(1.) "What does it mean that our sun is 93 million miles away? Often we need to translate a number to something more understandable. Light travels at an average speed of 186,281 miles per second. This means that it takes sunbeams about 8 minutes to reach the earth. The star Alpha Centauri is 4.35 light-years away. A light-year is the distance light travels in one year. Even if light year measures distance, it implies time.
If we were to ride on a beam of light, it would take us 4.35 years to reach our closest star. When we look up in the sky we see the past -the star as it appeared 4.35 years ago. Always look at what numbers mean. For example, ask: Does the magnitude make sense? In relation to what? Also, think about what counts -not how it is counted.
There was a sign hanging in the physicist Albert Einstein's office at Princeton that said: 'Not everything that counts can be counted, and not everything that can be counted counts.'"
- Peter Bevelin, Seeking Wisdom: From Darwin to Munger, 3rd Edition
(2.) “How does gravity produce an orbit and prevent the Moon from hitting Earth? According to Newton’s first law, in zero gravity, an object launched perpendicular to the direction of Earth results in motion along a straight-line at constant speed. According to Newton’s second law, releasing that object from rest with gravity causes it to accelerate toward Earth because of gravity’s force. Combining these two motions, we can examine them along small time steps. The Moon falls toward Earth during each moment it takes to travel a short distance perpendicular to Earth, but its new distance from the center of the Earth remains unchanged. During the next moment, the same thing happens, but now, the tangential motion (perpendicular to the direction of Earth) is in a slightly different direction because of the acceleration during the preceding moment. Again, the new distance remains unchanged. Thus, along numerous tiny steps of time, a smooth curved orbit is formed. In other words, the Moon really does fall toward Earth, but it never hits because the tangential motion keeps it away. It keeps missing the Earth, in a sense.
If Earth’s gravity were eliminated, the Moon would continue to move with constant speed and direction along the tangent to the orbit at the instant gravity disappeared. Newton illustrated his point by demonstrating that as the speed at which an object is thrown or projected increases, the object travels a farther distance before falling to Earth because of gravity. For example, a cannonball fired at great speed will eventually fall to the ground, but the distance will be greater than expected because the surface of the Earth partially curves away from the ball’s trajectory. Similarly, if an object were fired sufficiently fast, it would follow a trajectory that matched the Earth’s curvature, never hitting the ground but, rather, orbiting the Earth.”
- Alex Filippenko, The Great Courses - Understanding the Universe
(3.) "If an object travels fast enough, it can escape the Earth's gravitational pull. This speed is known as the escape velocity. At Earth's surface, the escape velocity is 11.1 kilometers per second [or 24,830 miles per hour].
- Alex Filippenko, The Great Courses - Understanding the Universe
(4.) "At [Newton's paper] Principia's heart were Newton's three laws of motion (which state, very baldly, that a thing moves in the direction in which it is pushed; that it will keep moving in a straight line until some other force acts to slow or deflect it; and that every action has an opposite and equal reaction) and his universal law of gravitation. This states that every object in the universe exerts a tug on every other. It may not seem like it, but as you sit here now you are pulling everything around you - walls, ceiling, lamp, pet cat - toward you with your own little (indeed, very little) gravitational field. And these things are also pulling on you. It was Newton who realized that the pull of any two objects is, to quote Feynman again, "proportional to the mass of each and varies inversely as the square of the distance between them." Put another way, if you double the distance between two objects, the attraction between them becomes four times weaker."
- Bill Bryson, A Short history of Nearly Everything
(5.) "There are three main sources of energy for life on earth: the sun, the heat from fissioning atoms deep inside earth, and the primordial spin of earth itself. These sources provide energy throughout the day. The sun provides the most energy. It's a fusion reactor releasing 10^26 watts every second. Earth's core also provides energy in the form of heat. The spinning of our planet keeps shifting the energy inputs and adds acceleration to the wind and the waves."
- Bill Nye, Undeniable: Evolution and the Science of Creation