The Grand-Cycle Timeline

What you are seeing

Most of the site's tools work in days and years. This one pulls back to the scale of centuries and millennia, where the slowest wheels of the sky become visible. Each row is one cycle, drawn on the same timeline so you can compare their speeds at a glance. When a cycle beats faster than the screen can resolve, its row turns into a soft band labeled with the period; zoom in and the individual beats separate out.

Precession, the Great Year

Earth's axis swings in a slow circle once every 25,920 years, so the star that marks the north pole keeps changing. The top band names the pole star of each era: Thuban in the age of the pyramids, Polaris today, and Vega far in the future. Zoom all the way out, or press One Great Year, to take in a full turn. This is the same wobble explained on the precession page.

The planetary conjunctions

Jupiter and Saturn meet about every 20 years in the great conjunction, the slow drumbeat that medieval astronomers tracked for centuries. Uranus and Neptune, the two outermost planets, meet only about every 171 years, the rarest pairing of the major planets. Zoom in to a few centuries and you can count the individual meetings; zoom out and they blur into a steady rhythm. See the dates on the conjunctions pages and the alignment calculator.

The Saros, river of eclipses

A single Saros series produces a near-twin eclipse about every 18 years, and dozens of series run at once, which is why eclipses come a few times a year. The Saros row shows one series' beat. Follow it forward on the Eclipse & Saros Explorer, which walks an actual series eclipse by eclipse.

How to read it

Drag the timeline left or right to travel through time. Use Zoom in and Zoom out, the scroll wheel, or the preset buttons to change the span, from a couple of centuries out to a full Great Year. The green dashed line marks the present. The cycles are drawn from their known mean periods, so the spacing is exact; the pole-star eras and history markers are placed at their accepted dates.

Build your own timeline

Below the timeline you can design your own view. Pick a single planet to add its orbital period, the time it takes to go once around the Sun, or pick a second planet to add their synodic cycle, the average time between one meeting and the next as seen from Earth. Each new row drops into place sorted by length, longest at the top. Remove any row with its ×, or press Reset cycles to return to the core set. Added rows are spaced from a reference near the year 2000, so their rhythm is exact even though the starting phase is illustrative.

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