Sun and Calendar · 2 of 9

The Ecliptic

Why do the Sun, the Moon and the planets all keep to one track?

One line across the sky carries the Sun, the Moon and every planet. Find it once and half of astronomy gets easier.

You’ll be able to trace the ecliptic across the sky, the line every planet stays close to.

11 steps · about 8 min

Before this: Celestial Coordinates (8 min)

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Why does every planet keep to one track? The line across this sky, past Aldebaran and Pollux, is the ecliptic: the Sun's path against the stars over a year, and the plane of Earth's orbit seen from inside it.

The same minute, pulled out past the orbit. The ring is Earth's path round the Sun, and the line you were looking at is this ring seen edge-on from a seat on it — a line, not a band. The four marks are where the seasons turn; the globe is you, drawn hundreds of times oversize.

Now the planets' rings too, seen from just above them. Every planet's orbit lies in nearly one plane, because they all formed from one flat, spinning disc round the young Sun. Seen from inside the disc, they share one line.

Lay the ecliptic flat and the celestial equator is the line that tilts: it closes on the ecliptic toward one side, crossing it at an equinox, and is furthest from it at the measured gap, 23° — Earth's axial tilt, 23.44° to be exact.

Where the ecliptic crosses the celestial equator is an equinox; the Sun is there about 20 March and 22 September, a day either way by year and time zone. Tonight the March one is setting due west, as every point on the equator does, from everywhere.

The constellations the line runs through are the zodiac — thirteen, counting Ophiuchus. Follow it past Aldebaran, just south of Castor and Pollux, straight through , Praesepe — the Beehive — and on to Regulus. The Beehive sits on the line, so planets keep drifting through it.

The whole sky at once, laid flat. The line is one wave all the way round: over the equator at the equinoxes, 23.44° either side at the solstices. The strips are the thirteen constellations it crosses. The price of flat: the poles are stretched sideways.

The marked body is the Moon, at its next first quarter. The second line is its own path, tilted 5.15° to the ecliptic. They cross at two , and only a new or full Moon near a node can make an eclipse: Eclipse Seasons tells the rest.

The nodes, from Eclipse Seasons

A planet on the line: Jupiter, on the night of its next . Its orbit tilts just 1.3° to Earth's, and no planet's tilts more than Mercury's 7°. A bright stranger on this line deserves a second look; off it, it is no planet.

Run seven hours and watch the line swing over: fixed to the stars, it pivots against your horizon. At any instant half of it is above the ground — and which half decides which planets you can see tonight.

Back at the opening hour. The Sun's year along it, and the seasons that makes, is next.

Planets keep close to the line
The Moon stays within 5° of it
Eclipses come where the two cross

The Ecliptic

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