Ages 9–10 · 8 little discoveries
Piko’s Space Evidence: Motion, Scale and Models
Eight lessons for ages 9–10 on how Earth turns and travels, how big space is, what gravity does, why the Moon has phases, how eclipses line up, how stars differ from planets, how a spacecraft is planned, and how to choose a model from evidence.
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Turning and travelling
Explain how a spin makes days and an orbit makes years, and read a turn from a drawing.
Earth spins like a spinning top. It turns once about every 24 hours. When the side where we live faces the Sun, it is day. When it turns away, it is night.
Earth also travels around the Sun. One trip takes about 365 days, and that is one year. Earth’s tilt makes our seasons change during that trip.
The Moon goes around Earth in about 27 days. It also turns once in that time, so we always see the same face of the Moon.
A model shows these motions, but it is not to scale. Use it to predict what you will see, then check the prediction with evidence.
Scale and distance
Explain why space distances are measured with the time light takes to travel them.
Space is far too big to draw at full size. We use a scale: one small unit on paper stands for a very large distance in space.
Light is very fast, but space is very wide. Sunlight takes about 8 minutes to reach Earth. So the sunlight we see left the Sun about 8 minutes ago.
Light from the Moon takes about one second to reach us. Light from the nearest star outside our Solar System takes about 4 years.
Size and distance are different. A big object can be far away and look small. A small object nearby can look big. Ask two questions: how big, and how far?
Pull and orbit
Explain an orbit as a pull toward a bigger object plus sideways motion.
Gravity is a pull between objects. Every object pulls on every other object. The more mass an object has, the stronger its pull.
The Moon is always pulled toward Earth. But it also moves sideways very fast. It keeps falling around Earth instead of falling straight into it.
If the Moon stopped moving sideways, it would fall straight down to Earth. Its sideways speed keeps it going around Earth.
Try it in your mind. Throw a ball sideways: a harder throw lands farther away. A satellite is like that ball, falling around the Earth.
Moon phases
Explain phases as changing views of the lit half of the Moon.
The Sun always lights half of the Moon. The lit half faces the Sun, just as half of Earth is in daylight.
As the Moon goes around Earth, the part of its lit half that we see keeps changing. That change is called a phase.
A full Moon shows its whole lit half to Earth. A new Moon shows almost none of it. Crescents and quarters are in between.
Do not confuse phases with Earth’s shadow. Earth’s shadow on the Moon is an eclipse, and it is rare. Phases come every month.
Eclipses and lining up
Explain solar and lunar eclipses by the order of the Sun, the Moon and Earth.
An eclipse happens when three objects line up. One object blocks the light, and another gets the shadow.
In a solar eclipse, the Moon sits between the Sun and Earth. Its shadow falls on Earth, and a small area sees the Sun blocked for a few minutes.
In a lunar eclipse, Earth sits between the Sun and the Moon. Earth’s shadow falls on the Moon, which can look dim and reddish.
Eclipses do not happen every month, because the Moon’s path is tilted. Most months the Moon passes above or below the line.
Star or planet?
Explain whether an object is a star or a planet, using its light and its movement.
A star makes its own light. Inside a star, nuclear fusion releases energy as light and heat.
A planet shines by reflecting sunlight, like a mirror. Its light looks steady, and it drifts slowly against the background stars over days.
Stars look like points and can twinkle, because air moves the light on its way to us. Planets shine steadily, so do not rely on twinkling alone.
Do not decide from one night. Look again on another night, compare with stars that stay in place, and then decide.
A spacecraft mission
Explain why a spacecraft aims ahead of a planet, and why messages from far away are slow.
A spacecraft to Mars must aim where Mars will be on arrival. Earth and Mars both move, so the target keeps shifting during the trip.
Radio messages travel at the speed of light. A message from Mars can take between about 3 and 22 minutes to reach Earth.
The path is a curve, not a straight line. Gravity from the Sun and planets bends it, and engines fire only a few times, because fuel is limited.
Check every claim against evidence. Sound needs a material to travel through, and space has almost none, so it cannot carry sound like air.
Choosing a model from evidence
Explain why a model is kept or rejected by checking what it predicts.
A model is an idea that predicts what we will observe. A good model makes clear predictions that can be checked.
Imagine a model where Venus always stays between Earth and the Sun. Then Venus would only ever look like a thin crescent.
A small telescope shows Venus with a full, bright face when it is on the far side of the Sun. That observation does not fit that model.
Keep a model only while it fits the evidence. If new evidence does not fit it, change the model, even if you liked it.