<P> The IAU also uses the right - hand rule to define a positive pole for the purpose of determining orientation . Using this convention, Venus is tilted 177 ° ("upside down"). </P> <P> Earth's orbital plane is known as the ecliptic plane, and Earth's tilt is known to astronomers as the obliquity of the ecliptic, being the angle between the ecliptic and the celestial equator on the celestial sphere . It is denoted by the Greek letter ε . </P> <P> Earth currently has an axial tilt of about 23.4 ° . This value remains about the same relative to a stationary orbital plane throughout the cycles of axial precession . But the ecliptic (i.e., Earth's orbit) moves due to planetary perturbations, and the obliquity of the ecliptic is not a fixed quantity . At present, it is decreasing at a rate of about 47" per century (see details in Short term below). </P> <P> Earth's obliquity may have been reasonably accurately measured as early as 1100 BC in India and China . The ancient Greeks had good measurements of the obliquity since about 350 BC, when Pytheas of Marseilles measured the shadow of a gnomon at the summer solstice . About 830 AD, the Caliph Al - Mamun of Baghdad directed his astronomers to measure the obliquity, and the result was used in the Arab world for many years . In 1437, Ulugh Beg determined the Earth's axial tilt as 23 ° 30 ′ 17" (23.5047 °). </P>

How much is the tilt of the earth's axis