How To Photograph The Solar System

I’ve been ending all my recent newsletters by reminding you of the various ways you can support this newsletter — by upgrading to a paid subscription, making a one-time donation, or even just sharing it with a friend. But this time I want to start by simply thanking all of you who have already done so. Your contributions keep me going!

Next week, parts of North America will get to see a total eclipse, made possible by the coincidental fact that the moon and sun are almost the same size in the sky when seen from Earth. This gets me thinking again about a question I ponder sometimes: Where in our galaxy would you need to be to to get the best group photograph of the entire solar system, when would you need to be there, and how long a lens would you need?

An eclipse happens even though the moon and sun aren’t the same size. But since the moon is much closer to us, its relative size appears bigger than if it were closer to the sun. You can illustrate this principle yourself by holding your hand in front of your face and moving it back and forth between your eyes and your monitor. The apparent size of your hand gets bigger relative to the monitor as you bring it closer to your face.

Now imagine someone just walked into the room through a doorway behind you and is watching you do this from around 20 feet away. From there, the relative size change of your hand isn’t nearly as apparent. From there, it just looks like you’re being weird. But to them, your hand stays pretty much the same size as you move it around, and remains more accurate compared to the size of the monitor.

Using that principle — that the further you are from objects, the more their relative size appears consistent even when they’re not exactly side by side — we could in theory get far enough away from our solar system that all the planets appear pretty accurate in size relative to each other. We’d just need a really long lens to see them, at just the right time and angle.

Getting the planets to cooperate

Now, one obvious question is how often the planets actually line up like they are in the image above. The answer is, not that often.

By one estimation, if you define alignment as being within 1° from each other, then perfect alignment may occur once in about 13.4 trillion years, which is older than the universe itself, and much longer than it will take for our sun to become a red giant and swallow a few of the planets in the process anyway.

Plus, as astronomer Phil Plait wrote in his book Bad Astronomy:

So they’ll never all be in a straight line. That ruins our photo op, right? Wrong.

We actually don’t want them to all be in a straight line. If they were all in a straight line, and we took the photo in profile, they would be so far away from each other that our picture would mostly be of all the empty space between them with the planets as little dots. They wouldn’t look like they’re next to each other at all.

We could still take advantage of a perfect alignment by shooting from an angle to compress the apparent distance between them. From far enough away with a long enough lens, that would work and give us almost exactly what we want. But the math says it’s not going to happen before the sun explodes, so it’s not an option.

But that’s okay because all we really need are for the planets to be at different points in their orbits so they appear to be lined up from wherever we are.

I must have something around here I can illustrate this with. Oh, I know. I have a bunch of LEGO minifigures. Let me grab a few.

Okay, this overhead view of three minifigs represents three planets at different points in their orbit:

But when viewed straight on, they appear to all be almost lined up in a straight line. If I were even further back with a longer lens, they would look even more aligned.

And there’s another reason why we don’t want them all to be lined up in a straight line: shadows. If all the planets were lined up with the sun on one side, then each planet would cast a shadow on the one next to it. In some cases, the shadow would be bigger than the planet it’s falling on, making it invisible.

Have you ever taken a photo of two people lit by a window, and the person closer to the window casts a shadow on the other person, so you have them take a small step back? Same thing.

So it’s less important that they be in perfect alignment and more important that they all just be on the same side of the sun (so the sun doesn’t end up in the middle of the photo) and at places in their orbit where there’s an angle we can shoot from where they appear close together and in the correct order.

Oh, but also we want them to be at points in their orbit where they are further from us than the sun so that they aren’t back-lit. It would look weird if some are lit from the front and some are lit from behind. But if the only way it can be done means some planets are backlit, I will accept it.

Now all that’s left to figure out is when that will happen, and where we need to be, and what lens our camera needs.

That sounds pretty simple

No, wait. Shoot. We have other problems to solve.

To get all the planets to appear relatively correct in size, we need to be really far away. I don’t know how far, but let’s imagine we need to be ten times further than Pluto’s orbit. At that distance, will there be other space stuff obscuring our image? When do dust, gas, etc. begin to be an issue? I don’t know but it sounds like something to solve for ahead of time. That might even factor into where we can shoot from.

Also, at that distance, the planets are going to be pretty dim. We need to make sure the sensor in our camera is sensitive enough to capture them well. But we also need to consider the fact that we’ll be pointing our camera at the sun, which we want included in the image. So it needs to have some sort of filter we can put over part of the lens to make sure the sun isn’t so bright that we can’t see the planets.

And we need to figure out what size lens to use. Are we talking about a lens the size of the Empire State Building? Or something smaller like the James Webb Space Telescope? We probably first need to figure out how far away we’ll be, and then we can figure out what our angle of view needs to be and what focal length is required. But whether it’s a traditional glass lens, or whatever magic makes the JWST work with all those gold hexagons is beyond me to figure out.

Maybe I have a smart reader who’s a mathematician and astronomer and can do all the work to figure out where we need to be, when, and how we can get there. Like, do we need to launch decades ahead of time to be ready for our close-up? Will we not even have a good photo opportunity for thousands of years? Or does my limited understanding of astronomy and astrophysics mean I’m asking all the wrong questions to begin with?

If you know, the answer, chime in! And if you have a friend who might know, forward them this newsletter! Otherwise, that’s it for another edition and I’ll see you next time. Thanks for reading as always!

David

Correction: An earlier version of this newsletter says that 13.4 trillion years is three times as long as our solar system has been around. That would be accurate if we were talking about billions of years, not trillions. But the whole universe is only about 13.7 billion years old. I’m sure I’ve probably made more mistakes, but at least that one has been caught!

_{Solar system image adapted from illustration by CactiStaccingCrane (CC BY-SA 4.0 DEED)}