At the Italian liceo linguistico (language-focused high school), in Year 1, the natural sciences programme is leaving (or has just left) the basic chemistry part to move into earth sciences, often coordinated with geography: the motions of the Earth, atmosphere, hydrosphere, geomorphology. The motions of rotation and revolution and the seasons are typically explained between the end of September and October, and the first test arrives between the end of October and mid-November, so five to eight weeks from now. At the liceo linguistico this test is almost always multiple-choice: closed questions plus two open ones, not an essay to write in prose. It is worth setting up the topic properly now, not catching up on it the night before.
The concrete problem: “closer to the Sun” explains nothing
What I see most often, when a student brings me this topic, is a mistaken belief that seems reasonable: in summer it is hot because the Earth is closer to the Sun, in winter it is cold because it is farther away. It is an intuitive idea, but it is wrong, and in a multiple-choice test it is exactly the trap answer that makes you lose the mark.
The Earth-Sun distance varies very little during the year, and for a reason that always surprises: in early January, in the middle of winter in the northern hemisphere, the Earth is at perihelion, that is, at the point of its orbit closest to the Sun. It is the exact opposite of what intuition would suggest. If the seasons depended on distance, January should be the hottest month of the year in the northern hemisphere, and it is not.
The real cause of the seasons is something else: the tilt of the Earth’s axis of 23.5° relative to the plane of the orbit (the plane of the ecliptic). This tilt stays fixed in space while the Earth goes round the Sun, and for this reason, depending on the point of the orbit where the Earth is, the northern or the southern hemisphere points more directly towards the Sun. Keep it in mind as a key sentence to put in an open question: the seasons exist because of the tilt of the axis, not because of the distance from the Sun.
Step by step: from the two motions to the seasons
1. The motion of rotation: day and night
The Earth spins on itself, around an imaginary axis passing through the two poles, in about 24 hours (the solar day). It is this motion that produces the alternation of day and night: at the moment when one part of the Earth’s surface is lit by the Sun, the opposite part is in shadow. The rotation goes from west to east, and this is also why the Sun appears to us to rise in the east and set in the west.
2. The motion of revolution: the year
The Earth also travels along an elliptical orbit around the Sun, completing it in about 365 and a quarter days (the solar year, hence the famous leap years, which make up the quarter of a day accumulated every four years). It is the motion of revolution, not that of rotation, that determines the year and, together with the tilt of the axis, the seasons.
3. The tilt of the axis and the Sun’s rays
This is where the two motions meet to produce the seasons. Imagine the Earth’s axis as a rod tilted at 23.5° from the perpendicular to the plane of the orbit. While the Earth travels along the orbit, the axis always keeps the same direction in space: it always points, more or less, towards the Pole Star. The result is that:
- when the northern hemisphere is tilted towards the Sun, the Sun’s rays hit it more perpendicularly (more energy on the same surface) and the day lasts longer than 12 hours: it is the northern summer;
- six months later, the Earth is at the opposite point of the orbit and the same northern hemisphere is tilted away from the Sun: the rays arrive more obliquely (less energy on the same surface) and the day lasts less than 12 hours: it is the northern winter.
While it is summer in the northern hemisphere, it is winter in the southern hemisphere, and vice versa: that is why in Australia or Argentina Christmas falls in the middle of summer.
4. Solstices and equinoxes
Along the orbit there are four points that mark the (astronomical) start of the seasons:
- summer solstice (around 21 June in the northern hemisphere): the longest day of the year, maximum tilt towards the Sun;
- winter solstice (around 21 December): the shortest day of the year, maximum tilt away from the Sun;
- spring equinox (around 20 March) and autumn equinox (around 22 September): day and night have the same length, 12 hours each, everywhere on Earth. It is the moment when the Earth’s axis is tilted neither towards nor away from the Sun relative to the Earth-Sun line.
For the exact dates of this year, which can shift by a day from these general indications, check the calendar or your school textbook: what matters here is the mechanism and the order of events, not the precise day.
5. Why the length of the day changes with latitude
Another point that is often dismissed by rote, without understanding it, is why in Milan in summer the sun sets at 21:00 and in Norway, in the same period, it hardly sets at all. The explanation is always the tilt of the axis: the further you move towards the poles, the more the difference between the length of the summer day and that of the winter day grows. At the equator, on the other hand, day and night always last about 12 hours each, in every season, because there the tilt of the axis has a minimal effect on the angle of illumination during the year. Beyond the polar circles, at certain times of year, the Sun never sets (midnight sun) or never rises for whole days: it is the same cause, taken to the extreme.
