Parallax view: Moon occults Mars

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Look up! There’s stuff to see in the sky!

On December 7/8 (Wednesday evening/Thursday morning depending on your location; see below)*, you have a chance to see a very special and extremely cool event: a lunar occultation of Mars.

This is when the Moon passes directly in front of Mars, blocking it from view. It’s not a rare event in general, in that it happens once or twice per year somewhere on Earth. But it is rare at any specific location, so it’s like a solar eclipse in that way. I’ve seen lunar occultations of many stars, and even Venus, and they’re fun to watch. However I’ve never seen one of Mars so I’m really looking forward to this.

I wrote about it for Scientific American, so you can get the background there. In fact, please click that link! The more traffic I get there the more likely the folks at the magazine will keep me around. J

I had to cut some stuff out of my article for length, though, and I hate to do that when it’s fun info. But hey, I have a newsletter! So I can talk about it here.

For example, in the SciAm article I linked to a page that has info on when the occultation is visible from various sites, and cautioned folks that the exact timing depends on your location. It turns out, too, that whether you can even see it at all depends on your location, so let’s dive into that, shall we?

The Moon orbits the Earth every 27 days, or roughly once per month. If some event happens on the Moon — say, a lunar eclipse or an asteroid impact — you’d be able to see it as long as the Moon is up in the sky, because the event is happening there, on the Moon itself. But some things, like solar eclipses, happen on Earth, so if you’re in Boulder Colorado and the Moon’s shadow during the eclipse falls in Kyiv, you’re out of luck. You’re just in the wrong spot.

In the case of an occultation, the first requirement to see it is that the Moon is up in your sky. So that limits the event to roughly half the Earth. For this occultation that favors the western hemisphere.

However, it’s even more limited than that. While all of Canada will be able see it (weather permitting), and most of the US, it can only be seen some parts of Mexico, Africa, and Europe, and not at all in South America. Why is that?

Parallax.

Oh, you know parallax. It’s the apparent change in the position of an object if your viewing angle to it changes. I gave a brief description of it in the SciAm article:

That’s the overview, but how does this work in detail?

The Moon is about 3,500 kilometers in diameter and 380,000 km away. That means its apparent size in the sky is about half a degree, where a degree is the same unit you use to measure angle, where there are 360° in a circle. We use the same units on the sky: There are 360° around the horizon, and 90° from the horizon to the zenith, the point directly over your head. The Moon is actually pretty small on that scale; you could fit 180 Moons stacked from horizon to zenith.

The Earth is much bigger than the Moon, about 13,000 km wide, so about four times larger. Given the Moon’s distance, if you move around on Earth you’ll see it change its position in the sky due to parallax by quite a bit. Someone on the North Pole would see the Moon shifted in the sky by about 2° relative to someone looking at it from the South Pole!

That’s four times more than the Moon’s 0.5° size. Changing your position on Earth can change the Moon’s apparent position by more than its diameter!

That’s why this occultation is limited to a range of latitudes on Earth. If you’re too far north or south then the Moon will appear to miss Mars, and the planet remains visible. I drew this diagram to help visualize it:

The Earth is on the left, the Moon in the middle, and Mars on the right. Note this is not to scale! It’s exaggerated to make the effect more obvious.

In the diagram, the little xkcd figure standing near the Earth’s equator will see the Moon blocking Mars (short dashed line); in fact anyone standing along the thicker arc on the Earth will see the occultation. The solid lines connecting the Earth’s surface to the Moon and past it to Mars show the extremes of latitude the event can be seen.

But the person on the North Pole has an unobstructed view of Mars. They see the Moon pass well south of the planet, so no occultation for them.

It’s a little more complicated than this IRL, though. The Moon doesn’t orbit over Earth’s equator for one, but instead the orbit is tipped so the occultation range isn’t usually centered on the equator as I drew it. Also, the Earth is curved and rotating, so the upper and lower latitude bounds of visibility are curved (you can see that in the visibility map above). Also, for some people the occultation happens as the Moon is rising or setting, complicating matters further.

Even worse: There’s also east/west parallax, so two observers in the same time zone see the event at different times! Yikes. I’ll note that the east/west limits of the visibility zone are not limited by parallax, since the Moon moves slowly in a westerly direction as it orbits the Earth. Everyone at a given latitude would at some point see an occultation… if the Earth weren’t opaque. Unfortunate. But the Moon travels enough in its orbit that it eventually moves far enough away from Mars that more occultation is possible.

Still, there are more such events to come. I’ll note that the Moon occults Uranus today (Monday, December 5) for folks in the US, but it happens in the daytime so it’s not possible (or at least seriously difficult) to observe. The International Occultation Timing Association website will have the listings for 2023 up soon, so check them out to see what else the Moon will hide in the coming year. Occultations are very cool to see, whether by eye or through binoculars or a telescope, so I hope you get to see this one!

* CORRECTION: In the original version of this I said the occultation was Tuesday night/Wednesday morning, which is wrong. My apologies for the confusion. I do suggest you go out every night anyway to take a look; you can see the Moon getting closer to Mars between now and the event, which is also cool and will help build anticipation. :)

Et alia

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