Ideas
Short ideas. Two of them are theories. We have not found aliens.
1
Measured
Gravity
Gravity pulls things together. It pulls you to the ground. It also holds the Moon in orbit around Earth. Drop a ball. That is a test.
2
Measured
Orbits
An orbit is a path. Earth orbits the Sun. The Moon orbits Earth. The space station orbits Earth too.
3
Measured
Day and night
Earth turns once each day. The Sun lights one side. That side has day. The other side has night.
4
Measured
Seasons
Earth is tilted. When your half leans toward the Sun, days are longer and warmer. That is summer. Tilt makes the seasons.
5
Measured
Light-years
A light-year is a distance. It is how far light travels in one year. Stars are many light-years away. Sunlight takes about eight minutes, not one light-year.
6
Measured
Rockets
A rocket pushes gas out the back. The rocket goes forward. It carries its own gas, so it can work where there is no air. We can test this.
7
Theory being tested
Life we have not met
There may be life we have not met. Scientists are testing this idea on Mars, on icy moons, and on far planets. We have not found aliens.
8
Theory being tested
A new kind of travel
A wormhole or a warp drive would be a new kind of travel. People study the idea. We do not have a ship that can do it. It is a theory, not a ride.