June Solstice

Today, it takes place the June Solstice, which is the Summer Solstice in the Northern Hemisphere, and in the Southern Hemisphere is the Winter Solstice.

A solstice (from Latin solstitium, meaning "sun-stopping") happens when the sun's zenith is at its furthest point from the equator.

On the June solstice, the sun reaches its northernmost point at about 23.5ºN. In addition, the Earth’s North Pole tilts directly towards the sun. It is also known as the northern solstice because it occurs when the sun is directly over the Tropic of Cancer in the Northern Hemisphere.

On this day, the midnight sun is visible throughout the night, in all areas from just south of the Arctic Circle to the North Pole. Meanwhile, on the other side of the planet, south of the Antarctic Circle there is the polar night: no sunlight at all, on the June Solstice.

Here (Barcelona, Catalonia), the sun is rising at 6:18 am and setting at 9:28 pm, thus making the duration of the day 15 hours and 10 minutes. It is, indeed, the longest day of the year.

However, the earliest sunrise already took place at 6:17 am on June 12th-18th, while the latest sunset will take place at 9:29 pm on June 23rd-July 1st.

What is the reason for this?

It all comes down to how we measure time.

The elliptical shape of the Earth’s orbit around the Sun and the tilt of the rotation axis of the Earth makes the Earth run faster when it is closest to the sun in January (perihelion), and slower when it is furthest away from the sun in July (aphelion).

This means that the length of a solar day — the time between two solar noons (when the sun is at the highest point in the sky) — is not always the 24 hours we measure on a clock. Indeed, it is slightly more than 24 hours toward the solstices; and slightly less than 24 hours toward the equinoxes. For instance, the longest natural day is about 51 seconds longer than the shortest.

Therefore, the clock time at which the sun reaches its highest point slowly drifts backward and forwards as the months progress. This has an effect on the times of sunrise and sunset. The earliest sunrise occurs a number of days before the longest day and the latest sunset a number of days after the shortest.

Just as a solar day is not always exactly 24 hours, a solar year is slightly longer than 365 days — 365.242 days to be exact. This irregularity prompted the invention of leap years to stop our calendars falling too far out of sync with the seasons.

You can read more about it here and here.