`tau = 23^o 26' 11.57"`

23.4365472133

The Tilt of the Earth is 23° 26′ 10.7″ from the normal vector of the ecliptic.  

The Earth's Tilt

The Earth's tilt simply refers to the fact that the line through the North and South poles are not perpendicular to the path that the Earth flies around the Sun.  That path is on a plane known as the Ecliptic and all of the planets, now that Pluto is no longer a planet, fly on elliptical orbits around the Sun on the Ecliptic plane. 

This constant represents the Earth's Obliquity to its orbit about the Sun, which is it's axial tilt relative to the plane of it's orbit.¹

1. ^ Earth Fact Sheet