Why Is It Cold in Winter?

The Earth goes round the Sun once a year in a path called its orbit. The orbit is very nearly a circle, but not quite, so the distance changes a little through the year:

	Date (2026)	Distance from the Sun
closest, perihelion	3 January	147.1 million km
farthest, aphelion	6 July	152.1 million km

The difference is 5 million km. That sounds like a lot, but it is only about 3% of the distance. Sunlight does get stronger closer to the Sun, and you can work out by how much.

A 7% change is real, but small. You will see later in this topic that the real reason changes the sunshine Bhaktapur gets by about a factor of two between June and December. And here is the clearest evidence that distance can’t be the cause: the whole Earth is at the same distance from the Sun at any moment, yet in January it is winter in the north and summer in the south. Whatever makes the seasons must be different for the two halves of the Earth.

You learned in Grade 6 that the Earth spins on its axis, an imaginary line through the North and South Poles, once a day, and that the axis is tilted. Tilted compared with what? Imagine the Earth’s orbit as a huge flat sheet with the Sun in the middle. If the axis stood straight up from the sheet, it would be upright. It isn’t: it leans 23.4° away from upright. (Some books give the same tilt as 66.5° measured from the flat sheet instead: 90° − 66.5° = 23.5°. Today’s exact value is 23.44°.)

The important part is this: the axis keeps pointing the same way in space all year. Its north end points almost exactly at Dhruva Tara, the Pole Star, in June and in December, by day and by night Grade 8: Grade 8: The Pole Star. A spinning top behaves like this too: once it is spinning, its axis holds its direction.

So as the Earth goes round the Sun, the tilted axis stays parallel to itself, and something changes:

• Around 21 June, the north end of the axis leans towards the Sun. The northern half of the Earth, with Nepal in it, faces the Sun more directly. It is summer in the north.
• Around 21 December, the Earth is on the other side of the Sun. The axis still leans the same way in space, so now the north end leans away from the Sun. It is winter in the north.
• In March and September, the axis leans sideways, neither towards the Sun nor away. Both halves get the same share.

When the southern half leans towards the Sun, the northern half must lean away. That is why the seasons in the two halves are always opposite: Australia’s summer is December to February, and its winter is June to August.

These four days have names. The June solstice (about 21 June, in early Asar) and the December solstice (about 21 December, in early Poush) are when the axis leans most towards or away from the Sun. Solstice is Latin for “the Sun stands still”: around these days the noon Sun stops getting higher (or lower) and turns back. The March equinox (about 20 March) and the September equinox (about 23 September) are when it leans sideways. Equinox means “equal night”: on these days, day and night are about 12 hours each, everywhere on Earth.

Leaning towards the Sun does two things. The first is about the Sun’s height: how high it climbs in the sky.

You have felt this already. In the early morning and late afternoon, when the Sun is low, its light hardly warms you. At midday, when it is high, it is hot. The light is the same light. What changes is how it falls on the ground.

Hold a torch straight above a table, pointing down, and it makes a small, bright circle. Tilt it, and the same light spreads over a long oval, and the oval is dimmer. The same amount of light is shared out over more table, so each part of the table gets less. Sunlight does exactly the same. A high Sun’s light falls nearly square on the ground and is concentrated on a small area. A low Sun’s light comes in at a slant and is spread out thinly over a large area.

The numbers in the widget come from the shape of the slanting beam. Here they are for a few heights:

Sun’s height	90°	80°	70°	60°	50°	40°	30°	20°
Ground lit by a 1 m beam	1.00 m	1.02 m	1.06 m	1.15 m	1.31 m	1.56 m	2.00 m	2.92 m
Sunlight on each m²	100%	98%	94%	87%	77%	64%	50%	34%

The second row is just 1 ÷ the first: if the beam is spread over 2 m, each meter gets half. (If you have learned trigonometry, the share is the sine of the Sun’s height.) Notice that a high Sun loses very little by being a few degrees off overhead, but a low Sun loses a lot.

There is a smaller second effect: light from a low Sun also passes through much more air on its way to you, and the air scatters and absorbs some of it. That is why you can look at the setting Sun for a moment, but never at the midday Sun.

How high does the Sun get?

On the equinoxes, the noon Sun is straight overhead at the equator. If you go north from the equator, the noon Sun is lower by the same number of degrees as your latitude, your distance north of the equator in degrees. On the June solstice, the north leans 23.4° towards the Sun, so the noon Sun is 23.4° higher than on the equinox; on the December solstice it is 23.4° lower.

$$\text{Sun’s height at noon} = 90^\circ - \text{latitude} \;\; (+\,23.4^\circ \text{ in June, } -\,23.4^\circ \text{ in December})$$

The second thing the tilt does is change how long the Sun stays up.

The Earth is a ball, and the Sun can only light one half of it at a time. The edge between the day half and the night half is a big circle around the Earth. As the Earth spins, each place is carried round once a day on a circle of its own latitude: part of the way through the day half, part through the night half. If more of your circle is in the day half, your day is longer than your night.

Without any tilt, the edge between day and night would run through both poles, cutting every circle of latitude exactly in half: 12 hours of day and 12 of night, everywhere, all year. With the tilt, in June, the north end leans into the day half. More of every northern circle is in daylight, so northern days are long. In December, the north leans into the night half, and northern days are short. Try it.

For Bhaktapur, the model gives (from sunrise to sunset, over a flat horizon; the hills round the valley take a little off each end):

	21 June	20 March, 23 September	21 December
Day length in Bhaktapur	13 h 54 min	about 12 h 8 min	10 h 23 min
Noon Sun’s height	86°	62°	39°

On the equinoxes, the day is a few minutes longer than 12 hours because sunrise and sunset are counted from the moment the Sun’s top edge touches the horizon, and because the air bends the Sun’s light, so it is seen a few minutes before it has really risen.

