Unearthing Mars: Helping Students Understand the Thinking Behind the Lab

Let me tell you a secret: I kind of like the Unearthing Mars lab.

But I also think it has so much more to offer than getting the points plotted and answering the questions. There is a pretty remarkable scientific puzzle hiding underneath all those angles and lines: How do you figure out another planet’s orbit while standing on a planet that is also moving?

As I worked through it, I kept coming back to a question of my own. We’re given Earth’s position so we can start locating Mars. Okay—but how did astronomers know where Earth was in its orbit in the first place?

Nobody had popped outside the solar system to take a picture. There was no convenient “You Are Here” arrow. So I did some research. And this is the part I wanted to share with my students.

How Did Astronomers Know Where Earth Was?

The first piece is something we already teach: the sky follows patterns. As Earth travels around the Sun, the Sun appears in different directions against the background stars, tracing an annual path called the ecliptic. We cannot normally see the stars beside the Sun in daylight, but astronomers could connect daytime measurements of the Sun with nighttime observations of the stars to map that path.

Astronomers also needed a shared starting point for describing positions. The March equinox provided an observable reference: around the equinox, the Sun rises due east and sets due west for most locations on Earth. That recognizable pattern helps connect an abstract orbital diagram to something we can actually observe from the ground. The March equinox direction became the 0° reference for measuring celestial longitude—essentially, the starting mark on an enormous protractor.

With a reference direction and careful observations of the Sun’s annual motion, astronomers could calculate Earth’s angular position around the Sun. The numbers in an orbital-position table came from observations and mathematical models. They were not information someone could simply read off an overhead picture of the solar system.

And the coolest part? My students already had a connection to this. We had spent time looking at how the constellations visible at night change throughout the year as we investigated evidence that Earth revolves around the Sun. So when we got to recognizing those patterns as clues to our position in orbit, they had something to build on.

I would love to tell you I thoughtfully planned that connection from the very beginning. I did not. This was a happy accident—and one of those moments when something we had already explored became useful in a new way. Those constellations were helping us tackle another question: Where are we in our orbit when we’re making these observations of Mars?

Recognizing a seasonal constellation is not the same as measuring a precise orbital position, of course. But it gave my students a way into the idea. Our changing view of the sky could help us make sense of where we were.

If your students need a starting point for picturing Earth’s motion, try this day-and-night modeling activity.

Wait—Wouldn’t Mars Be Moving, Too?

Yes! And that is exactly the question worth stopping for.

If we observe Mars from one place and then move to another place, we cannot simply assume Mars stayed put while we were traveling. Two sight lines aimed at two different locations of Mars would not locate one shared position.

The clever solution was to compare observations about 687 days apart—one Martian year. During those 687 days, Mars completes one trip around the Sun and returns to approximately where it started. Earth keeps moving too, but it takes a different amount of time to complete an orbit—so when Mars returns, we’re looking at it from a different location. That gives us two different viewpoints of approximately the same Mars location. Repeating that reasoning with other pairs of observations helps reconstruct its orbit.

And there was our constellation connection again! The background stars gave astronomers reference points for tracking Mars’s motion. We had already explored how our view of the stars changes as Earth revolves, so this gave us another reason to revisit those familiar sky patterns.

That changes what the lines on the page mean. They represent carefully chosen observations, separated by time, made from a moving planet.

Making Sense of the Vocabulary Before the Lab

Before the lab, I also spent time using my science centered (that phrase will make sense in a moment!) Frayer model to distinguish heliocentric longitude from geocentric longitude. I wanted students to connect each term to what they were actually measuring: Where is the measurement centered? What position or direction does it describe?

There’s a fun word connection here, too. Helio- comes from the Greek word for Sun, helios—also the name of Helios, the Greek Sun god. Geo- means Earth. So heliocentric means Sun-centered, and geocentric means Earth-centered. Those word parts give us a useful clue about where to center our measuring tool.

During the lab, I started saying “our heliocentric measurement” and “our geocentric measurement” instead of repeating “longitude” every time. We had introduced the formal vocabulary; now I wanted the language to help students focus on what they were doing. Here is our Sun-centered measurement locating Earth around the Sun. Here is our Earth-centered measurement showing the direction toward Mars.

“Measurement” was classroom shorthand for the angular measurements we were making. The important question remained: Are we measuring from the Sun or from Earth—and what does that measurement tell us?

So I Brought Mars Onto the Classroom Floor

I wanted my students to be able to visualize this before they were managing all the plotting directions in the state lab. So I created a classroom triangulation demonstration.

A small object represents Mars. Two marked locations represent different positions of Earth, and a student takes on the role of the astronomer. From the first location, the astronomer measures the direction toward Mars using a 360° measuring tool. Everyone transfers that observation onto a paper model.

Before adding the second observation, I want to pause on what we actually know: “We know Mars is in this direction. Do we know how far along that line it is?”

This is the thinking I want to make visible. A line of sight gives us a direction, but that one measurement does not tell us where along the line Mars belongs. A string stretched from the observer toward—and beyond—the object helps show that.

Then we move our astronomer to the second Earth position and measure again. On the paper model, students draw the second sight line and locate the intersection. The second observation supplies information the first one could not.

The classroom object stays still to represent Mars returning to the same orbital position at the two selected observation times. It is a simplification worth naming out loud: we are modeling the geometry of those observations, not suggesting that Mars waited patiently on the floor for almost two years. Very cooperative of classroom Mars, though.

The Question I Want Students to Answer

For me, the important check is not just whether students found the intersection. It is whether they can explain why that intersection means something.

Try asking: “What did the second observation tell us that the first one couldn’t?” Then have students point to their model as they explain. Which point represents the observer? What does each line show? Why does the place where they cross help locate Mars?

This is also a chance to return to the vocabulary. Can students explain why they centered their measuring tool on the Sun for one measurement and Earth for another? Can they connect those choices to what they were trying to find?

And finding one location of Mars is not the same as finding its whole orbit. That next problem requires additional observations—the reason to keep investigating.

Giving the State Lab More Meaning

This is why I made my Unearthing Mars pre-lab. The classroom demonstration, paper models, background readings, work with ellipses and eccentricity, and Kepler’s laws reference give students something to connect the state-lab procedures to.

If you want the materials to bring this into your classroom, the pre-lab pulls that work together before students begin the official NYS investigation. It gives us time to explore what the measurements mean and why the procedure works.

And again - I still kind of like the lab. I just want students to have access to the interesting thinking inside it. I want students to understand how those observations help us figure out where Mars is—because this lab has so much more to offer than a round of celestial connect-the-dots.