Shape of the Earth How a scientific proof works. What is Earth’s shape? Is it flat like a coin, or spherical like a ball? Perhaps it’s like a tin can, flat on the ends and round in the middle? Describe the evidence that best supports you answer. Spherical Earth? Scientists claim that Earth is much like a slightly flattened ball - an oblate spheroid. How do they know or do the merely have faith in the claim? What is the evidence that supports a scientific belief in a roughly spherical Earth? Ships appear to sink gradually below horizon from bottom up. Polaris, the North Star is located practically over the North Pole of planet Earth. If you stood at the North Pole, Polaris would be almost directly overhead. Polaris Why do observations of Polaris help determine Earth’s shape? The North Star gradually appears lower and lower in the sky as you travel toward the equator. When the North Star is visible at the horizon, you are located at the equator. South of the equator the North Star is not visible at all. These observations are possible only for a roughly spherical Earth. Lunar Eclipses Only a sphere can cast a shadow that always appears round. During lunar eclipses, Earth casts its shadow on the moon and the shadow is always round. Many other indications… Circumnavigation of the globe. Existence of time zones. Earth-orbiting satellites. Photographs from space. Eyewitness reports of astronauts. Other objects - sun, moon, planets - are roughly spherical. Summary: How do we know the Earth to be spherical? Ships appear to sink GRADUALLY below the horizon as they travel away from observer and over the horizon - in all directions. Polaris (North Star) changes altitude (angle measured in degrees above earth’s surface) with latitude – at all longitudes. Lunar Eclipses - Earth’s shadow on the moon is always round. Satellite imagery and much more. Is Earth perfectly spherical? The Earth is NOT a perfect sphere It is flattened at the poles and bulges at the equator due to its spinning. Earth is very slightly out of round or OBLATE. Saturn is an even better example of being oblate. What proof is there that the Earth is slightly oblate? Gravity measurements. Gravity is the force of attraction between any two objects. Increase mass of objects = increase gravity Decrease distances = increase gravity If Earth were a perfect sphere, it would be expected to exert an equal force on objects at equal distances from the center of earth. The pull of gravity is weaker farther from the center of Earth You weigh just a bit less at the equator than at the pole! The Oblate Sphere The difference with a sphere is small We can not see it with the naked eye Earth still appears like a sphere Why is Earth not perfectly round? Earth’s spinning (rotation) causes… – bulging at equator and – flattening at the poles. Be careful on multiple-choice tests!