June solstice

21 June will be the longest day of 2019 in the northern hemisphere, midsummer day.

This is the day of the year when the Sun’s annual passage through the constellations of the zodiac carries it to its most northerly point in the sky, in the constellation of Cancer at a declination of 23.5°N.

On this day, the Sun is above the horizon for the longer than on any other day of the year in the northern hemisphere. This is counted by astronomers to be the first day of summer.

In the southern hemisphere, the Sun is above the horizon for less time than on any other day of the year and astronomers define this day to be the first day of winter.

Sunrise and sunset times

The table to the right lists the sunrise and sunset times in London around the solstice. In June the Sun’s right ascension changes slightly faster than at other times of year, which means that each solar day lasts fractionally longer than 24 hours and the time of noon moves around a minute later each day.

This phenomenon is described by the equation of time.

The shift also affects sunrise and sunset times, and means that the latest sunset and earliest sunrise do not occur on the day of the solstice itself. Instead, the earliest sunrise occurs a few days beforehand, and the latest sunset is a few days later.

Solstice geometry

Solstices occur because the axis of the Earth’s spin – its polar axis – is tilted at an angle of 23.5° to the plane of its orbit around the Sun.

The direction of the Earth’s spin axis remains fixed in space as it circles around the Sun, while the Earth’s sight line to the Sun moves through the constellations of the zodiac. As a result, sometimes the Earth’s north pole is tilted towards the Sun (in June), and sometimes it is tilted away from it (in December). This gives rise to the Earth’s seasons:

The date of the solstice

The Earth orbits the Sun once every 365.242 days, and this is the time period over which the cycle of solstices and equinoxes, and consequently all the Earth’s seasons, repeat from one year to the next.

In any year which is not a leap year, the solstices occur roughly 5 hours and 48 minutes – just under a quarter of a day – later from one year to the next.

This is why the seasons would drift later in the year if it was not for an additional day being inserted inserted into every fourth year on 29 February.

The chart below shows the day of the month when the June solstice falls in each year. The gradual drift of the four-year cycle earlier in the month is due to the equinoxes repeating 12 minutes less than a quarter of a day later each year.

In the Gregorian calendar, this is fixed by omitting leap years in three out of every four century years, e.g. 1700, 1800 and 1900, but not 2000.

Measuring the radius of the Earth

At the solstice, the Sun appears overhead at noon when observed from locations on the tropic of Cancer, at a latitude 23.5°N.

This fact was used by the ancient Greek astronomer Eratrosthenes in around 200 BC to work out the radius of the Earth for the first time. He knew that at midsummer, the Sun appeared exactly overhead in the Egyptian city of Swenet (now Aswan), because its light shone right down to the bottom of deep wells.

He travelled to Alexandria, on the Egyptian north coast, at a distance of 5,000 stades from Swenet. Here, he used a stick in the ground to determine that the Sun was seven degrees away from the zenith at midsummer, implying that a distance of 5,000 stades around the circumference of the Earth corresponded to a distance of seven degrees around the Earth’s curved surface.

Thanks to this experiment, the ancient Greeks were well aware that the Earth was spherical and had a very good idea exactly how big it was, long before anyone had circumnavigated the globe.

The 2019 solstice

The exact position of the Sun when it reaches its most southerly declination in 2019 will be (J2000.0 coordinates):

This entry in the observing calendar was provided by In-The-Sky.org

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