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Changes that come back

Change and motion · level 1, lesson 5 of 6

A question to start

Every evening the sky gets dark. Every morning it gets light again. You have seen this happen every single day of your life. So here is a strange question: how do you know that the Sun will come up tomorrow?

Nobody has seen tomorrow yet. But you are completely sure. In this lesson you will find out what kind of change makes you so sure, and why it lets you predict the future, which means to say what will happen before it happens.

The idea

Changes that go one way, and changes that come back

In lesson 1 you saw changes that go one way: a puppy grows into a dog and never becomes a puppy again. A candle burns down and never grows back.

Some changes are different. They change, and change, and then come back to where they started, and then do it all again. Grown-ups call this a cycle (say "SY-kul"), like the wheels of a bicycle that go round and round.

Day and night

The most important cycle is day and night.

day night day night day night

Day, night, day, night... It is like a repeating pattern of beads on a string: the same thing over and over.

Why does it happen? Our Earth is a giant ball that spins, like a top, all the time. It turns around once every day. When your side of the Earth faces the Sun, it is day. As the Earth keeps turning, your side turns away from the Sun, and it is night. Then it turns back to face the Sun, and it is morning again.

You can try this with a lamp and an orange. Stick a sticker on the orange to be "you". Turn the orange slowly in front of the lamp. The sticker goes into the light (day), round to the dark side (night), and back into the light (the next day).

So the answer to the question at the start: you know the Sun will "come up" tomorrow because the Earth keeps spinning, the same way it has spun for a very, very long time. The pattern repeats, and a repeating pattern lets you predict.

On Mars, there is day and night too, because Mars spins as well. A day on Mars is called a sol, and it is a little longer than our day: about 40 minutes longer.

The seasons

Another cycle is the seasons: winter, spring, summer, autumn (also called fall), and then winter again.

winter spring summer autumn

The seasons take a whole year to go round once. A year is a long time: 365 days (366 in a leap year). In many places:

  • winter is cold, and the days are short (it gets dark early);
  • spring gets warmer, and plants start to grow;
  • summer is the warmest, and the days are long;
  • autumn gets cooler, and many trees drop their leaves.

Then winter comes back, and it all happens again. (In some warm places near the middle of the Earth, the seasons are more like a "rainy season" and a "dry season". They still come back every year.)

Why does it happen? The Earth travels around the Sun in a big loop, and one trip around takes one year. The Earth is also a little tilted, like a top leaning over. For part of the year your half of the Earth leans towards the Sun and gets more sunshine: summer. For the other part it leans away: winter.

On Mars there are seasons too, because Mars is tilted like the Earth. But Mars takes much longer to go around the Sun, almost two of our years, so each Mars season lasts almost twice as long as ours.

The shape of the Moon

Look at the Moon on many nights in a row, and you will see its shape change.

new half full half growing, then shrinking, then back to new

First you can hardly see the Moon at all (a new moon). Then a thin curved sliver appears, like a fingernail (a crescent). Night after night the bright part grows, to a half moon, then nearly round, then a full moon, a bright round circle. Then it shrinks again in the same way, back to new. The whole cycle takes about a month, just over 29 days.

(The picture shows how it looks from the northern half of the Earth, where Canada is. From the southern half, like Australia, the Moon looks flipped: it grows from the left.)

Why does it happen? The Moon does not really change shape. It is always a round ball. The Sun always lights up half of it, the half facing the Sun. As the Moon travels around the Earth, we see more or less of that lit-up half. When the Moon is between us and the Sun, its lit side faces away from us: new moon. When the Moon is on the far side of us from the Sun, we see all of its lit side: full moon.

Why cycles let you predict

All three cycles have something in common: the same changes happen in the same order, again and again, and each round takes about the same time. So once you know the cycle, you can predict:

  • If it is night now, then day comes next.
  • If it is spring now, then summer comes next.
  • If the Moon is full tonight, then in about two weeks it will be new, and in about four weeks it will be full again.

That is the power of a cycle. It is a pattern that goes round in a loop, and patterns can be continued.

Small cycles all around you

Cycles are everywhere, not only in the sky:

  • a swing goes forward, back, forward, back;
  • a traffic light goes green, yellow (amber), red, green, ...;
  • your heart beats: squeeze, rest, squeeze, rest;
  • you breathe in, out, in, out;
  • a week goes Monday, Tuesday, ..., Sunday, and then Monday again.

Worked examples

Example 1. What comes next? The traffic light is green, then yellow, then red. What comes after red?

  1. The traffic light is a cycle: green, yellow, red, and back to the start.
  2. After red comes green.

Example 2. Days of the week. Today is Friday. What day will it be in 3 days?

  1. Count forward: Saturday (1), Sunday (2), Monday (3).
  2. It will be Monday. The week went round its loop and started again.

Example 3. Seasons. It is summer now. What season will it be in two seasons?

  1. After summer comes autumn (1 season), then winter (2 seasons).
  2. Winter.

Example 4. Growing up is not a cycle. Is "baby, child, grown-up" a cycle?

  1. A cycle comes back to the start.
  2. A grown-up does not turn back into a baby.
  3. So no, growing up goes one way. It is not a cycle. (But here is a nice idea: a grown-up can have a baby of their own, so in a family, babies come again and again!)

Try it

  1. It is morning now. What comes after day?
  2. Name the four seasons in order, starting with spring.
  3. Tonight the Moon is a thin crescent and getting bigger. In about a week, will it be bigger or smaller?
  4. Today is Wednesday. What day was it yesterday? What day will it be tomorrow?
  5. Which of these are cycles, and which go one way? A swing going back and forth; an apple getting eaten; the seasons; a candle burning down; the days of the week.
  6. Name one cycle in your own day (something you do again and again in the same order).
  7. On Mars, a day (a sol) is about 40 minutes longer than on Earth. If you lived on Mars, would you get more or less time each day than on Earth?
  8. Grown-up question: why do we always see the same face of the Moon, but different amounts of it lit up?
Answers
  1. Evening and then night, and then day again.
  2. Spring, summer, autumn (fall), winter.
  3. Bigger. A growing crescent becomes a half moon in about a week.
  4. Yesterday was Tuesday. Tomorrow will be Thursday.
  5. Cycles: the swing, the seasons, the days of the week. One way: the apple getting eaten, the candle burning down.
  6. Many answers, for example: wake up, eat breakfast, go to school, come home, have dinner, sleep, and then again the next day.
  7. More time: about 40 minutes extra every sol.
  8. The Moon turns around once in the same time it takes to go around the Earth, so the same side always faces us. But the Sun lights it from different directions as it travels, so the lit part we see changes.

Common mistake

Thinking the Moon really changes shape, or that the Earth's shadow makes the phases. The Moon is always a round ball, half lit by the Sun. We just see different amounts of its lit half. (The Earth's shadow only falls on the Moon in a lunar eclipse, which is rare.)

Calling every repeat a cycle. A cycle comes back to the same place and then goes round in the same order. Growing up, a candle, or a melting snowman do not come back.

Where it leads