- Is there a reason that the Earth has the tilt that it does (~23°)?
- How do we know which way is supposed to be 0°?
- Does this tilt have major consequences on the planet?
- Has it changed and will it change in the future? (If so, would there be any consequences?)
First up, the tilt is exactly 23.45 degrees.
The reason for Earth's tilt is still not yet really proven, but scientists at Princeton stated on August 25, 2006 that planet Earth may have 'tilted' to keep its balance. Quote:
Same goes for the lack of proof about the exact consequences on the planet or if it'll happen again. All that is still being researched and debated, but those Princeton scientists have thrown in some interesting perspectives which you'll discover when you visit the link I provided and completely read what they wrote.
Besides "consequences" for Earth as a planet, it should be noted that the tilt of Earth is the reason why we have seasons. So even when we yet have to find out what the consequences for Earth (as a planet) have been and/or will be, we do know that the tilt surely has consequences for all beings that live on planet Earth… the seasons that influence us all: summer, fall, winter, and spring.
And - last but not least - as for your question how they know where 0 degrees would be, you just have to look at the Earth's rotation around the sun. Take a 90° angle of the planet's orbit and you know where 0° would be. But I guess a picture explains more than a thousand words, so I quickly created the following graphic:
All this theories of top scientist are ignoring the enormous gravitational forces that rule the central Sun of the galaxy, influencing our Sun and Earth and Moon alike. If it was, like top scientists claim, that our Earth was struck in its young days with big object that influenced Earth tilting (for millions of years?) actually Earth would quickly find its equilibrium, couple of hundreds of years would be enough if the object is even moon sized one. Gravitational forces are so strong that even if Earth was out of so strong composition like for example egg sized diamond, gravitational forces would grind that diamond shaped Earth in a perfect ball in just couple of centuries. The main reason of Earth tilting and seasons on it is the Moon and its unnatural gravitational field that influence Earth tilting. Some theorize about time the Moon is in Earth gravity. Let's say Moon is in Earth gravity for 10.000 Years. That 4 times smaller object even if he came very violently to Earth gravitational hug, making earth flip totally in its axis, he would still stabilize in perfect orbiting around Earth equator for a maximum of 200 earth years’ time. Earth without a Moon or with stabilized Moon in Earth rotation would cause Earth flipping of 24 degrees disappear, we would have around 22 Celsius, day time, all around Earth and only one Season. Almost nonexistent extreme weather conditions we have now including earthquakes. We would have less rains but more humidity and ozone in the air because Earth energy balance would stabilize. That would cause plants to grow faster and bigger and humans would require less food to sustain itself because higher earth energy that does not dissipate because of Earth flipping would mean more energy for living beings on Earth. Smarter people less wars. The Moon has unnatural gravity that is making havoc on Earth we are experiencing. HOWL….
The tilt of the rotation axis of the Earth is probably twofold. First, it is probably due to Earth formation history. Even though, by angular momentum conservation, planets should have a rotation axis aligned with that of the Sun, their formation history, characterized by accretion and collision of planetisimals, should have perturbated the initial axis. Second, it is due to gravitational perturbations. And actually, regarding this effects, Earth axis should be much more tilted; hopefully, the Moon is there to help.