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This activity introduces students to the concept of angular distances between celestial objects through the construction and use of a simple, handmade astronomical tool called the "astronomical rake", using everyday materials. The rake is used to measure angular distances between stars in the night sky, focusing on the asterisms such as the Big Dipper and results are compared with actual measurements taken from star charts or Stellarium. The activity is adaptable for different educational levels: younger students can follow simplified instructions and focus on observation, while older students can explore mathematical reasoning, unit conversions, and error analysis. This activity develops spatial reasoning, observational precision, and a deeper appreciation of the night sky, while reinforcing concepts in geometry and astronomy.
Materials needed:
Other materials needed:
Image: the attachments to be printed and distributed to students
When we observe the night sky, stars and planets may appear close together or far apart. However, these distances are not measured in meters or kilometers, but in angular distances. Angular distance describes how far apart two objects appear in the sky from our point of view on Earth, and it is usually measured in degrees (°), arcminutes ('), and arcseconds (").
A full circle around the observer corresponds to 360°, so the angular separation between two stars represents a fraction of that circle. For example, the apparent diameter of the Moon in the sky is about 0.5°, while the distance between two bright stars in an asterism such as the Big Dipper can span several degrees.
Astronomers use angular measurements because celestial objects are extremely far away, making direct distance measurements impractical for observations with the naked eye. Instead, angular distances allow us to map the sky, compare positions of stars, and track their apparent motion.
Simple tools can help estimate these angular distances. Even without instruments, the human body can serve as a reference: for example, the width of a finger at arm’s length corresponds to about 1–2°, while a fist spans roughly 10°. These approximations have been used for centuries in observational astronomy.
Image: a representation of the angular distanceUsing basic geometry, a physical tool like the astronomical rake allows us to visualize these distances. The radius of 57.3 cm is used because 1 radian equals approximately 57.3 degrees—so each centimetre on the arc corresponds to one degree.
The activity can be done individually or in groups: print one copy of the Students Observation sheet and Assessment sheet for each student/group and prepare all material needed. Part1 can be done in the classroom, while Part 2 will need to be done at night, observing the night sky. Part 3 is optional and will need a computer and an internet connection.
Images from left to right: Tools and parts of the astronomical rake; Drawing the circle diameter; Hammering the nails
Images from left to right: The assembled astronomical rake; The correct way to hold the instrument during measurements
Image: the Big Dipper without and with angular distances (https://nightsky.jpl.nasa.gov/news/236/)
Croatia
5th grade: Maths- circle, diameter
6th grade:Maths - degreesAstronomy - horizontal and equatorial coordinate systemTechnology and design - working with materials (wood)
7th/8th gradePhysics - scientific method, measurements, analytics and error calculations
1st grade - high schoolPhysics - circular motion
2nd grade - high schoolMaths-Trigonometry
3rd grade - high schoolPhysics- angular momentum
