what is the earth tilt ? how can u define it?

angle of difference between the earths equator and the sun's.

Or between the celestial equator and the ecliptic

23 degrees, It's the difference between the angle of rotation of the earth and the orbit of the earth.

Earth's Tilt

In astronomy, axial tilt is the inclination angle of a planet's rotational axis in relation to its orbital plane. It is also called axial inclination or obliquity. The axial tilt is expressed as the angle made by the planet's axis and a line drawn through the planet's center perpendicular to the orbital plane.

The tilt of Earth's axis is about 23.5 degrees.

The earth makes two kinds of major movements, rotation and revolution. The earth rotates around the imaginary line called the  axis. The northern end of the axis is the North Pole, and the southern end is the South Pole. The earth rotates once everyday-24 hours- around the axis. The rotating earth travels about 1,000 miles per hour (1,609km/hr)! (Don?t forget that you are traveling along on this 1,000 mile-an-hour journey)

The rotating earth also revolves around the sun. It takes the earth one year- 365 days- to complete its journey around the sun. The earth speeds along at 64,000 miles an hour (102,995km/hr) for its revolution. (You are still on this journey, right? Traveling at 64,000 miles an hour through space while simultaneously rotating at 1,000 miles per hour.)

As the year progresses and the earth revolves around the sun, the tilt of the earth?s axis doesn?t change. The axis is 23 ½ degrees away from a straight up and down line relative to the sun?s position. This means that for some of the year, the North Pole is aimed toward the sun, and some of the year it is aimed away from the sun.

The axial tilt may equivalently be expressed in terms of the planet's orbital plane and a plane perpendicular to its axis. In our solar system, the Earth's orbital plane is known as the ecliptic, and so the Earth's axial tilt is officially called the obliquity of the ecliptic. In formulae it is abbreviated with the Greek letter ?.

The Earth currently has an axial tilt of about 23.44° (23° 26?). The axis remains tilted in the same direction throughout a year; however, as the Earth orbits the Sun, the hemisphere (half part of earth) tilted away from the Sun will gradually become tilted towards the Sun, and vice versa. This effect is the main cause of the seasons (see effect of sun angle on climate). Whichever hemisphere is currently tilted toward the Sun experiences more hours of sunlight each day, and the sunlight at midday also strikes the ground at an angle nearer the vertical and thus delivers more energy per unit surface area.

Lower obliquity causes polar regions to receive less seasonally contrasting solar radiation, producing conditions more favorable to glaciation. Like changes in precession and eccentricity, changes in tilt influence the relative strength of the seasons, but the effects of the tilt cycle are particularly pronounced in the high latitudes where the great ice ages began [1]. Obliquity is a major factor in glacial/interglacial fluctuations (see Milankovitch cycles).

The obliquity of the ecliptic is not a fixed quantity but changing over time in a cycle with a period of 100,000,000 years. It is a very slow effect known as nutation, and at the level of accuracy at which astronomers work, does need to be taken into account on a daily basis. Note that the obliquity and the precession of the equinoxes are calculated from the same theory and are thus related to each other. A smaller ? means a larger p (precession in longitude) and vice versa. Yet the two movements act independent from each other, going in mutually perpendicular directions.

[edit] Measurement

Knowledge of the obliquity of the ecliptic (?) is critical for astronomical calculations and observations from the surface of the earth (earth-based, positional astronomy).

To quickly grasp an idea of its numerical value one can look at how the sun's angle above the horizon varies with the seasons; this was the way the Chinese astronomers determined it in 1000 BC. They measured the difference between the angles of the Sun above the horizon at noon on the longest and shortest days of the year. That difference in the angles is twice the obliquity.

The extreme northern and southern declination of the Sun during the year are equal to the obliquity. On the longest day of the year the earth is tilted toward the sun and we say that the sun's declination is + 23° 26?. To an observer on the equator standing all year long looking above, the sun will be directly overhead at noon in March (Vernal Equinox), then swing north until it is ? degrees away from the zenith in June (Summer Solstice). In September (Autumnal Equinox) it will be back overhead, then at the Winter Solstice in December it will be ? degrees away from the vertical again.

Example: an observer at 50° latitude (either north or south) will see the Sun 63° 26? above the horizon at noon on the longest day of the year, but only 16° 34? the shortest day. The difference is 2? = 46° 52?, and so ? = 23° 26?.

(90° - 50°) + 23.4394° = 63.4394° when measuring angles from the horizon (90° - 50°) -

the tilt of about 23 and a half degrees that is the difference between the equatorial perpendicular and the oblique imaginary line around which it rotates(axis).