Venus, or Earth?

Here’s a question:

Which planet is closer to Jupiter: Earth or Venus?

It’s pretty easy, right? Venus is the second planet from the sun, earth is the third, and Jupiter is the fifth. The answer is clear. Well, let’s have a closer look before we start writing any comically gigantic checks, and throwing confetti everywhere. Balloons are fun, though.

Earlier this week, and over the weekend, I made my way outside while most people were sleeping so I could look at the sky. It was the high point of the gorgeous show Mars, Venus, and Jupiter have been putting on in the mornings lately. This is the sequel to their well-reviewed performance in the earlier part of the year, when Jupiter and Venus slid past each other by less than the width of a finger at the end of an outstretched arm, convincing lots of surprisingly unconcerned people that they were UFOs, and eventually disappeared below the horizon. It was terrific then, and it was well worth getting up early to see this part of it, too.

As I stood outside in the chilly morning air trying to figure out how to work the tiny buttons on my camera, which I usually keep in my pocket and use to take photos of my kids’ birthday parties, I got thinking about what I was really looking at. Conjunctions like these happen from time to time, developing slowly, over weeks and weeks. They’re always quite a sight.

Eventually, I was able to squeeze out this photo. Pretty great, right?

Jupiter-Venus Conjunction - October 2015
Jupiter & Venus in conjunction, October 26, 2015

That’s Jupiter on the left, and Venus on the right. Mars was there, too, but it was too dim to be  picked up by the camera. Mercury was also around, but it was hidden behind the houses across the street and the hillside behind them. It’s an amazing sight when they look to be so close together. This is all an illusion, though; a trick of perspective. They look very close together, but in reality, they’re about 500 million miles apart. That’s pretty far. So, how does this work?

When you look at something in the sky, a planetary conjunction, an eclipse, even any of the constellations, you’re seeing them how these objects look from earth. That might sound obvious but it’s very important. The view I had from the front of my house is different from the view someone standing on the moon or on Saturn or at KIC 8462852 would have had of the same scene.

If we we were able to see the view of this conjunction from overhead, we’d see something interesting. Here’s a drawing I whipped up. To make things a little easier to see, I only included the sun, the earth, Venus, and Jupiter. It’s not to scale, and save your jokes about a color-blind guy making the sun green for the comments.

Overhead View of the Solar System
The solar system seen from above – Oct. 26, 2015.

The line of sight goes from your eye, to Venus, then to Jupiter, passing through the middle of the solar system on the way. The sun was still below the horizon – it hadn’t yet risen – so the line of sight misses it. The reason we see that conjunction is because the planets are in a nearly straight line with us here on earth. Imagine walking through the woods. As you walk, you see nearby trees moving from one side of more distant ones, sliding in front of them, and then passing to the other side. For a moment or two as you walk, the trees are in straight lines with your eye. As we orbit the sun, we see nearby planets move from one side of more distant ones, slide in front of them, and then pass to the other side.

You can see, at the moment, Venus is between the earth and Jupiter. So, it appears in the same part of the sky as Jupiter does. If I were to draw a different version of the diagram, with the planets aligned differently, the earth between Jupiter and Venus, they would appear very far apart in the sky. This would represent the way the planets will be aligned on some other day, as everything goes around their orbits. Maybe one would be up in the daytime, the other in the night. You wouldn’t be able to look across the same 500 million miles of space to see both planets. You’d have to turn around to see them both.

Everything in the solar system, including the solar system itself, but more on that in another post, are moving all the time. So, the alignment of the planets relative to each other is always changing. From one day to the next, everything is in a different place, and it’s these changes that we see playing out in the sky.

I suppose I’ve already tipped my hand with my drawing, but what’s the answer? Which planet is closer to Jupiter: Venus or The Earth? Surprisingly enough, as we’ve learned, the answer is Venus, for now. Over time, this will change, and earth will be closer, but for now, it’s Venus.

As I write this, Venus is 5.3 astronomical units* (about 490 million miles, 792 million km) from Jupiter, while still being only about 68 million miles (109 million km) from the sun. The earth, meanwhile, is 6 AU (about 557 million miles, 897 million km) from Jupiter. That’ll change, though, and hopefully, there’ll be something great to see as it does.

* One astronomical unit (AU) is the average distance from the earth to the sun, about 93 million miles (150 million km).

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