MC lines are power lines. There are ten major power lines in your astrolocality map -one for every planet.
Most astrolocality maps show 20 power lines -10 conjunctions or MC points and 10 oppositions or IC points. There are in reality
a total of 100 such lines when considering all the 30° based aspects, and ideally there should be 144, but were not at 12 planets yet.
A good astrolocality program will be able to show all of these 100 lines and allow you to display them individually as well.
Programs like Janus, Kepler, Solar Fire, Matrix Horizons, and ZET to name a few. Each of these programs will have different or limited
astrolocality capabilities, but many are complementary when used together. I am partial to Janus in terms of accuracy and versatility and
ZET in terms of landscape esthetics.
A good astrolocality program will be able to show all of these 100 lines and allow you to display them individually as well. Programs like Janus, Kepler, Solar Fire, Matrix Horizons, and ZET to name a few. Each of these programs will have different or limited astrolocality capabilities, but many are complementary when used together. I am partial to Janus in terms of accuracy and versatility and ZET in terms of landscape esthetics.
Why do most maps only show conjunctions and oppositions of planets to the MC? Because it is easy or convenient to do so.
Are they the most important lines? This is debatable. While it is true that the planetary conjunctions or oppositions to the Asc and MC
are critical, this does not mean that we should omit the other aspects. Every aspect is arguably of equal importance, even the semi-sextile or quincunx.
If we employ all seven aspects(only those based on 30° are legitimate in my view) one may argue that there will be too many lines on the map. This is true when looking at the world map all at once, but they make sense when we zoom in considerably. The solution here is to use the primary aspect lines(conjunction and/or opposition) for an overall view, and to add in the other aspects when focusing on a much smaller area. Or one can select only one or two planet lines or aspects at a time.
While the MC power lines designate the longitudes of power, where an individual can rise to prominence according to the planet, sign, or aspect involved, the intersecting Ascendat lines narrow in on the most significant latitudes along those lines. As an example, let's take Sun conjunct Midheaven which supposedly will bring the native to social or professional prominence. This line will run perfectly vertical from 90°North to 90°South on the map along a particular longitude. That's quite a range to choose from and that's where the intersecting Ascendant lines come in narrowing down that range. Choose a favorable planet and aspect line that isn't too far north or south and that isn't running through the ocean or desert or some other undesirable area. This will narrow down your latitude choices considerably. Sun, Moon, Venus, and Jupiter lines and the conjunction, opposition, trine or sextile are best. If there are no desired Ascendant lines you will still reap the effect of your MC line.
A crossing or intersecting Midheaven and Ascendant line creates what is called a "paran" or horizontal effect, so that the influence of that intersection is felt all along that particular latitude(within one degree or 60 miles), but is of course strongest at its point of origin. There may even be a variation along that paran, like every 30 degrees from it. Parans are secondary in power to the planet lines and give added influence and/or information.
Perhaps the most powerful power line is Sun conjunct Midheaven which supposedly elevates the native to fame or power. Sun oppose Midheaven(same as Sun conjunct IC) is also
to be noted. Then there are Moon, Jupiter, or Venus conjunct or oppose MC which also bring fame or prominence. Moon/MC is good for both public and domestic affairs and dealings
with women. Mercury/MC is good for writers and speakers and learning or training. Venus/MC brings money, pleasure, art, and relationships. Mars/MC is good for athletes,
competitors, fighters, and warriors. Saturn builds governors, economists, politicians, builders, managers, and industrialists. Uranus/MC spawns electricians, inventors,
aviators, astrologers, and reformists. Neptune/MC, inspires dreamers, artists, romantics, and occultists. Pluto/MC fosters investigators, researchers, elitists, sex workers,
criminologists, surgeons, and executioners.
When relocating, some people feel the effects of their power lines right away, while for others it may take weeks or months. Living between two power lines will give you the effect of both provided they are relatively close(within a few hundred miles of the other). Moving closer to one line weakens the other. Within 50 miles of a line on either side is strongest, weakening considerably after 200 miles. Double or triple lines will of course have more impact and range. Transits and progressions can also affect your lines.