Planets, Moons & Stars

Gr 9 Science

Composition:Terrestrial = Made of rock, mineralsGaseous = Made of gases

Stars are always gaseous.Moons are always terrestrial.

Composition:

Planets can be bothSome planets are gaseous.Some planets are terrestrial.

Distances & SizesStars look small, but are huge.They are the largest … just very far.

Distances & SizesMoons 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 lightLuminous = makes light

Reflective = reflects light

Stars are luminous, making light through nuclear fission. Seen from Earth, they ‘twinkle’.All planets & moons are reflective. Seen from Earth, they ‘glow’.

PhasesPhases = an effect resulting from how light strikes a moon.Lunar phases = Earth’s shadow on the moon.

Some planets have phases. Venus has phases.Stars do not have phases.

PhasesHere is an illustration of moon phases over one

month

Motion & Orbital PeriodRotation = spinning around an axis

Revolution = orbiting around a star.Planets, moons and stars all rotate, but only planets & moons revolve or orbit.

Motion & Orbital PeriodOrbital Period = time for 1 revolution.

The orbital period of a planet is called its year.The orbital period of a moon is called its month.

PositionPlanets 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 trackThey look to their side and see:the inside runners ahead of themthe outside runners behind them

PositionBecause 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

ParallaxStars shift because change in angles, or Parallax

ParallaxHere’s how Parallax works:

AtmospheresAtmospheres = layers of gas

surrounding a planet, held in place by gravity.

Moons & Stars don’t have atmospheres..Some planets have atmospheres, but a few don’t. Only Earth’s atmosphere can support life.

OriginsPlanets form from gases, stone or metals pushed together by gravityMoons form from pieces of debris in space, trapped in a planets orbit. Planets can have many moons, if large enough..

OriginsStars form when a Nebula condenses.

A Nebula is a kind of dust & gas cloud. When it condenses enough, a nuclear reaction begins.