Parallax
Parallax is the shift in where an object appears when you view it from two different spots.
Stand a tall glass jar in front of a short, wide bowl on a kitchen counter. From the left side, the jar appears to the right of the bowl. Move to the right side, and the jar appears to the left of the bowl. Neither object moved — your viewing spot changed, so the jar seems to shift.
Explaining parallax by grade level
Hold one finger up in front of your face. Close one eye, then switch and close the other. Your finger seems to jump side to side. It did not really move. Your two eyes see from two spots, so each one sees your finger in a different place. Things close to you jump more than things far away.
Projects that explore parallax
How do you measure the distance to something you cannot reach? Parallax — the shift in where an object appears when you view it from two different spots — is one answer astronomers use to measure distances to stars. In this experiment, you set up six candles on stools and a distant reference light in a dark room. Sitting in one chair, you hold graph paper at arm's length and record the spacing between each candle and the reference light. When you move to a different chair and repeat, each candle's position shifts against that reference. From those two sets of measurements, you calculate how far away each candle is — the same method astronomers use to determine stellar distances.
Once you see the shift, you can use it to figure out how far away something is. This project does exactly that. You view a target from two spots on a wall, measure the angle of the shift, and then calculate the distance.