Venus: the Morning Star, the Evening Star & the Pentagram

Earth’s inner neighbor

Venus circles the Sun once every 224.7 days at a distance of about 0.72 AU, roughly 72 percent of the Earth’s distance from the Sun. Its orbit therefore sits entirely inside Earth’s larger orbit. (Venus is not somehow inside the planet Earth. Both worlds orbit the Sun, and Venus simply keeps to the smaller, inner track.) Because Venus is always nearer the Sun than we are, the Sun and Venus can never sit on opposite sides of our sky. A planet farther out, such as Mars or Jupiter, can ride high overhead at midnight, directly opposite the Sun, but Venus never can. From our moving viewpoint on Earth it stays tethered close to the Sun, swinging out to one side of it and then back to the other.

Morning star and evening star

When Venus sits to the west of the Sun, it rises a little before the Sun does, so we catch it low in the east in the hour before dawn. That is the morning star. When Venus sits to the east of the Sun, it sets a little after the Sun, so it shines in the west in the hour after sunset. That is the evening star. We never see both at once, because Venus can only ever be on one side of the Sun at a time. The ancient Greeks did not realize this at first. They named the dawn apparition Phosphorus, the light-bringer, and the dusk apparition Hesperus, the evening one, and understood only later that the two were a single world.

How far from the Sun can Venus stray? At most about 47°, a separation astronomers call its greatest elongation. (It varies between roughly 45.4° and 47.1° as the two orbits carry the planets nearer and farther apart.) There is a simple reason for the limit. Picture the line of sight from Earth that just grazes the edge of Venus’s orbit. The angle that line makes with the direction of the Sun depends only on the size of Venus’s orbit, and it works out to the arcsine of 0.72, or about 46°. A planet on an even smaller orbit is held even tighter to the Sun, which is exactly why Mercury, the innermost planet, is harder to glimpse than Venus.

The eight-year rhythm and the pentagram

Because Venus is closer to the Sun, it moves faster than Earth and repeatedly laps us, the way a runner on an inner lane overtakes one on an outer lane. The time from one lap to the next, measured by when the Sun, Earth, and Venus return to the same alignment, is the synodic period. For Venus it is 583.92 days, a little over a year and seven months.

Now for the remarkable part. Five of those synodic periods add up to 2,919.6 days, which is almost exactly eight Earth years (2,922 days), and almost exactly thirteen Venus years as well. Because these three separate clocks so nearly agree, the entire Sun, Earth, and Venus arrangement very nearly repeats every eight years.

The clearest fingerprint of that rhythm is where Venus passes closest to Earth. Each close pass happens at an inferior conjunction, the moment Venus slips directly between Earth and the Sun. Five of them occur in every eight-year span, and they fall about 72° apart around the sky. Seventy-two degrees is exactly the spacing of the five points of a star, so joining the conjunctions in order draws a pentagram. If instead you follow Venus’s position relative to Earth without lifting the pen, as the right-hand view does, those five loops open into the famous rose of Venus.

The agreement is close but not exact. With each eight-year cycle the pattern slips by about 2.4°, so the rose turns slowly, taking roughly 1,200 years to come all the way around. You can reproduce the 584-day synodic period yourself on the Synodic-Period Calculator, and find it listed among the cycles by length.

The phases of Venus

Venus shows phases, just as the Moon does, and for the same reason. As the angle between the Sun, Venus, and Earth changes, we see different fractions of Venus’s sunlit half. When Venus is on the far side of the Sun, near superior conjunction, almost its whole lit face is turned toward us, so it looks nearly full, yet it is also at its most distant and appears small. When it swings around to pass between us and the Sun, near inferior conjunction, its lit half faces mostly away from us and Venus thins to a crescent, though it is now closest to us and looks large. At greatest elongation, halfway between, we see it exactly half lit.

This changing shape carried real historical weight. When Galileo pointed his telescope at Venus in 1610 and watched it run through a full set of phases, including a plainly gibbous, nearly full Venus, he had direct evidence that Venus must travel around the Sun rather than around the Earth. That single observation helped overturn the old Earth-centered picture of the heavens, and it became one of the cornerstones of the Sun-centered model we hold today.