why is the rotating axis of earth tilting 23.5 degree from vertical ? Which is considered vertical line?

Answer 1

The odds that the axis would not be tilted is extremely remote. The net angular momentum (spin) of the planet is a composite of a lot of events. Also, it is not just tilted, but precesses with a wobble. That affects which star is our current north star, when the equinoxes will occur, and things like that.

Answer 2

The momentum of the particles that made up the earth when it was formed were the cause of the earth's inclined axis of rotation. But "vertical" is in the eyes of the beholder, and the usual definition is in line with the local gravitational force, rather than in a particular direction such as normal to the ecliptic plane.

Answer 3

You are correct. When you refer some angle there should be a reference line to the axis of earth.Then only you can call it as a tilt. The line of axis drawn perpendicular to its path of movement to the sun is 23.5 degree. This is helping for 4 types of seasonal changes.

Answer 4

In astronomy, this angle is called the "the obliquity of the ecliptic". The ecliptic is the plane of the Earth's orbit. The angle is between the axis of the earth and a line perpendicular to the ecliptic and passing through the center of the Earth.

Answer 5

The tilted axis is what gives the planet our four seasons.
When the Northern hemisphere is tilted toward the sun, it is summer in the north, and winter in the south.

Answer 6

http://en.wikipedia.org/wiki/Axial_tilt

I think this might explain it better than I could.