20 June will be the longest day of 2021 in the northern hemisphere, midsummer day.

This is the day when the Sun's annual journey 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. This day is counted by astronomers to be the first day of summer in the northern hemisphere.

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 to be the first day of winter.

Sunrise and sunset times

The table to the right lists the sunrise and sunset times in Cambridge around the solstice. At this time of year, noon – the moment when the Sun appears highest in the sky – moves around a minute later each day.

This phenomenon is described by the equation of time, and is caused by slight variations in the length of each day depending on the time of year.

In some months, days can be up to 20 seconds longer or shorter than 24 hours, in a predictable pattern which repeats every year. This arises from two effects:

1. The rate of the Sun's eastward movement through the constellations changes over the course of the year. It is fastest at the solstices, and slowest at the equinoxes.
2. The Earth's orbit around the Sun is not a perfect circle, but is slightly elliptical. This means that its orbital speed changes through the year.

Both these effects very slightly alter the rate of movement of the Sun across the sky, adding or subtracting a few seconds from the time it takes to get from noon on one day to noon the next day.

Clocks, however, continue to run at a constant rate at all times of year. This means that, at those times of year when days are shorter than 24 hours, noon drifts earlier in the day. When days are longer than 24 hours, the noon comes later each day.

In June, each solar day lasts fractionally longer than 24 hours, and so the time of noon moves around a minute later each day.

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 at other times 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 time 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 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 even had a good idea of its size, long before anyone had circumnavigated the globe.

The 2021 solstice

The exact position of the Sun when it reaches its most southerly declination in 2021 will be (J2000.0 coordinates):

The sky on 20 Jun 2021

Source

The circumstances of this event were computed using the DE430 planetary ephemeris published by the Jet Propulsion Laboratory (JPL).

This event was automatically generated by searching the ephemeris for planetary alignments which are of interest to amateur astronomers, and the text above was generated based on an estimate of your location.

Image credit

The Earth, as seen by the Apollo 17 astronauts. © NASA

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