The ring of the zodiac (the ecliptic) was in a certain location AT the moment of your birth.
We set the Midheaven and the Ascendant for what WAS there.
And then for the rest of the houses it is a division of time .. how long it takes for the Sun to go from the Ascendant to the Midheaven.
- in summer, when the day is long, there is a lot of the zodiac between the Asc and the MC... you can have as many as 4 signs in that space (in those 3 houses).
- in winter, when the day is short, there is not much time between sunrise and noon ... so you have fewer signs in that space .. as few as 2.
No, there is no rule that consistently tells you what signs your house cusps will be.
It is a factor of the time of year .. .and also how far north/south from the equator your birthplace was.
The chart is calculated, based on the relative movement in the sky. It is not an "automatic" thing, EXCEPT for the Sun sign.
The reason the Sun sign is automatic and matches calendar dates, is because our calendar is based on Earth's orbit around the Sun. Every May 1st, when you look at the Sun, the SIGN of Taurus is behind it . because every May 1st, Earth will be in the same place on its orbit.
Now, there are different house systems. Different ways of dividing the ecliptic up into houses.
The "Equal House System" DOES have one different sign for each house, and you would have Leo on the cusp of your 12th if you had Virgo Rising.
But most Westerners use the Placidus system of House division, which is based on movement and time .. and that is what your chart has been done up for, and that is the system I have explained here for you.
We use Placidus because that system seems to produce more-accurate results.
As for Libra in the first house .. think of the zodiac as a wheel. The 1st house starts AT 26 Virgo, and ends AT about 22 Libra (hard to read the small print there). Everything between 26 Virgo up to 22 Leo (or about) is IN the 1st house.
Houses are man-made, man-decided. They are not natural phenomenon. Planets are natural phenomenon. So are the aspects between any two planets (shown by the red or blue lines connecting some planets to other planets).
Houses and signs are not natural phenomenon. (the zodiac signs are distances from where the Sun is at the time of the Vernal Equinox .. and don't actually match the constellations they are named after).
As for your Mercury .. let's start big and hone in on Mercury.
The zodiac is (appears as) a circle around Earth.
Like all circles, it is 360 degrees around.
360 degrees in a circle
60 minutes in a degree
60 seconds in a minute
The zodiac circle has been broken up into 12 equal areas, each area named after a constellation (yes, the "signs" are not constellations).
So the closest any planet In Cancer CAN get to "being in Leo" is if it is at 29:59:59 Cancer (29 degrees, 59 minutes, 59 seconds). Add one more second-of-arc, and NOW the location is no longer in Cancer, but it is in Leo.
Sure your Mercury can be in Cancer.
In fact it WAS in Cancer ... assuming you used an accurate and precise birthtime when you entered your birthdata into the website's calculation program.
Thank you for bearing with me
It is not easy to discuss astronomical data without diagrams to point to, and it is even harder to READ it.