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Measuring Time with the Sky
Every clock, calendar, and time zone we use today is rooted in the sky. Long before mechanical clocks or GPS, people watched the rising and setting of the Sun, the phases of the Moon, and the shifting stars to keep track of time and navigate the world.
During this lesson, we’ll uncover how the motions of celestial objects define our days, months, and years—and how modern astronomers still rely on these ancient rhythms. As you read, think about this: What can the sky tell us about where we are, and when we are?
Watch the Science Lesson
RECORDING WILL BE POSTED 10/01/2025
Do the Assignments
Homework
Each week, you'll be assigned a set of chapter review questions, quantitative problems, and quiz questions. Begin with the review questions to make sure you're understanding the material presented in class. Then move into the homework problems, skipping any that are challenging or you’re not sure how to start (saving them for after you’ve worked on other problems first.) Continue working on these problems each day until you’ve completed the set.
Space Podcast
Each week, students choose a space-related podcast episode to listen to and write a short summary in their journal. These reflections help students stay connected to current science news while developing their ability to explain scientific concepts in their own words.
Lab Project Activity
This week, you have two lab projects! The first is our regular weekly lab project, and the second goes much more in-depth (called the Discovery Project). If you don't have time or inclination to do both labs, choose the one you'd like to do this week.
Below you'll find several short videos about telling time using the stars, and even create a scientific instrument to measure the location of stars! Download your lab project, and use the step-by-step instructional videos to walk you through every part of the assignment.
Discovery Project: Celestial Sphere
Astronomy is built on a foundation of key observations—not just of objects, but of celestial events and phenomena. The great astronomers of the past didn’t simply observe the sky; they measured, recorded, and carefully analyzed what they saw. Our modern understanding of the universe rests on the work they left behind. The remarkable part is that we can still replicate many of their foundational observations ourselves. These projects take effort and often involve math, but they’re worth it.
Why? Because truly learning a subject means more than just memorizing facts—it means understanding how those facts were discovered and why they matter. Just as biology students dissect frogs and chemistry students run experiments, astronomy students must engage with the sky directly. It's through doing that real understanding begins.
Nightly Motion of the Stars
In this project, you’ll measure star positions using two different coordinate systems. We’ll need this in future labs when we need to know precisely where stars are at specific times in order to do further investigative work!
Materials:
- A simple theodolite (DIY from a previous lab or 3D-printed)
- Tripod or stable mounting surface
- Compass (or compass app on your phone with true north feature)
- Index cards (2) or a sheet of paper cut in half
- Pencil and eraser
- Protractor, string, and a washer to build a clinometer (if your theodolite lacks built-in scales)
- Stellarium (or similar sky simulation software)
- Star chart
- Access to a clear night sky
- Flashlight (with red filter) to read your scales in the dark
Supplementary Videos
The videos below are additional explanations of topics discussed in class. This week, we've included a couple of (entirely optional) hands-on activities in case you're interested in doing more of these types of projects. These use very simple materials that you should have easily available to you.
The top three videos explore making different kinds of sundials, and the bottom four videos demonstrate how motion of the sun, stars, and planets change over the course of a year.