Lesson 1 of 4

Day and night.

About 20 minutes · 5 practice questions

Read the full article: Seasons

The Earth spins on its axis: why half of it is always lit, how many degrees the Sun moves in an hour, why day and night are equal everywhere at the equinox, how high the noon Sun stands, and what the midnight Sun is.

Step 1 of 5 · A turning globe

Step 1 of 5

A turning globe

In the Scene the Earth goes round the Sun once a year, with its axis tilted 23.4° and always pointing the same way in space. But day and night come from another, much faster motion: the Earth spins about that axis.

The Sun is very far away, so its rays arrive almost parallel and light exactly half the globe at any moment. The line between the lit half and the dark half is called the terminator.

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Step 2 of 5

Degree by degree

Look inside is open now to Sunlight up close: the Scene closes in on the Earth. The white line is the terminator, the dashed line is the latitude where the Sun stands overhead at noon, and the bright ring is the latitude you chose, $32^\circ$ ⁠ north, about Tel Aviv. Its part on the lit side is the day, and its part on the dark side is the night.

The Earth turns from west to east, so the Sun seems to rise in the east, cross the sky and set in the west. How fast?

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Step 3 of 5

Equal night

Day $78$ ⁠ of the year is the March equinox. The dashed line, over which the Sun is overhead, coincides with the equator. The latitude chosen now is Singapore, $1.3^\circ$ ⁠ north, and the label under the globe prints the length of the day and the height of the noon Sun.

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Step 4 of 5

The noon Sun

The length of the day is one thing. The height of the Sun at noon, its angle above the horizon, is another. The rays are parallel, but the Earth’s surface curves: for every degree of latitude you move away from where the Sun is overhead, it stands one degree lower. If the Sun is overhead at latitude $\delta$ ⁠, then at latitude $\varphi$ ⁠:

$h = 90^\circ - |\varphi - \delta|$ ⁠

The Scene is closed back to the orbit. Work it out yourselves.

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Step 5 of 5

The midnight Sun

Now it is the June solstice, day $171$ ⁠. The dashed line has moved north to $23.4^\circ$ ⁠, the Tropic of Cancer, and the terminator no longer runs through the poles: it reaches $23.4^\circ$ ⁠ beyond the North Pole. In Tel Aviv the ring is no longer cut evenly, and more than half of it is on the lit side.

What happens far to the north?

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Interactive scene

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Each step sets up the Scene for you, and a control marked “fixed” stays put for that step.

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Practice · Choose

Why are there day and night?

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Practice · Calculate

The Earth turns $360^\circ$ ⁠ in 24 h. By how many degrees does the Sun seem to move across the sky each hour?

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Practice · Predict, then look

It is the March equinox, and we are in Singapore, almost on the equator, with a day of about 12 h. How long is the day on the same date in Tromsø in Norway, at latitude $69.6^\circ$ ⁠ north?

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Practice · Calculate

On the same equinox, how high above the horizon does the noon Sun stand in Tel Aviv, at latitude $32^\circ$ ⁠ north?

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Practice · Predict, then look

Now it is the June solstice, and the latitude is $80^\circ$ ⁠ north, in Svalbard. How long will the Sun be above the horizon?