Observatory · the wanderers
Morning star, evening star
The brightest "star" in the evening sky is often not a star. It is Venus, and the Greeks thought it was two: Hesperus in the evening and Phosphorus in the morning, until they worked out that it was the same light, never far from the Sun. The stars keep their places; five lights did not, and wandered slowly through the constellations, sometimes even turning back. The Greeks called them planētai, wanderers, and that is where the word planet comes from. In Dutch they were dwaalsterren.
Morning star, evening star
Fig. I. The inner planets from above, and the twilight sky over Eindhoven, west and east.
- Venus
- Mercury
On the left the inner solar system from above, worked out from NASA JPL's orbital elements for every day. The lines from the Earth show where we have to look: at the Sun, and at Venus and Mercury. However they turn, the angle between them stays small, because both orbits lie inside ours: Venus gets at most 47 degrees from the Sun, Mercury 28. When Venus runs ahead of the Sun, east of it, it is still up after sunset: the evening star. When it lags behind, west of it, it rises before the Sun: the morning star. In between it passes in front of or behind the Sun and is lost in the glare for weeks.
On the right is what that looks like from Eindhoven, the same spot as the orrery's "tonight", in the dusk half an hour or so after sunset and before sunrise, when the Sun is six degrees below the horizon, with the planets drawn bigger for the brighter ones. Mercury is the hard one: it never climbs far out of the twilight and is only easy to catch on a few evenings or mornings a year. Mars, Jupiter and Saturn show up in these windows too when they happen to be there, but they are not on a leash: they can stand anywhere, even opposite the Sun at midnight.
Wandering through the stars
Fig. II. A planet's loop against the stars of the zodiac, and the Earth overtaking it.
- Date
- Moving
- In
Night by night a planet creeps eastwards through the stars, left on this strip, as you see it facing south. But for a few weeks around the time it stands opposite the Sun it stops, goes backwards, stops again and carries on: a loop, or a zigzag. For two thousand years astronomers explained that with wheels on wheels, the epicycles of Ptolemy. In 1543 Copernicus gave the simple answer you see in the bottom view: the Earth is on the inside track and overtakes the slower outer planet, and while we pass it, it seems to slide back against the far-off stars, the way a slower car seems to go backwards when you overtake it on the motorway.
Coming together
When two wanderers pass each other in the sky they can stand so close that they look like one double star: a conjunction. They are not near each other at all, just in the same direction. These are the close ones in the next few years, worked out in your browser, when both are bright and far enough from the Sun to see. Click one to set the sky above to that day.
Who worked it out
Where it already matters
Under the dome. I first watched the planets walk their paths across the sky in the planetarium of the Nagoya City Science Museum, one of the biggest in the world, its dome 35 metres across, with an astronomer presenting live, in Japanese. I followed it fine; half the audience slept through it.
Two diamonds in the desert. Two weeks later, in June 2026, Venus and Jupiter came within less than two degrees of each other in the evening sky. A few evenings after the closest pass I saw them from the desert outside Abu Dhabi, one above the other, and knew what they were thanks to Nagoya.
Venus has phases. In 1610 Galileo turned his telescope on Venus and saw it wax and wane like the Moon: a small full disc when it was far behind the Sun, a large thin crescent when it was close to us. That only works if Venus goes round the Sun, and it was one of the strongest arguments for Copernicus.
Crossing the Sun. Now and then Venus or Mercury passes exactly in front of the Sun, a small black dot crossing the disc. Venus did it in 2004 and 2012 and will not do it again until December 2117. Mercury is next on 13 November 2032.