8th GradeAngles and parallel linesabout 3 minCCSS 8.G.A.5Use informal arguments to establish facts about the angle sum and exterior angle of triangles, about the angles created when parallel lines are cut by a transversal, and the angle-angle criterion for similarity of triangles.CCSS 4.MD.C.5.aAn angle is measured with reference to a circle with its center at the common endpoint of the rays, by considering the fraction of the circular arc between the points where the two rays intersect the circle. An angle that turns through 1/360 of a circle is called a "one-degree angle," and can be used to measure angles.+0
Guess the shadow 800 km north, then see how a stick measured the whole Earth.
Lesson Plan
Use the angles formed by parallel lines and a transversal to find an arc's share of a circle, as Eratosthenes did.
Question: Ask what students spot and what they are curious about. Read “What shadow does the stick cast?” aloud. Have each student guess and say why.
Question: Read “Earth twice as big around: the shadow?” aloud. Have partners explain their guesses to each other.
Real-world application: Ask: Where else have you seen this?
Your turn: Have students try their own. Ask them to predict before they test.
How to run it
Whole classPut the lesson on the board. At each choice the class votes, then you select the winner.
SoloShare the link, QR code, or post to Google Classroom. Each student gets to take their own path.
