Ecliptic

The ecliptic is the great circle on the celestial sphere that the Sun appears to trace from Earth's perspective over the course of a year. It is not a path the Sun actually walks; it is the projection of Earth's orbital plane onto the sky. The Sun appears to move along it because we are moving around the Sun.

The ecliptic is the reference plane for astrology. The twelve signs of the zodiac are equal 30-degree segments of the ecliptic, beginning at the vernal equinox point (0 degrees Aries). When an astrologer says Mars is in Scorpio, what is meant is that Mars' ecliptic longitude falls within the segment between 210 and 240 degrees of ecliptic longitude.

The ecliptic is tilted approximately 23.4 degrees from the celestial equator. This tilt is what produces the seasons and what makes the Sun appear to rise higher or lower in the sky depending on the time of year. The tilt is also why planetary positions in astrology are measured along the ecliptic rather than along the celestial equator used by astronomers for most other purposes.

All major planets in our solar system orbit close to the ecliptic plane, which is why they all appear to move through the same band of constellations. The Moon's orbit is tilted about 5 degrees from the ecliptic, which is why eclipses do not happen every month — they require the Moon to be near one of the two points where its orbit crosses the ecliptic.