Obliquity of the Ecliptic

• The Obliquity of the Ecliptic is a phrase that refers to the ‘Tilt of the Earth’, which is currently 23.4°.
• This is the angle formed between the Ecliptic (the Plane of the Earth’s Orbit), and the Celestial Equator (the Plane of the Earth’s Equator).

The Tilt of the Earth

• The Earth’s Tilt is currently 23.4 degrees.
• The tilt was first recorded by Eratosthenes (c.275-174 BCE), then by Hipparchus (c.190-120 BCE) and later by Ptolemy (c.150 CE).

Ecliptic

• The Ecliptic is the path of the Sun in the Sky, a line along which the Sun always travels during the Year.
• The Moon and the visible Planets nearly all follow a line close to the Ecliptic.

Celestial Equator

• The Celestial Equator is a Great Circle or disc that divides the Earth’s Sphere into exactly two Hemispheres.
• It is on precisely the same plane as the Equator.