Imagine a future where aliens living on asteroids near the center of the galaxy want to visit you. Excitedly, you tell them, “Sure! I’m free any day this week.” But there’s a problem—they don’t understand what a “day” means because their calendar is completely different. To them, the idea of a “day” is very alien.
You start by explaining that a day is the time it takes for Earth to complete a full rotation on its axis. The aliens input this into their computer simulation, but you quickly realize there’s a flaw. As Earth rotates, it also moves around the sun. By the time Earth reaches the other side of the sun, our “daytime” and “nighttime” have switched places! This isn’t what we mean by a calendar day. What you’ve described is a “Stellar day,” which is measured with respect to a distant, stationary point in space. However, our concept of a day is more about the sun, not the galaxy.
You give it another shot. This time, you explain that a day is the time it takes for the sun to return to its highest point in the sky. The aliens adjust their instruments, but their day counter doesn’t match your clock. It’s starting the new day earlier and earlier, then later and later. This isn’t a mistake in their programming—it’s a feature of Earth’s orbit. What you’ve described is a “Solar day,” which isn’t the same as a day kept by a clock. The length of time between when the sun is highest changes throughout the year because of Earth’s elliptical orbit and tilted axis.
If we used solar days in everyday life, our calendars and clocks would have to change the number of minutes and seconds in a day depending on the time of year. Sundials naturally do this, but they have other drawbacks. Changing the length of a second or the number of seconds in an hour isn’t practical for regular or interplanetary use.
So, you tell the aliens that a day is an invented time period that is 24 hours long. Each hour is defined by 33 trillion oscillations of a special kind of photon emitted by a cesium atom. If they want to know why a day is defined this way and how it relates to Earth’s rotation, you can direct them to an interactive video on MinuteLabs. This video explains solar, stellar, and standard 24-hour days, how they’re related, and how Earth’s orbit affects them. It even lets you experiment with different orbits to see how they change the length of days!
With this knowledge, you’ll be ready to coordinate a visit with aliens, no matter what day it may be.
Using a globe or a ball, simulate Earth’s rotation and orbit around the sun. Mark a point on the globe to represent your location. Rotate the globe to show how a “Stellar day” differs from a “Solar day.” Discuss with your classmates how Earth’s tilt and orbit affect the length of a day.
Watch the interactive video on MinuteLabs about solar, stellar, and standard 24-hour days. Experiment with different orbits in the simulation to see how they change the length of days. Share your findings with the class and discuss how this knowledge could help coordinate activities with the aliens.
Work in groups to design and build a simple sundial. Use it to track the sun’s position throughout the day. Compare the time shown on your sundial with a standard clock and discuss any differences you observe. Reflect on how ancient civilizations might have used sundials to measure time.
Research how time zones work and why they are necessary. Create a presentation explaining how time zones are related to Earth’s rotation and the concept of a 24-hour day. Consider how time zones would be important for coordinating a visit with the aliens.
Learn about how cesium atoms are used to define a second. Conduct a simple experiment or demonstration that illustrates the concept of atomic timekeeping. Discuss why this method is more reliable than using solar or stellar days for keeping time.
Day – The period of time it takes for the Earth to complete one full rotation on its axis, typically 24 hours. – The length of a day on Earth is about 24 hours, which is the time it takes for the planet to spin once on its axis.
Earth – The third planet from the Sun in our solar system, which is home to all known life. – Earth is the only planet in our solar system that has liquid water on its surface, making it unique for supporting life.
Sun – The star at the center of our solar system that provides light and heat to the planets orbiting it. – The Sun is a massive ball of gas that emits light and heat, which are essential for life on Earth.
Solar – Relating to or determined by the Sun. – Solar energy is harnessed from the Sun’s rays and can be used to generate electricity.
Stellar – Relating to stars or celestial objects. – The stellar explosion known as a supernova can outshine an entire galaxy for a short period.
Orbit – The curved path of an object around a star, planet, or moon, especially a periodic elliptical revolution. – Earth orbits the Sun in an elliptical path, taking about 365.25 days to complete one revolution.
Axis – An imaginary line about which a body rotates. – Earth’s axis is tilted at an angle of about 23.5 degrees, which is why we experience different seasons.
Time – A measured or measurable period during which an action, process, or condition exists or continues. – In physics, time is a fundamental quantity used to sequence events and compare the durations of events and the intervals between them.
Photon – A particle representing a quantum of light or other electromagnetic radiation. – Photons are the basic units of light and are responsible for transmitting electromagnetic energy.
Calendar – A system for organizing and dividing time, typically based on the cycles of the moon or the Sun. – The Gregorian calendar, which is widely used today, is based on the Earth’s orbit around the Sun.
