Why doesn't Mercury travel straight across the Sun's equator during it's transit?

Answer 1

Mercury would go straight across the Sun (although not technically its equator) if Mercury's orbit was not tilted relative to Earth's orbit. Since it is tilted 7 degrees relative to Earth's orbit, it's actually pretty rare for Mercury to appear right in front of the Sun as seen from Earth to begin with - even though Mercury only takes 88 days to orbit the Sun, Mercury transits only happen about a dozen times a century!

Earth's orbit is also tilted relative to the Sun's axis of rotation, so even if Mercury's orbit wasn't tilted compared to Earth's orbit, Mercury still wouldn't go straight across the Sun's equator - it would form an angle with the equator, intersecting the equator in the middle.

Answer 2

Mercury's orbit is inclined at 3.38° relative to the Sun's equator.

When small bits of dust and material coalesced to form Mercury, their combined momentum determined the inclination. It would take considerable force to move the orbit to follow the sun's equator.

Answer 3

Neither Mercury's orbit nor Earth's lay exactly on the Ecliptic (the plane of the Solar System). The Ecliptic is defined by the AVERAGE planes of all the planetary orbits.