what it is, when it falls and the astronomical explanation

The summer solstice 2024 will fall on Thursday June 20th at 10.51pm Italian time: this is the moment when the sun’s rays fall perpendicular to the Tropic of Cancer. marking the beginning ofastronomical summer in the Northern Hemisphere and astronomical winter in the Southern Hemisphere. It’s about the earliest solstice in the last 228 yearssince 1796. But what specifically happens between Earth and Sun and why is this anniversary called the “longest day of the year”?

What happens during the summer solstice

Basically, the summer solstice is the time during the year when the Northern Hemisphere is most “leaning” towards the Sun, resulting in a maximum of daylight hours and the height of the Sun.

The Earth, in addition to rotating around its own axis – the imaginary line that connects the North Pole to the South Pole – also orbits around the Sun on an imaginary plane, whose intersection with the celestial sphere is called ecliptic. The Earth’s axis is tilted by 23°27′ with respect to the perpendicular to the plane of the Earth’s orbit, which is why we have thealternation of seasons. If the Earth’s axis were perpendicular to the plane of the Earth’s orbit, the sun’s rays would reach the Earth perpendicular to the rotation axis and there would be no seasons, no equinoxes or solstices!

The importance of the summer solstice lies in the fact that at that time solar rays reach their maximum inclination with respect to the horizon in the northern hemisphere. This has three main consequences in the Northern Hemisphere:

  • the Sun reaches its maximum height in the sky;
  • the day has its maximum duration;
  • the points where the Sun rises and sets reach their greatest proximity to the north.

The opposite happens in the southern hemisphere, where the Sun reaches its minimum inclination, the day its minimum duration and sunrise and sunset occur at maximum proximity to the south.

In particular, in the December solstice the Sun reaches the zenith along the Tropic of Capricorn in the southern hemisphere (the light falls perpendicular to the ground and objects do not send it further), while above the Arctic Circle the Sun is unable to rise above the ‘horizon. If on the day of the summer solstice at midday we were exactly along the Tropic of Cancer, the Sun would be right above our head; this is because the sun’s rays are perpendicular to this parallel, which has a latitude (north) exactly equal to the inclination of the Earth’s axis, 23°27′.

Credit: NASA

This moment of transition marks thestart official ofsummer in the Northern Hemisphere and winter in the Southern Hemisphere. During the June solstice the Arctic Circle is completely illuminated, while the Antarctic Circle is in darkness. Consequently, in all locations south of the equator the day lasts less than 12 hours while in all locations north the day lasts more than 12 hours.

During the solstices, the Earth reaches a point where its tilt is at a maximum angle to the plane of its orbit, causing one hemisphere to receive more daylight than the other. Credit: NASA – Genna Duberstein

When does the summer solstice fall 2024: the date and time

In common speech it is used to say that the summer solstice (like the winter one) is an entire day, but in reality from an astronomical point of view it is a precise moment. The same goes for the two equinoxes (the spring and the autumn). In 2024 we will witness this event on Thursday 20 June and more preciselymind you 10.51pm (Italian time).

Why the summer solstice 2024 falls on the 20th and not the 21st

The summer solstice n0n occurs on the same date and time every year (may vary between June 20th and 21st) because the Earth completes one rotation around the Sun not in exactly 365 days, but in approximately 365 days and 6 hours. This small gap causes the solstice moment to occur when the Earth is in different positions along its orbit. This gap is also at the origin of the need to have a leap year every 4 years. In fact, it is no coincidence that the solstice falls on June 20th, typically in leap years: the last time was in 2020 and the next one will be in 2028. The summer solstice 2024 is particular because it is the one that happens first (in terms of time and date) from 1796.

Meaning and origins

The beginning of summer has always been a symbol of returning to life and the height of the summer, especially for the people of the far north and far south of the globe where periods of prolonged darkness are regularly experienced throughout the year.

The holidays linked to the summer solstice originate in very ancient times but are still celebrated today throughout the world with rites, parties and banquets. The term solstice in fact derives from Latin solstitium which means “stopping of the Sun” and which indicates the fact that twice a year (during the summer and winter solstice) the Sun seems to stop since it reaches the points of respectively maximum And minimal declination in the celestial vault (i.e. the angle of the sun’s rays with the equatorial plane).

Although Stonehenge is considered the most famous place in the world linked to ancient astronomical rites, there does not seem to be enough evidence to attribute to this site a certain meaning linked to the passage of time and the advancing of the seasons.

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