Venus, if you Will!

It’s mid February, Whabbloggers, and very soon, the evening sky is going to be graced with a rather stunning addition. Yes, our closest planetary neighbor Venus, known since antiquity as the Goddess of Love, is about to assume her position as the dominant “Evening Star”, brightest of all celestial objects save the Moon.

Where has Venus been lately? And why isn’t she the “Evening Star” all the time? To begin to answer these questions, suppose, about a month ago, you headed out to your back yard (or equivalent) at our preferred visualization time, right around sunset. You had taken pains to pick a clear day, of course, so the Sun was clearly visible along the western horizon.  The rest of the sky was still bright blue, with no stars visible just yet. Now, where was Venus? Even though invisible, you could have easily visualized her location by imagining a straight line joining you to the Sun and extending out behind it. Venus was almost directly on that line, some 67 million miles past (beyond) the Sun!

Of course, we know from the earlier Sun blog that the Sun is a stationary object, so there’s nothing special about having gone out near sunset as far as imagining the location of Venus was concerned; you could have just as easily imagined the line connecting the Earth, the Sun, and Venus when the Sun was halfway across the sky at noon, or just clearing the eastern horizon at sunrise, or even when the Sun was below your feet at night. To make this point more concretely, take a look at Figure 1 below, which depicts the green/blue Earth, the yellow Sun, and four half-black circles. These circles represent the position of Venus at four points in her orbit around the Sun. Last month, at the time you made your hypothetical trip to your back yard, Venus was in the position corresponding to the circle on the far right of the figure, on the other side of the Sun from the Earth.

In the Figure, you’re looking straight down on the Solar System from directly above the Sun. Another very useful perspective from which to visualize the relative positions of the Sun, Venus, and the Earth is from a vantage point just behind and slightly above Earth, as in Figure 2 below. A month ago, Venus was in the position labeled “Superior Conjunction”. The Sun occupies the middle position, so the side of Venus that was turned toward the Earth was fully illuminated. We couldn’t see it, though, due to a combination of two factors. First, Venus was fully 160 million miles away, so its disk was quite small. Second, and more important, since Venus rose and set at almost exactly the same time as the Sun, it was never in the sky at night, and during the day, it was completely washed out by the Sun’s glare.

It is interesting to compare the source of Venus’ invisibility last month to the invisibility of the New Moon. As we found out in the lunar blogs, the Moon is located between the Sun and the Earth when it is New, so the side facing us is completely dark. Venus is invisible for entirely different reasons.

Like every other body in the solar system, Venus is never stationary. Day by day, our sister planet travels in the standard leftward (counterclockwise) direction all inner solar system objects travel as they orbit the Sun. The closer the object to the Sun, the faster it moves, and Venus actually whips along at over 75,000 miles per hour. You read that right: Venus covers 21 miles every second! You can see from Figure 2 that over the past month, this movement has been taking Venus gradually away from, and to the left of, the sun.

As Venus moves leftward, the distance she is putting between herself and the Sun (as viewed from our perspective here on Earth) is about to pay dividends for sky watchers. Fast forward in your visualization exercise to about three weeks from now, towards the middle of March. Take some time to settle down in your back yard just after sunset, maybe with a cocktail to keep you company. Sipping slowly, you enjoy the gathering dusk while waiting for the first star to appear. Presto: I guarantee that a bright object will pop into view over near the western horizon, and you will be the first on your block to welcome the Evening Star!

Every day thereafter, Venus will slip a little further to the left along its orbital path, taking her ever further away from the Sun. As she climbs higher in the sky, she will hang around for a longer period after sunset before descending down to the western horizon and disappearing from view (this rapid movement of Venus towards the western horizon is entirely due to the Earth’s leftward rotation; it has nothing to do with Venus’s own movement at all).

Let’s continue to follow Venus around in her orbit. As she travels ever further “up, up and away” from the Sun, eventually she reaches the half-way point, equivalent to the Moon’s “half full” position, the point of maximum displacement from the Sun (again, that statement pertains only to our perspective; from her own perspective, Venus stays remarkably close to 67 million miles from the Sun throughout her orbit). The period of maximum displacement will happen over this summer. The black and white ball at the top of Figure 1, and the extreme left of Figure 2, illustrates what position she will take.   By then, Venus has swung far enough from the Sun that she is quite a ways from the Western horizon at sunset, and remains visible for several hours after sunset.

Let’s pause for just a moment, consider Venus at this point in her orbit, and visualize her behavior in our sky over a 24-hour period beginning at sunset (i.e., through a full rotation of the Earth). Once it has disappeared, the Sun commences following the standard path below your feet (the path we visualized in the Sun blog) due to the Earth’s eastward rotation. Meanwhile, that same earthly rotation is driving Venus ever rightward, down to and eventually below the western horizon. By the time the Sun pops back into view along the eastern horizon in the morning, where is Venus? Still below your feet, of course, tracing out pretty much the same path as the Sun, just several hours behind. Since Venus doesn’t rise for several hours after the Sun, the event goes completely unnoticed because the planet rises into full daylight.

