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Course: Cosmology and astronomy>Unit 1

Lesson 1: Scale of earth, sun, galaxy and universe

Scale of earth and sun

Scale of Earth and Sun. Created by Sal Khan.

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• Around you were talking about the size of an AU, is that from the corona of the sun to the atmosphere of earth, from the surface of each, or from the center of earth and the sun?
• Until 1976 the AU was defined as the semi-major axis of earth's orbit: 149,598,261 km. Where the center of mass of the solar system is at one focus, and the center of mass of the earth is on the path of the ellipse. See the video "Conic Sections: Intro to Ellipses" for more information on what the semi-major axis and the focus is.

In the 1970's there was a major push to define all of our constants against natural laws of the universe, in ways that could be reproduced in laboratories. As a result, the AU was given an even more complex definition.

In 2012, the IAU re-defined it to be 149,597,870,700 m. Independent of the exact orbital parameters of earth.
• Is the Earth orbit elliptical or cicular? and if elliptical how is the AU compared to the minimum and the maximum distances?
• It is elliptical but its almost circular. At farthest, Earth is 1.01671388 AU, at closest, Earth is 0.98329134 AU.
• Does the Sun have an orbit?
• Yes, the sun does orbit around the center of the Milky way galaxy at a speed of roughly 828,00 km/h.
• What is dark energy and dark matter?
• Dark energy a proposed force to explain why the universe is expanding at an accelerated rate. Its an energy that seems to come from space itself. Dark matter is a strange type of matter that does not interact with light and would undetectable if not for its gravity.
• why do we not go to mars now ?
• We still don't have the technology ready to go to Mars. One of the biggest concerns is radiation shielding.
The Earth's magnetic field protects us from a lot of solar radiation. However, a trip to Mars would involve astronauts being exposed directly to this radiation for long periods of time, posing a significant health risk. Shielding this amount of radiation using current technology would require significant weight, and therefore significant cost.
So, we must figure out a better way to shield without increasing the weight, to keep an already expensive endeavor more manageable.
• how fast is the speed of light in hours
• See if you can work it out yourself. 3 x 10^8 m/s = ? km/hr? It's not hard.
• why doesn't the sun suck in any of the planets?
• An object orbiting the sun is falling towards the sun but the orbital velocity is enough that it keeps missing the sun.
• I dont really know if this would belong here but, if Mars hit the Earth how big would the crater be?

I mean i spent dozens of hours doing numbers and math but i get a hole about 60% of the planet, is that right?
• You would not get a crater from that type of event. What will happen is that the entire crust of the Earth will be vaporized an launched into space. The entire surface will now consist of magma and any crater would disappear because it would be liquid.
• how do you now how many miles the earth is