So on 21 June, Bhaktapur has three and a half hours more sunshine than on 21 December, and at a higher angle. Both effects come from the same tilt, and both work the same way: long days with a high Sun in summer, short days with a low Sun in winter.

Where the Sun doesn’t set

Move the widget to Tromsø, a town in the north of Norway at 69.7° N, inside the Arctic Circle (66.6° N). In June, its whole circle is on the day side: the Sun goes round the sky without setting. From about 20 May to 22 July, Tromsø has the midnight Sun. (Counted from the moment the Sun’s top edge stays above a flat horizon, as the widget does, it is a few days longer; the mountains round the town make it a few days shorter.) In December, its whole circle is on the night side, and from late November to about 15 January the Sun doesn’t rise at all: the polar night. It isn’t completely dark at midday, though: the Sun is only just below the horizon, so there is a few hours of blue twilight.

At the poles themselves, the year is one long day of six months and one long night of six months.

How much do the two effects add up to? You can add up the sunshine falling on each square meter over a whole day, from sunrise to sunset, at every height the Sun passes through. (This is sunshine above the air; clouds and haze take some away.)

Bhaktapur	21 June	21 December
Day length	13 h 54 min	10 h 23 min
Noon Sun’s height	86°	39°
Sunshine on 1 m² in the day	11.3 kWh	5.8 kWh

Twice the sunshine in June. So why isn’t June twice as warm? And why isn’t 21 December the coldest day?

Look at Figure 22.3. The days are shortest on 21 December, but the coldest month is January, not December, and February is still colder than November. The ground, the air and the lakes and seas take time to cool down and warm up. All through December and early January, the ground and air are still losing more heat each night than the short days give back, so they keep getting colder. Only once the days have grown long enough for the gain to match the loss does it stop getting colder. The same happens every day: the warmest part of a sunny day is not noon, when the Sun is highest, but about 2 to 3 in the afternoon. This delay is called the lag of the seasons.

At the other end, Kathmandu’s summer is flattened by the monsoon: from June to September, clouds block much of the sunshine and the rain cools the ground, so June, July and August are all about 24.5 °C, and the hottest days of the year usually come in late May and June, before the rains set in.

The tilt itself changes slowly too, between about 22.1° and 24.5° every 41,000 years, and these slow changes helped start and end the ice ages: see Why Has the Climate Changed Before? in Going Further 9–12.

The model you have built, a lit ball going round the Sun, also explains the Moon. You learned in Grade 7 how the Moon’s phases come about. Briefly: the Sun always lights one half of the Moon. As the Moon goes round the Earth, once in about 29.5 days, we see different amounts of its lit half. At the new moon (aunsi), the Moon is between the Earth and the Sun, and its lit half faces away from us. At the full moon (purnima), the Moon is on the far side of the Earth from the Sun, and we see its whole lit half.

You also learned in Grade 7 that an eclipse of the Sun happens when the Moon comes exactly between the Sun and the Earth, so it can only happen at a new moon; and that an eclipse of the Moon happens when the Moon passes through the Earth’s shadow, so it can only happen at a full moon. But there is a new moon and a full moon every month. Why isn’t there an eclipse every month?

The Moon’s tilted orbit

The answer is another tilt. The Moon’s orbit round the Earth is not on the same flat sheet as the Earth’s orbit round the Sun: it is tilted to it by about 5°. That seems small. But the Moon is 384,000 km away, 60 times the Earth’s radius, and at that distance 5° lifts it about 34,000 km above or below the sheet. The Earth’s shadow, out where the Moon crosses it, is only about 9,000 km across in its dark middle. So at most full moons the Moon passes above or below the shadow and misses it completely, and at most new moons the Moon’s shadow passes above or below the Earth.

The Moon’s orbit crosses the Earth’s flat sheet at two points, on opposite sides. Only when the line joining those two points points at the Sun, or nearly, can the new or full Moon be in line with the Earth and the Sun. As the Earth goes round the Sun, that happens about twice a year, for about five weeks each time. These are the eclipse seasons. Every eclipse season has at least one eclipse of the Sun somewhere on Earth (often only partial) and at least one of the Moon two weeks before or after it (sometimes only a faint one, when the Moon passes through just the pale outer edge of the Earth’s shadow). That is why eclipses come in twos or threes, and the groups come about six months apart.

An eclipse of the Moon can be seen from the whole night side of the Earth at once: anyone who can see the Moon sees it darken. An eclipse of the Sun is different. The Moon is small, and its dark shadow on the Earth is at most a few hundred kilometers wide, so a total eclipse of the Sun is seen only along a narrow path; outside it, people see a partial eclipse, with only part of the Sun covered.

There is also a third kind. The Moon’s distance from the Earth changes, between about 356,000 and 407,000 km. When the Moon is far, it looks a little smaller than the Sun and can’t quite cover it, and a ring of the Sun shows round it: an annular (ring) eclipse. That is what happened on 21 June 2020. The ring was seen along a narrow path across northern India, through Dehradun and Joshimath in Uttarakhand, just west of Nepal; in Kathmandu nearly nine-tenths of the Sun was covered.

• Is the Earth Really Getting Warmer? The seasons come and go every year. How scientists tell a real, lasting warming from the ups and downs of the weather.
• Why Is Pokhara So Wet and Mustang So Dry? The Sun heats the Earth unevenly, at the equator more than at the poles. How that sets the winds, and how the winds and mountains decide a place’s climate.
• How Do Rocks Tell Time? The Earth’s year and day are its clocks; rocks keep a much longer one.