How the Earth is recognised
- Water Surface!
Why is the sky blue?
The Earth's atmosphere causes the light from the Sun to scatter, which means we only see the blue light.
What are the benefits of the Earth's atmosphere?
- It blocks out most of the ultra-violet rays, infra-red radiation and X-rays.
- Gives us atmospheric pressure.
- It traps heat from the Sun and keeps Earth warm.
What is the main cause of light pollution?
- The main cause of light pollution is HUMAN ACTIVITY (street lights, lights from houses etc)
- Astronomers do not like this because it obscures their vision of the stars and other objects in the Solar System, meaning that for accurate observations they must set up telescopes outside the Earth's atmosphere.
Eratosthenes and the circumference of the Earth
Eratosthenes knew that on the summer solstice at local noon on the Tropic of Cancer, the Sun would appear at the zenith, directly overhead (sun elavation of 90°) - though Syene was in fact slightly north of the tropic. He also knew, from using a vertical stick and measuring the cast shadow, that in his hometown of Alexandria, the angle of elevation of the Sun would be 83° or 7° south of the zenith at the same time. Assuming that Alexandria was due north of Syene - Alexandria is in fact on a more westerly longitude - he concluded, using geometry of parallel lines, that the distance from Alexandria to Syene must be 7/360 of the total circumference of the Earth. The distance between the cities was known from caravan travellings to be about 5,000 stadia. He established a final value of 700 stadia per degree, which implies a circumference of 252,000 stadia. The exact size of the stadion he used is no longer known (the common Attic stadion was about 185 m), but it is generally believed that Eratosthenes' value corresponds to between 39,690 km and 46,620 km. The circumference of the Earth around the poles is now measured at around 40,008 km.
Diameter of the Earth and other planets
- Mercury - 4900km
- Venus - 12,100km
- Earth - 12,800km
- Mars - 6800km
- Jupiter - 143,000km
- Saturn - 121,000km
- Uranus - 51,000km
- Neptune - 50,000km
Evidence that the Earth is spherical
Aristotle first discovered that the Earth is spherical. He did so, firstly by watching a ship on the horizon. The ancient Greek skies were much clearer than our skies today so he could see that when the ship reached the horizon, the bottom of it began disappearing as though it was going downhill. If the Earth was indeed flat, he would still see the whole of the ship. His second piece of evidence was how the constellations appeared to move around the sky at night. If the Earth was flat, the Earth would not turn and the constellations would not move. His third piece of evidence was looking at the shadow of the Earth on the Moon at a lunar eclipse. It cast a round shadow on the Moon, rather than a straight shadow, proving that the Earth is round.
Important Astronomical terms
- Equator - The imaginary line around the middle of the Earth that is of equal distance to the North pole and the South pole.
- Latitude - The angular distance of a place north or south of the equator.
- Longitude - The angular distance of a place east or west of the Earth's meridian at Greenwich.
- Poles - The points that the axis of rotation of the Earth pass through.
- Horizon - The imaginary line where the sky and the Earth meet.
- Zenith - The point directly above us in the sky.