prime Vertical toward the northern horizon. The ecliptic is at a 23 1/2 degree angle to the equator, with Aries through Virgo north and Libra through Pisces south of the equator. Which parts of the ecliptic and equator are visible above the horizon depends on the sidereal time and the latitude. As soon as you shift the point from which you are observing, you shift the angle between the horizontal system and the equator and ecliptic.
Because we are dealing with sets of spherical coordinates at varying angles to each other when we work with latitude and longitude on earth, celestial latitude and longitude, and altitude and azimuth, it is very difficult to give simple or intuitive rules for how large a shift in each set of coordinates will be produced by a given shift in one of the other coordinates. Moving by one degree in either latitude or longitude on the earth can change the azimuths of different planets by as little as a few minutes up to as much as two degrees. Because the planets’ azimuths don’t all shift by the same amount and in the same direction when you move, the aspects between the azimuth positions change as you move. (As an example, my Los Angeles local azimuth angle between Ceres and the South Node is 179° 46’, and my New York angle is 179° 54’, a change of only 8 minutes of arc. In contrast, my L.A. azimuth angle between Mars and Jupiter is 55° 57’, and my NY angle is 92° 58’, a change of 37° 1’. Both of these azimuth changes are for the same change of 6° 41.6’ in latitude and 44° 22’ in longitude.) Because of the complexity of the changing angles, I suggest that if you want to work with “local space” charts you let a computer calculate the relocated azimuths and azimuth aspects as well as the natal.