Solstice: The Celestial Harmony That Marks Our Time

The Earth Rotates: Rotation, Revolution and the Secrets of the Seasons

The seasons do not depend on Earth-Sun distance (in summer we are actually further away!), but on the Earth's axial tilt of approximately 23.5 degrees. This tilt determines how perpendicularly solar rays strike the surface, creating the cycles of solstices and equinoxes that govern our calendar.

The movements of the Earth-Sun system are fundamental in governing the rhythm of life on Earth: days, months, seasons and years are all based on these phenomena. They influence both the climate and the hours of daylight, allowing us to distinguish the dark winter from the bright summer and to calculate the civil calendar.

Over the course of a day, the Earth completes a full rotation on its axis, generating the day-night cycle. Over the course of a year, it completes a revolution around the Sun following an elliptical orbit with a small eccentricity of 0.017. This elliptical shape means that, at certain points in its path, the Earth is at its maximum distance from the Sun (aphelion) -- which occurs at the beginning of July -- and at its minimum distance from the Sun (perihelion) -- which occurs at the beginning of January.

Yes, you read that correctly. Contrary to what many people think, during summer the Earth is at the point of its orbit farthest from the Sun. This may seem counterintuitive, since we associate summer with warmth and winter with cold. However, if we lived in the opposite hemisphere, our perception of seasons would be reversed: in the southern hemisphere, when it is summer in the northern hemisphere, it is winter! This means that the shortest distance between Earth and Sun does not correspond to summer in the northern hemisphere but to winter.

So, what is the true mechanism that determines the seasons?

The Earth rotates on its axis with a tilt between 22.1 degrees and 24.5 degrees relative to the orbital plane. The direction of this tilt undergoes a precession, moving slowly along a circle over a period of approximately 26,000 years. The tilt of the Earth's rotational axis, combined with its orbit around the Sun, means that at any given moment one hemisphere is more tilted towards the Sun than the other. It is this tilt that determines the intensity of solar heating: when a hemisphere is more tilted towards the Sun, it receives a greater quantity of perpendicular solar rays on its surface, giving rise to summer.

This is why areas near the equator, where the sun's rays are always perpendicular to the surface, are less affected by the change of seasons, while those closer to the poles, where the angle of solar rays varies more significantly throughout the year, are more affected.

Solstices and Equinoxes: The Celestial Rhythms That Mark Time

The Earth's axis is tilted, and this affects not only our seasons but also the length of day and night. Although it may seem like the Sun moves around the Earth, in reality this is an apparent motion called the ecliptic.

The ancients observed that the Sun's height in the sky varied throughout the year, reaching its maximum in June and its minimum in December in the northern hemisphere. This change in height appeared to stop for a few days at the maximum and minimum points, hence the term "solstice", which literally means "the Sun stands still."

At the winter solstice, the sun's rays are perpendicular to the Tropic of Capricorn (latitude 23.27 degrees South) and at the summer solstice they are perpendicular to the Tropic of Cancer (latitude 23.27 degrees North). Because of these variations, the arc that the Sun traces across the sky is greater in summer and smaller in winter, thus affecting the length of day and night.

Even though we consider the solstice as a whole day, it actually lasts only an instant. For example, in 2023 the summer solstice was on June 21st at 2:58 PM, while the winter one occurred on December 22nd at 3:27 AM, Italian time.

The dates of the solstices can vary between December 20th and 23rd for winter, due to the imperfect correspondence between the solar year (the time it takes for the Sun to return to illuminating the same point on Earth) and our calendar. This discrepancy is corrected every four years by adding a day in the leap year (the famous February 29th).

If we imagine the solstices as the maximum and minimum points of a cycle, there must be intermediate points, right? Exactly! There are days when the length of day and night is the same. These are called equinoxes and they occur in the transitional seasons of spring and autumn. During an equinox, anywhere in the world, there are exactly 12 hours of light and 12 hours of darkness.

The Midnight Sun: When Day Challenges Night

Imagine for a moment looking at the sky at midnight and seeing the sun shining high. This is not a dream, but a real phenomenon called the "midnight sun", a unique experience lived by people who inhabit the polar regions.

The midnight sun is a fascinating phenomenon that occurs at latitudes equal to or above 66 degrees 33 minutes in both hemispheres. This happens when the sun, instead of following the usual east-to-west path above our heads, completes an incredible 360-degree rotation over the course of 24 hours. Yes, you understood correctly -- the sun goes around in a circle in the sky!

