Worked example - how deep is a planet's transit dip? The fractional dimming is (R_planet / R_star)^2. For Jupiter crossing the Sun: R_J / R_sun ~ 0.1, so the dip is 0.1^2 = 0.01 = 1% - easily detectable from the ground. For Earth crossing the Sun: R_E / R_sun ~ 1/109, so the dip is (1/109)^2 ~ 8.4 x 10^-5, just 84 parts per million. Step 3: that is why Kepler needed space-based precision - an 84 ppm dip is like detecting a flea crossing a car's headlight from kilometres away. Yet Kepler found hundreds of such Earth-sized dips.