Ecliptic
The ecliptic or ecliptic plane is the apparent path of the Sun on the celestial sphere, resulting from Earth's orbit around the Sun. It was a central concept in a number of ancient sciences, providing the framework for key measurements in astronomy, astrology and calendar-making. The ecliptic is so named because the ancients noted that eclipses only occur when the Moon is crossing it.
Sun's path can be traced against the background of stars, especially the Zodiac constellations. The planets of the Solar System can be seen along the ecliptic because their orbital planes are very close to Earth's. The Moon also appears near the plane, with the Moon's orbital plane inclined only 5.1°, intersecting the ecliptic at the lunar nodes.
The ecliptic is an important reference plane and is the basis of the ecliptic coordinate system. Ancient scientists were able to calculate Earth's axial tilt by comparing the angle of the ecliptic (about 23.4°) to that of the equatorial plane.
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