Planets, Moons & Stars Gr 9 Science

Composition : Terrestrial = Made of rock, minerals Gaseous = Made of gases Stars are always gaseous. Moons are always terrestrial.

Distances & Sizes Stars look small, but are huge. They are the largest … just very far. Moons look large, because they’re close to a planet. They’re smallest. The planets are different sizes, but are all in between the size of moons & stars.

Source of light Luminous = makes light Reflective = reflects light St ars are luminous, making light through nuclear fission. Seen from Earth, they ‘twinkle’. All planets & moons are reflective. Seen from Earth, they ‘glow’.

Phases Phases = an effect resulting from how light strikes a moon. Lunar phases = Earth’s shadow on the moon. Venus also appears to have phases, when seen from Earth. Because they are luminous, stars do not have phases.

Phases Here is an illustration of moon phases over one month

Motion & Orbital Period Rotation = spinning around an axis Revolution = orbiting around a star. Orbital Period = time for 1 revolution. All three rotate, but only planets & moons revolve or orbit. The orbital period of a planet is called its year. The orbital period of a moon is called its month.

Position Planets seem to change position when observed from Earth. This is because we are seeing them from Earth, which is also moving. It’s like runners racing on a track They look to their side and see: the inside runners ahead of them the outside runners behind them

Position Because they orbit planets, moons change position frequently. Earth’s moon changes position every night. Stars seem to stay in their position within constellations, but shift during the night due to Earth’s rotation They also shift when observed from different angles on Earth, due to parallax.

Parallax This is an example of parallax you’ve experienced:

Parallax An illustration of parallax in astronomy:

Atmospheres Atmospheres = layers of gas surrounding a planet, held in place by gravity. Stars don’t have atmospheres. Their intense heat would burn off gases. Moons are too small. They don’t have enough gravity to hold onto an atmosphere. Some planets have atmospheres, but a few don’t. Only Earth’s atmosphere can support life.

Origins Planets form when gravity pushes together a mass of gases or other particles. Moons form out of pieces of debris in space, trapped in a planet’s atmosphere. Planets can have many moons, if large enough. Stars form when a cloud of gases & dust called a Nebula condenses enough to begin a nuclear reaction.