When the Moon swings into view at the half full point in it’s cycle, our satellite is clearly visible in the daytime sky.  Believe it or not, Venus is, too, if you know exactly where to look. The best opportunity comes when the half moon and Venus rise at almost exactly the same time, very close together, and then move in unison as the Earth’s rotation carries them up and across the daytime sky. I’ll be sure to point this opportunity out when it happens this summer. If you can spot Venus, you’ll be one of the few human beings on the planet who’ve ever seen it in broad daylight!

Now, let’s return to the main event, the changes in Venus’ appearance that accompany its orbit around the Sun. You can see from the highly informative Figure 2 that, at the time of greatest elongation, Venus is actually “rounding the curve” and about to start swinging back to the right along our line of sight: A path that will bring it back into alignment with the Sun. Venus is still drawing closer to Earth during this time, so it continues to grow larger. However, as late summer gives way to autumn, each night the distance between our brilliant Evening Star and the Sun will shrink steadily, and the interval between when the Sun sets and Venus sets will shrink as well.

By Halloween, Venus’s orbital motion will have brought it right back into alignment with the Earth and the Sun, but between the two, rather than on the far side of the Sun as it is now.   Once again, rising and setting in tandem with the sun, Venus will fade into invisibility in our night sky. This time, her invisibility is caused both by the fact that Venus and the Sun will rise and set together, and also because our point of view is now aligned with the dark side.   In that sense, the Halloween “New Venus” and the New Moon of November 6th will have a lot in common!

And what then? Once again, Figure 2 tells the story: Venus’s incessant movement will cause it to move past the sun and off to the Sun’s right. Remember from our Moon blogs what happens to celestial objects that are located to the right of the Sun? When the Sun sets, objects to the right are already below the western horizon, and have a head start when it comes to being whisked around by the Earth’s rotation toward their appointment with the Eastern horizon. So it is that Venus will start to rise before the Sun, and take her appointed place as the blazing Morning Star. Over the course of next winter, the planet will swing further and further to the right, giving her more and more of a head start on the Sun. Consequently, Venus will rise earlier and earlier than the Sun, and stay visible for a longer period of time before dawn arrives and she fades into the morning twilight.

To really imagine this Morning Star behavior, let’s go back to Figure 1 and its “God’s Eye perspective” above the Sun. Pretend you are a specific location along the Earth’s equator (along the very edge of the disk), and the entire green ball representing the Earth is spinning in a counterclockwise direction. Venus is at the bottom of the figure. Now imagine spinning throughout the night. As you pass midnight, which is when you are exactly opposite the Sun on the Earth’s far side, you can see from the geometry in the Figure that Venus is soon going to swing into your view along the Eastern horizon. The Earth will have to complete almost another quarter turn before the Sun does its own appearance act, also to the East.

And that’s pretty much it! When Venus is located to the right of the Sun (from our perspective) she forms the Morning Star. When she is located to the left of the Sun, she forms the Evening Star. Over the course of this year, I encourage you to follow the progress of Venus, starting about three weeks from now along the western horizon. Check out how high she gets in the sky over the summer. In the autumn, track her movement back in the direction of the Sun, before she disappears completely. And then, next winter, if you find yourself with insomnia some night, or happen to get up unusually early, check out the Morning Star!

Figure 2 illustrates one more notable change in Venus brought on by her orbital motion, although you need a telescope or good pair of binoculars to see it. Right now, in the middle of February, the side of Venus that faces Earth is almost fully illuminated. As the planet swings in our direction, the proportion of the lit side that is visible from our vantage point shrinks steadily. When Venus is at her maximum elongation from the Sun, she is exactly “half full”, just like the Moon. Then, as Venus swings back into alignment with the Sun, late next summer and into fall, the proportion of her disk that is illuminated from our vantage point shrinks to a thin crescent (again, these phases are just like the Moon). You will see these phases of Venus clearly if you ever examine the planet in a good pair of binoculars or a small telescope.

Before leaving Venus completely, I’d like to take this opportunity to talk about the place for just a little bit. Virtually a twin of the Earth in terms of size, Venus has always been my favorite planet. I don’t really know why. She certainly isn’t more likely to harbor life than Mars; the surface temperature is about 800 degrees Fahrenheit, 300 degrees hotter than the highest setting on your oven. That temperature is pretty constant across the entire surface, too, despite Earth-like variations in latitude and altitude, and despite the fact that nights on Venus are longer than 100 Earth nights. The long nights are because Venus spins on its axis very slowly, a lot more slowly than the Moon even (you do remember from earlier blogs how slowly the Moon spins, right)? As with many other features, the slow rotation rate of Venus is still a mystery. As for how Venus maintains such a constant temperature, the answer is that Venus’s atmosphere is 90 times as dense as ours. The air pressure at the surface is equivalent to the pressure 3000 feet down in our oceans! Just as there isn’t a lot of variation in water temperatures at that depth, so too is there little temperature variation on Venus.

There is a great deal more that is fascinating about our sister planet, such as the current thinking about why she has such a thick atmosphere, but I will save that discussion for another blog.

Until next time, then, Whabbloggers… when I think I’ll tackle a little astrology!