The Sun's yearly path across the sky is why our day lasts 24 hours
Earth spins once relative to the stars in about 23 hours 56 minutes. Our day is four minutes longer because, as we orbit, the Sun creeps a little under one degree eastward each day along the ecliptic, and the ground has to turn a bit further to catch up with it.
The ecliptic is the line the Sun appears to trace against the stars over a year, a reflection of Earth's own orbit. Ancient astronomers, astrologers and calendar makers built their measurements on it, and they named it for a telling observation: eclipses only happen when the Moon crosses this line. The Moon never strays far, since its orbit is tilted just 5.1 degrees, cutting the ecliptic at two points called the lunar nodes.
The planets hug the same path, and the zodiac constellations lie along it. That is because the Solar System condensed from a flat disc of gas and dust. Measured against the invariable plane, a reference set by the total spin of the whole system, Earth's orbit tilts a little more than 1 degree, Jupiter's barely half a degree and Mercury's, the most tilted, about 6. Jupiter supplies more than 60 percent of that total angular momentum. Since the invariable plane depends on knowing every object precisely, astronomers prefer the ecliptic as the working reference, accepting that it drifts slowly over geological time.
Earth's equator is tipped about 23.4 degrees to the ecliptic, a figure called the obliquity, and ancient scientists worked out the planet's axial tilt by comparing the two planes. Projected into the sky, the equator meets the ecliptic at two equinoxes. The Sun crosses northward at the March equinox, also called the first point of Aries, and southward at the September equinox.
The path is not perfectly smooth. Earth's orbital speed changes through the year, so the Sun spends roughly 185 days north of the celestial equator and about 180 south of it, which feeds into the equation of time. The Sun's track also wobbles monthly as Earth swings around its shared balance point with the Moon, and the other planets nudge that balance point further. Relative to the plane of our galaxy, the whole ecliptic is tilted by 60 degrees.
Source: Ecliptic