6. The climate zones
As a consequence of this mechanism, the Earth’s surface is divided into bands that receive, on average, different amounts of solar energy during the year:
- torrid zone (between the two tropics): the Sun’s rays fall almost perpendicular all year, little seasonal variation in temperature;
- temperate zones (between the tropics and the polar circles): here the seasons are lived as we know them, with a marked variation between summer and winter;
- frigid zones (beyond the polar circles): the rays always arrive very obliquely, and the extreme phenomena of continuous day or night appear at certain times.
7. Time zones: a consequence of rotation, not of revolution
Time zones exist because the Earth rotates: as it turns, different points of the surface face the Sun at different moments, so there can’t be a single “right” time for the whole planet based on the position of the Sun in the sky. To simplify, the Earth’s surface is divided into 24 zones, one for each hour of the day, each about 15° of longitude wide:
Moving east you gain time (local time is ahead), moving west you lose time (local time is behind). It is the same geometric principle behind the first worked exercise below.
The typical mistake to avoid
The mistake that almost always costs the mark in a multiple-choice test is exactly the one we opened with: attributing the seasons to the Earth-Sun distance instead of to the tilt of the axis. If in a question you read an option that speaks of “the Earth closer to the Sun in summer”, always discard it: it is almost certainly the distractor built on purpose on this misunderstanding, which is also the most common at the level of common sense, not just among students.
A second frequent mistake is confusing rotation and revolution when attributing the effect: rotation produces day and night (24 hours), revolution plus the tilt of the axis produce the seasons (365 days). If a question asks you “which motion produces the seasons”, the correct answer cites both elements, revolution and tilt of the axis, not rotation alone.
Two worked exercises
Exercise 1: difference in local time between two cities.
Two cities lie at longitude East and West respectively. Calculate the difference in local time between the two.
Method. The total longitude separating the two cities is obtained by adding the two longitudes, because one is east of the reference meridian (Greenwich, ) and the other is west:
Each time zone corresponds to and to one hour of difference, so:
The city at East is ahead (closer to dawn relative to the reference), the one at West is behind. Result: when it is 14:00 in the first city, it is 8:00 on the same day in the second.
A note before the second exercise: on earth sciences we don’t yet have a dedicated quiz, but if you need to practise answering multiple-choice questions against the clock (which is half the problem in a multiple-choice test), the high school practice tests in maths and physics serve that purpose. The correct answer is free, the guided solution unlocks with the first lesson.
Exercise 2: which hemisphere is it in?
On 21 December, in a certain city, the day lasts about 15 hours against 9 hours of night, and at midday the Sun is very high in the sky. Which hemisphere is that city in? And which season is it experiencing?
Method. Two pieces of data, to be read in this order.
- The day lasts longer than the night (15 against 9) and the Sun at midday is high: these are the conditions of a hemisphere tilted towards the Sun, that is summer. I rule out winter straight away, whatever the date.
- The date is 21 December, that is the northern winter solstice. In the northern hemisphere that day is the shortest of the year: 15 hours of daylight are impossible.
- The two facts fit together in only one way, because the seasons in the two hemispheres are always opposite: 21 December is the southern summer solstice.
Result: the city is in the southern hemisphere, and is experiencing summer. The 15 against 9 also says something more about the latitude: at the equator the day would last 12 hours even on 21 December, so that city is at a middle latitude, not an equatorial one: the further you move from the equator, the more the imbalance between day and night at the solstice grows.
Geography, and the other half of the Year 1 programme
This earth sciences module is usually coordinated with geography: the same concepts (time zones, climate zones, geographic coordinates) come back there with a more descriptive and less physical approach. Studying them together, with the same language (tilt of the axis, not “closer/farther”), saves you from having to learn them twice in different ways. To see where it leads in Year 5, there is Astronomical geography in the final year.
If in Year 1 you are also tackling the basic chemistry part of the programme (density, states of matter, mixtures), it is not the subject of this article: you will find it covered separately in Year 1 science at the classical high school and, for the technical track, in chemistry in Year 1 at the technical school. They are the “other half” of the Year 1 science programme, with different logic and testing from this one.
On earth sciences and basic astronomy we don’t yet have a dedicated quiz in our catalogue: in the high school practice tests you will find the maths and physics ones, where the correct answer is free and the guided solution unlocks with the first lesson. They are training in reasoning against the clock, not a revision of this topic: you do that on the seven sections above.
Science lessons are online, one to one, and often the block is not the theory itself but connecting the pieces (rotation, revolution, tilt) into a single piece of reasoning to use on any question in the test. If you want a hand, the first chat is free: contacts (a free 30-minute introductory call).
