Earth and Space Sciences: Earth Systems, Weather, and Sky Patterns
Core ideas
Earth has four main systems: the geosphere, hydrosphere, atmosphere, and biosphere. Water moves through the water cycle by evaporation, condensation, precipitation, and collection. Most of Earth's water is salt water in oceans; only a small fraction is fresh water available for living things. Weather describes short-term conditions such as temperature, wind, clouds, and precipitation. Climate describes long-term patterns. Earth's rotation on its axis causes day and night. Earth's revolution around the Sun, together with the tilt of its axis, causes seasons. The Moon orbits Earth, and we see phases because the amount of the sunlit side facing Earth changes. Eclipses occur when Earth, the Moon, and the Sun line up in specific ways. Gravity keeps planets in orbit and pulls objects toward Earth. Stars are distant suns; their apparent brightness depends on their actual brightness and distance.
Traps to avoid
Seasons are not caused by distance from the Sun; they are caused by axial tilt. Moon phases are not caused by Earth's shadow; that causes a lunar eclipse. Day and night are caused by rotation, not revolution. The Sun is a star, not a planet. Apparent motion of the Sun and stars across the sky is mostly due to Earth's rotation.
Worked example
A flagpole casts a long shadow in the early morning, a short shadow near noon, and a long shadow again in late afternoon. Rule of thumb: the lower the Sun is in the sky, the longer the shadow; the higher the Sun, the shorter the shadow.