This occurs near the geographic north pole, between the spring equinox and the autumn equinox. During these 6 months, the sun never sets, providing an entire semester of light. The intensity and duration of this phenomenon vary depending on latitude: it can last approximately 6 months near the pole, while near the polar circle, the experience can last 24 hours or even nearly 48 hours!

This strange and wonderful scenario is quite common for many people living beyond the Arctic Circle, for example in Siberia, Canada, Norway and Iceland. For these places, the term "Midnight Sun" identifies the day closest to the solstice when the Sun never sets. Scandinavian populations celebrate this phenomenon with festivals, songs and fairs: the beauty of this experience also has a beneficial effect on regional tourism.

However, there's a small twist. Due to a phenomenon called refraction, the midnight sun can also be seen from regions at latitudes below those of the polar circles, such as Iceland. In these regions, the sun remains visible above the horizon even when it shouldn't be possible.

Intriguing, right? But before packing your bags to go see this phenomenon, remember that there are consequences. Having the sun always high in the sky can cause imbalances in the sleep-wake rhythms of humans and animals. The human body regulates the sleep-wake cycle through the hormone melatonin, whose production is favoured by the absence of light. With the sun always present, falling asleep can become a bit more difficult!

Polar Night and Polar Twilight: A Meeting of Light and Shadow

While the "midnight sun" may seem fascinating, its opposite phenomenon, known as the "polar night", brings with it a period of total darkness during the winter months. Remember, however, that not all regions that experience the midnight sun face a complete polar night. For a true polar night, the sun must be at least 18 degrees below the horizon even during the day, which can only occur at latitudes above 84 degrees 33 minutes.

A place where a complete polar night can be experienced is the Svalbard Islands archipelago, which is part of Norwegian territory. Located north of Russia, these islands host numerous astronomical observatories and scientific stations. Here, during the long polar night (approximately 2 months per year), both astronomy enthusiasts and professional astronomers can observe the spectacular northern lights 24 hours a day.

At lower latitudes, we experience what is called "polar twilight". During this phenomenon, the twilight light allows outdoor activities despite the absence of the sun. Most human settlements experience a few days of polar twilight per year.

Polar Night and Health: A Complex Relationship

But what happens to those who live for months without sunlight, as occurs during the polar night? Sunlight is in fact fundamental for the functioning of various organic and psychological processes, including the release of hormones that regulate our sleep-wake cycle. Studying physics and biology helps understand these complex phenomena.

According to various studies, the lack of sunlight can produce symptoms similar to those of depression: fatigue, apathy, listlessness and a growing craving for sugars. This condition is known as Seasonal Affective Disorder (SAD). A recent survey revealed that approximately 50% of Finns experience these symptoms during winter. To combat SAD, many people use special phototherapy lamps that emit a light intensity similar to that of the sun at dawn. This solution appears to be effective in 75% of cases.

The sun is also fundamental for the production of vitamin D, which is essential for bone calcification. Vitamin D deficiency can lead to various bone problems and make teeth weaker and more vulnerable to cavities. Although vitamin D can be taken orally, most of it is produced by our skin after approximately 20 minutes of sun exposure. Consequently, populations living at extreme latitudes are often advised to take vitamin D supplements.

These incredible natural phenomena, like the polar night and the midnight sun, can bring unique challenges and opportunities. Have you ever experienced one of these phenomena in your life?

FAQ

Why don't seasons depend on Earth-Sun distance?

In summer, Earth is actually further from the Sun (aphelion in July). Seasons depend on Earth's axial tilt of 23.5 degrees: when a hemisphere tilts towards the Sun, it receives more perpendicular and intense rays, creating summer.

What happens during a solstice?

At the summer solstice (June 21st), the northern hemisphere tilts maximally towards the Sun, creating the longest day. At the winter solstice (December 22nd), the opposite occurs, with the shortest day. Technically, the solstice lasts a precise instant, not an entire day.

What is the midnight sun?

It is a phenomenon occurring beyond the Arctic Circle, where the sun does not set for months during summer. At the North Pole, the sun remains above the horizon for approximately six consecutive months, completing a full 360-degree circle in the sky every 24 hours.

Can polar night affect health?

Yes, prolonged lack of sunlight can cause Seasonal Affective Disorder (SAD), with symptoms similar to depression. About 50% of Finns suffer from it in winter. Phototherapy with special lamps is effective in 75% of cases, along with vitamin D supplements.