Kid Asked

Why is there sun at wake-up time and moon at sleep time?

Tap the yellow button to begin.

For the grown-up

The short answer. Because the Earth spins, once a day. The sun always lights half of the Earth, and that half has day. In the morning your side turns toward the sun; at night it turns away, and the other side of the world has day. The moon is out in the daytime on many days too. It just shines brightest against a dark sky, with light from the sun.

These pages answer a bedtime question with one of the biggest ideas there is: the sun stays put, and the Earth turns. Here is what really happens, a few numbers, the moon in the day, where the pictures stop being true, what to ask, and how to see it for yourselves.

Why day turns into night

  1. The sun lights half the Earth. Sunlight falls on only one side of the Earth at a time. That side has day. The side facing away has night.
  2. The Earth spins. It turns on its axis once a day, in about 24 hours, toward the east.
  3. Morning and evening. As your side turns into the sunlight, the sun seems to rise in the east. As it turns away, the sun seems to set in the west. The sun is not crossing the sky: we are turning under it. That is also why the moon, the planets and the stars rise in the east and set in the west.
  4. Somebody else's morning. When it is night where you are, it is day on the side of the Earth facing the sun. Bedtime for one kid is breakfast for another.

A few numbers

To turn once in 24 hours, the ground at the equator moves at close to 1,000 miles an hour (1,600 km/h). Farther from the equator it moves more slowly: near New York, about 750 miles an hour.

We do not feel it, because gravity holds us to the ground and we move with the Earth. Bodies feel a change in speed, not speed itself: a smooth train or car ride can feel like standing still.

The sun is about 93 million miles away, and about 100 times wider than the Earth.

The moon can be seen in daylight on about 25 days of a month, on average. For two or three days around the new moon it is too close to the sun in the sky to be seen. A full moon rises just as the sun sets.

The clocks in Beijing are 12 hours ahead of New York while New York is on daylight saving time, and 13 hours ahead in winter, because China keeps one time zone all year.

The moon in the day

The moon makes no light of its own. Moonlight is sunlight bouncing off it. In the daytime the moon still shines, but so does the sky: the air scatters blue sunlight all across it, and the moon has to shine through that glow, so it looks pale. At night the sky behind it is dark, and it seems to shine. The stars are up in the daytime too; the sun is simply too bright for us to see them.

Why we sleep when it is dark

Our bodies keep their own clocks, on a cycle of about 24 hours, and light and dark have the biggest say in setting them. In the evening the body's master clock tells the brain to make more melatonin, which makes us sleepy. So sleep time follows the dark side of the Earth around.

Where the pictures stop being true

The sizes
The sun is drawn close beside the Earth, and smaller than it. Really it is about 100 times wider and 93 million miles away. No page could fit both.
The view
The ball is seen from above the North Pole, tipped toward you, so the spin looks like a top's. From there the Earth turns counterclockwise, which is toward the east. The pictures leave out the tilt of the Earth's axis.
Two even halves
The pictures split the Earth evenly into day and night, with a soft edge. Really the edge is twilight, when the sky still glows after sunset and before sunrise, and the lengths of day and night change with the seasons and with where you live.
“The sun did not go away”
The Earth also travels around the sun, once a year. That trip is not what makes day and night: the spin is.
Your house and your friend's
Your house sits about where New York is, and your friend's about where Beijing is. The land is drawn as simple shapes. The kids on the last page live near London, New York, Vancouver, Mexico City, São Paulo, Nairobi, Mumbai, Beijing, Tokyo and Jakarta.
The moon at sunset
The first pages show a full moon rising as the sun goes down, which happens only around the full moon. On most days the moon rises at another time, about 50 minutes later each day, and for a few days around the new moon it cannot be seen at all.
“The moon is out in the day”
The half moon on page 6 is a first quarter moon, which rises around noon and is highest at sunset, so it is up all afternoon. In the picture it stays still while the sun goes down. Really it moves across the sky from east to west too, carried by the same spin.
The beam of light
At night the sun is below the ground, out of sight. The line drawn to the moon shows where its light comes from.

Questions to ask

Before:

  • “Where do you think the sun goes at night?”
  • “Is the moon only out at night?”

After:

  • “Does the sun move, or do we?”
  • “When it is your bedtime, who is waking up?”
  • “Why does the moon look pale in the day and bright at night?”
  • “Where does the moon's light come from?”

Try it

  • A lamp and a ball. In a dark room, a lamp with no shade is the sun and an orange or a ball is the Earth. Put a sticker on it for your house. Turn the ball slowly, counterclockwise as seen from the top, and say “good morning” when the sticker turns into the light.
  • Be the Earth. Your nose is your house. Face the lamp: it is noon. Turn slowly to your left. The lamp slides away past your right shoulder: sunset. Your back to the lamp: midnight. Keep turning, and the lamp peeks in from your left: sunrise.
  • Hunt for the daytime moon. The easiest times are afternoons in the week after a new moon, toward the east, and mornings in the week after a full moon, toward the west. Keep your eyes on the moon, with the sun behind you.
  • Call someone far away. If you have family or friends in another part of the world, ask what time it is there, and whether they are having breakfast or going to bed.

Where this comes from

Every word, page by page
  1. Look! The sun is going down. And there is the moon! Why is there sun at wake-up time, and moon at sleep time? Let's find out!
  2. Good morning! The sun is coming up. Pull the sun up, into the sky. Rise and shine! Now pull the sun across the sky, and down. Good night! Here comes the moon. But wait. Where did the sun go?
  3. Let's fly up high and look. Up, up, up! Tap to fly. Whoa! The Earth is a giant ball. The sun lights up one side. Day! The other side is dark. Night! Your house is right here. It is on the night side.
  4. The sun did not go away. The Earth moves! It spins, like a top. Spin the Earth! Turn toward the sun. Good morning! Turn away from the sun. Good night! One spin is one whole day and one whole night. And the Earth keeps on spinning.
  5. Here is your house. And far away, on the other side of the Earth, is your friend's house. It is nighttime there. Spin the Earth, and bring the morning to your friend! Good morning, friend! But look. Now you are asleep! When it is night here, it is day over there.
  6. What about the moon? Is it only out at sleep time? Look up at the day sky. Can you find the moon? Tap it! There it is! The moon is out in the day, lots of days. It looks pale in the bright sky. Now pull the sun down. Night! The sky is dark, so the moon shines bright. And its light comes from the sun!
  7. Your turn! Kids live all around the Earth. Spin it, and bring the morning to every kid! One spin, one whole day! On a sunny day, look for the moon in the sky. And tonight? Sleep tight!

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