If you’ve ventured outside this year in the early evening, you’ve surely noticed the brilliant “evening star” hanging over the western horizon. It’s not really a star at all, of course–it’s the planet Venus, and it is the most prominent feature in the evening sky.
As children, we all memorized the order of the planets from the sun: Mercury, Venus, the Earth, then Mars. But it probably wasn’t made clear that when we look at those other planets as seen from Earth, their order dictated when and where we could look to find them. Innermost Mercury’s orbit hugs the sun and can never stray more than 28 degrees away from it. From our vantage point, it is almost always hidden in the glare of the sun, and people can go their whole lives without ever glimpsing Mercury.
The situation for Venus is similar, but with a considerably bigger orbit: As seen from Earth, it can appear up to 47 degrees in angle away from the direction of Old Sol. That’s sufficient for it to clear the morning or evening twilight, where the planet’s reflective clouds make it dazzlingly obvious.
It is possible for the outer planets to appear opposite the sun in the sky and be visible to us at midnight, something not physically possible with the two innermost planets. Thus the terms “evening star” and “morning star” are reserved for these two–and are most often used in reference to Venus because it’s the only one we tend to notice.
To get a feel for the geometry of the inner solar system, imagine standing outside at noon looking at the Sun to your south. Your gaze will include the planet Venus, even though you can’t see it in broad daylight. Venus will lie somewhere to the east or west of the sun, along the imaginary line of the ecliptic (see “Seeing the Light“), and no more than halfway to the eastern or western horizon. If Venus lies to the west, then as time passes it will be first to set, and you’ll not see it in the evening. By waiting twelve hours, however, Venus will emerge before the sun in the morning, and we’ll dub it the “morning star.” If, on the other hand, Venus lies to the east of the sun (as it does now), Old Sol will set first–and as twilight fades, the planet will eventually become visible and dominate the western sky before it itself sets: our “evening star.”
If we could look down on the solar system from above, we would see the planets racing around the sun as if on a wide circular racetrack, all moving counterclockwise just like they do at a horse track. The sun is located at the very center of the track, and we’re riding Planet Earth along with all the other participants. Seven other horses are in this race (Pluto was recently disqualified), but let’s concentrate our thoughts on just one competitor: the beautiful filly named Venus.
Our race around the sun against Venus is rigged, and we have no chance of winning. For one thing, Venus gets to race along the inside rail of the track. We, by the rules set by the celestial mechanics racing commission, are restricted to hugging the outside rail. So even if all other things were equal, Venus would complete a lap around the sun faster than us, due to its advantage of having a shorter distance to travel.
But we’re doubly handicapped by the law of gravitation, which demands that those on the inside track must always run faster. So we have farther to go on a slower pony: Venus will always beat us around the sun. And the race never ends. We circle the track endlessly, requiring about 365 days to complete one lap, while Venus, with its dual advantage, completes a trip in only 225 days.
The graphic shows several recent snapshots of the race. Last August, Venus could be found directly opposite us on the far side of the track, indicated as position 1. We couldn’t see our competition then because it was hidden behind the glare of the Sun. However, in a couple of months Venus became visible to us as it rounded the far bend and emerged from behind the Sun (position 2). At about the start of this year, Venus became well clear of the glare and was only a quarter of a track behind us (3). And a short while ago, on March 24, it was right on our heels, and we had to crane our necks as far back behind us as possible to see its approach (location 4). Coming up in late May, we’ll watch Venus almost overtake us (5) as it seems to move more and more into the bright infield sunlight. And finally, on June 3, Venus will pass the Earth (6) and regain the lead, although we’ll no longer be able to see her do so because of the sun in our eyes.
Of course, the drawing simplifies the situation in that it makes it look like the Earth is standing still the whole time, when in fact we’ve galloped over three quarters of the way around the track. That doesn’t change the apparent relationship of Venus coming up on our flank with respect to the sun, however–it just changes the background scenery (the stars) against which it all happens.
It should be apparent that as Venus overtakes us, it appears to grow in size as its distance between us dramatically decreases. The biggest puzzle, then, is why Venus appears to stay almost the same brightness to the naked eye from start to finish. By a geometric quirk of fate, its increase in apparent diameter is almost exactly compensated by its ever-thinning illuminated profile, so that the overall apparent “surface area” of Venus stays almost constant from Earth’s perspective. Without a telescope to discern its changing size and shape, it all looks pretty much like the same unresolved dazzling blob to us.
Galileo was the first to observe the changes in the phases of Venus in 1610, although he didn’t publish his sketches until “The Assayer” in 1623. We may think it odd, but it wasn’t the thin crescent of Venus that excited him; that was to be expected if Venus always occupied the space between the Earth and the sun as required by the Aristotelian Earth-centered universe. It was the sighting of Venus as a distant, small round ball that proved that Venus passed on the other side of the sun from us–and therefore showed that the sun, not the Earth, was the center of it all.