For the sake of argument: Let’s say that a small, rogue black hole came blasting through our solar system and it unceremoniously tossed Mars out of its orbit. Let’s also imagine that Mars is heading towards Earth so that as it passes by it appears to be as big as the full moon. Other than presenting a really nice view in the night sky, what would be the consequences? (We’ll ignore the effects of that rogue black hole, which is probably going to really ruin everybody’s day and just consider Mars for the moment.)
Mars is almost 9 times as massive as the moon is, and at its closest approach to Earth it contributes a tidal force that is about .00003% that of the moon’s. Very small, indeed! For the planet Mars (diameter – 4076 miles/6794 km) to appear as big as a full moon, which is about .5 degrees wide, it would have to be about 467,000 miles (747,200 km) from us. That’s almost twice the distance between the earth and the moon. At that distance Mars would exert a tidal force (which is a function of 1/R3, where R is the separation of the two bodies) on the earth that is about 1.1 times that of the moon. So depending on the location of the moon relative to Mars, the tides could be more than twice as big if the moon and Mars were aligned on the same side of Earth, to almost nothing if they were on opposite sides of our planet, where the two forces would balance out. This tidal force would change direction as Mars swept by Earth, affecting the magnitude and duration of the tides. Mars would also tug on the Earth-moon system and could cause the moon’s orbit to change by some amount depending on the timing of the event.
We would have a beautiful view in the night sky for sure, but more importantly, we would probably have some very high tides and more seismic activity that we would not have had otherwise! This would also apply to the moon, as Mars would play a tug-of-war with Earth over it.