Unit Checkpoint: Solar System

There are 8 planets in our solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Pluto was reclassified as a dwarf planet in 2006.

Mars is called the Red Planet because its surface contains iron oxide (rust), which gives it a reddish appearance when viewed from Earth or space.

The moon phases go: New Moon (not visible) to First Quarter (right half lit) to Full Moon (fully lit) to Last Quarter (left half lit) and back to New Moon.

Jupiter is the largest planet, with a diameter of about 139,820 km. It's so big that over 1,300 Earths could fit inside it!

As the Moon orbits Earth, we see different portions of its sunlit side. The Moon doesn't produce its own light; it reflects sunlight. The phases are caused by the angle at which we see the illuminated portion.

The four inner planets are called terrestrial or rocky planets because they have solid, rocky surfaces. The outer planets (Jupiter, Saturn, Uranus, Neptune) are made mostly of gas and ice.

The Moon takes about 29.5 days to complete one full cycle of phases (from New Moon back to New Moon). This is called a lunar month or synodic month.

Saturn has the most spectacular and visible ring system. While Jupiter, Uranus, and Neptune also have rings, they are much fainter and harder to see.

Earth is in the "habitable zone" or "Goldilocks zone" - not too hot and not too cold for liquid water. Combined with its atmosphere and magnetic field, this makes Earth ideal for life.

During a New Moon, the Moon is between Earth and the Sun, so the sunlit side faces away from us. We see only the dark side, making the Moon invisible in the night sky.