Cosmos

• The Earth has a radius of 4,000 miles/6,378 km
• The circumference of the Earth is 40,000 km/25,000 miles
• The distance from the sun to the Earth is equal to one astronomical unit
• The speed of light is 300,000 km/s
• A lightyear refers to the distance light travels in one year
• It takes light 8.3 minutes to reach Earth
• Jupiter is 5 astronomical units away from the sun, it takes light 41 minutes to travel
• Pluto is 40 astronomical units away, it takes 5 hours and 30 minutes for the light to travel
• Proxima Centauri is 270,000 astronomical units away, that is 4.2 lightyears
• Alpha Centauri is 4.4 lightyears away
• Gliese 710 is 63.8 lightyears away, but in 1.4 million years it will be 1 lightyear away
• The Milky Way Galaxy is 100,000 lightyears across and contains over a 100 billion stars
• The Milky Way is a spiral galaxy
• The Andromeda Galaxy is 2.5 million lightyears away and is the nearest galaxy
• There are more galaxies than stars in the Milky Way
• The Earth spins on its axis once in about 24 hours
• The Earth orbits the sun once a year
• The sun orbits the center of the Milky Way once in 255 million years at the speed of 230 km/s
• The Milky Way is moving towards Andromeda and they will collide in 3 billion years
• Planets orbit in ellipses
• Ellipses have a semi-major axis and a semi-minor axis
• Comets have highly elliptical orbits
• Ou'muamua and Borisov are examples of interstellar objects, which were recently discovered
• Two stars orbit around a center of mass
• It was originally thought that planets moved in perfect circles
• Kepler discovered planets moved in elliptical orbits
• Kepler had three laws of planetary motion
• Kepler's first law was that planets moved in ellipses with the sun in one focus
• Kepler's second law is that planets "sweep out" in equal areas in equal times (in equal times they cover the same area)
• Kepler's third law is that the period of orbit squared equals the semi-major axis
• Newton had three laws of motion
• Newton's first law of motion is that an object in motion stays in motion unless acted upon by an external force
• Newton's second law of motion states that force is equal to that object's mass times its acceleration
• Newton's third law of motion is that forces always come in equal but opposite pairs
• Gravity equals the mass of one object times the mass of the second object divided by the distance squared
• Weight is a measure of force acting on an object due to gravity
• The gravitational force between two objects in space is the same for both objects (because forces occur in pairs)
• Tangential speed prevents planets from falling into the sun, and the sun from falling into the planets
• Tangential speed is equal to orbit speed
• Gravity is always attractive