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The Night Sky

Four short lessons for looking up: why the stars cross the night, why the Moon changes shape, how to tell a planet from a star, and how far the light has come.

Spark & Whetstone · sparkwhetstone.org/learn/the-night-sky · CC BY-NC-SA 4.0. Answers are at the end, so you can try each question first.

Lesson 1. Why the stars move

Watch the sky for an hour and the stars drift together, rising in the east and setting in the west, as the Sun does by day. They are not moving across the sky. We are turning.

The Earth spins once a day, so the whole sky seems to wheel around us. In the northern hemisphere one star, Polaris, sits almost exactly above the North Pole, so it barely moves while the rest circle it.

The stars keep the same patterns relative to each other over a lifetime — which is why the constellations people named thousands of years ago still look the same.

Question. Why do the stars seem to cross the sky during the night?

  1. The stars are moving around the Earth.
  2. The Earth is spinning.
  3. The Moon pulls them along.

Say it back. In your own words: why do the stars move across the night sky, and why does Polaris barely move?

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Lesson 2. Why the Moon changes shape

The Moon makes no light of its own. Half of it is always lit by the Sun, and half is always dark. What changes through the month is how much of the lit half we can see from here.

When the Moon is roughly between us and the Sun, its lit side faces away — that is the new moon. A week later we see half of the lit side. When the Moon is on the far side of us from the Sun, we see the whole lit face: a full moon.

It is not the Earth's shadow that makes the phases. The Earth's shadow only falls on the Moon during an eclipse, which is much rarer.

Question. What causes the Moon's phases?

  1. The Earth's shadow covering part of the Moon.
  2. How much of the Moon's sunlit half we can see from Earth.
  3. Clouds passing in front of the Moon.

Say it back. Say it back: explain the Moon's phases without mentioning the Earth's shadow.

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Lesson 3. Planet or star?

Some of the brightest "stars" in the evening sky are planets — Venus, Jupiter, Mars, Saturn. They shine by reflecting sunlight, as the Moon does.

An old rule of thumb: stars twinkle, planets mostly do not. A star is so far away it is a single point, and the air bends that point about. A planet is close enough to be a tiny disc, so the air's wobble averages out.

Planets also wander. Over weeks they shift against the background of stars — the word planet comes from the Greek for wanderer.

Question. You see a very bright point low in the west after sunset that shines steadily without twinkling. Most likely it is…

  1. a star
  2. a planet
  3. an aeroplane

Say it back. In your own words: give two ways to tell a planet from a star by eye.

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Lesson 4. Old light

Light is the fastest thing there is, but space is big. Sunlight takes about eight minutes to reach us, so we see the Sun as it was eight minutes ago.

The nearest star beyond the Sun is about four light-years away: its light set out four years ago. Many of the stars you can see by eye are dozens to hundreds of light-years away.

So looking up is also looking back. The night sky is not one moment — every point of light left at a different time.

Question. If a star is 100 light-years away, the light you see tonight left it…

  1. 100 years ago
  2. last night
  3. 100 days ago

Say it back. Say it back: why is looking at the night sky also looking into the past?

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At the table: The window-sky table

Players: 2 or more, any ages. You'll need: A clear evening, a window or a yard, paper and a pencil.

  1. Pick one bright star or planet and sketch where it sits next to a roof, tree or chimney.
  2. Look again an hour later and mark where it is now. Which way did it move?
  3. Talk it over: is it moving, or are we? Does it twinkle?
  4. Over a few weeks, sketch the Moon each night you can see it and line the drawings up.

Answers

  1. The Earth is spinning. — The Earth turns once a day, so every star seems to wheel around us — all together, keeping their patterns.
  2. How much of the Moon's sunlit half we can see from Earth. — The phases come from our changing view of the Moon's lit half as it circles us. The Earth's shadow only matters in an eclipse.
  3. a planet — A steady, very bright point that stays put is most likely a planet — often Venus in the west after sunset. Stars twinkle because they are single points.
  4. 100 years ago — A light-year is how far light goes in a year, so light from 100 light-years away has been travelling for a century.