Ten Most Important Basic Astronomy Facts

1. The Sun, Moon, stars and planets all appear to slowly rise in the East and set in the West each day because the Earth rotates once around its axis every 24 hours.
2. The phases of the moon are caused by the relative position of the Sun, Earth and Moon. As the Moon orbits the Earth, we see all the phases cycle through once per 29 days: new, waxing crescent, waxing gibbous, full, waning gibbous, waning crescent, then new again. Waxing phases are visible in the afternoon and evening, waning phases are visible in the morning, and the full moon is visible all night long.
3. The Earth orbits the Sun once per year. As it does, the constant tilt of 23.5 degrees of the Earth's rotation axis causes the seasons in the Northern and Southern high latitudes.
4. The Sun and stars are the same kind of object: huge spheres of hot Hydrogen gas, radiating heat and light. The reason the Sun appears so bright is that it is much, much closer to the Earth than the other stars.
5. The Moon and planets in our Solar System are cool spheres like the Earth, physically much smaller than stars. They shine because of reflected sunlight.
6. The Solar System includes the Sun, 8 planets (including the Earth, but not including Pluto/Charon, which is classified as a large double asteroid), plus many moons, asteroids and comets. The Solar System is an extremely small portion of the Milky Way Galaxy, which is a giant collection of stars 100 million times larger than the Solar System in size. All the stars we see at night are part of the Milky Way Galaxy.
7. The Milky Way Galaxy is a spiral galaxy containing about 400 billion stars, including the Sun. It is one of many galaxies in the Universe. The universe is everything.
8. The Sun and Solar System, including the Earth, all formed from a collapsing cloud of gas 4.6 billion years ago. The cloud collapsed due to its own self-gravity. The fact that this cloud was initially rotating led to the orbits of the planets around the Sun, and the rotation of the Sun.
9. The universe and all the galaxies in it began in an explosion about 14 billion years ago. Galaxies are still receding from eachother as a result of the initial explosion, and we can detect radio waves which show direct evidence of the initial "Big Bang". These radio waves are called the Cosmic Microwave Background Radiation.
10. A black hole is an object so dense that the gravity forces at its surface prevent anything from escaping. Not even light can escape out through the surface of a black hole. The surface is called the event horizon.