The Northern Solstice.

The Northern Solstice will fall on Sunday 21 June this year (2026), the day on which the Sun rises highest in the sky and the longest day of the year in the Northern Hemisphere (where it is the Summer Solstice) and the day on which the Sun rises lowest in the sky and the shortest day in the Southern Hemisphere (where it is the Winter Solstice). Up until this date the days have been growing shorter in the Northern Hemisphere and longer in the Southern Hemisphere since the Southern Solstice in December last year (which is the Summer Solstice in the Northern Hemisphere and Winter Solstice in the Southern Hemisphere), but after it the situation will be reversed, with days growing steadily longer in the Northern Hemisphere and shorter in the Southern Hemisphere until the next Southern Solstice in December. 

The solstices are entirely a product of variation in the Earth's rotation on its axis, which is at an angle of 23.5° to the plain of the Earth's orbit about the Sun. This means that in December the Earth's Southern Pole is tilted towards the Sun, while the Northern Pole is tilted away from it. This means that around the Southern Solstice the Southern Hemisphere is receiving radiation from the Sun over a longer part of the than the Northern, and at a steeper angle (so that it to pass through less atmosphere to reach the planet), creating the southern summer and northern winter.

The tilt of the Earth at the Northern Solstice. Wikimedia Commons.

The solstices are fairly noticeable astronomical events, and tied to the seasons which govern the life cycles of life on Earth, and they have been celebrated under different names by cultures across the globe, but most notably by those at higher latitudes, who are more profoundly affected by the changes of the seasons.

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The Earth approaches perihelion.

On Saturday 3 January 2025, at 5.15 pm, GMT, the Earth will reach its perihelion; the closest point on its orbit to the Sun, when it will be 147 099 894 km from the Sun. This is because the Earth's orbit is not a perfect circle, but varies by 3.3% over the course of a year. This perihelion distance varies each year; in 2025 the Earth reached 147 103 686 km from the Sun on 4 January and in 2027 it will reach 147 104 592 km from the Sun on 3 January.

The Earth's Aphelion and Perihelion. My Dark Sky.

This means that the Earth is at its closest to the Sun in the middle of the Northern Hemisphere's winter, counter-intuitive to most of the planet's population. This is, however, purely coincidental; the Earth's seasons are not caused by its distance from the Sun, which only varies by 3.3%, but rather by the tilt of the planet. The Earth is currently tilted at an angle of 25.5° to its plane of orbit (this varies on a timescale of tens of thousands of years, but remains fixed from the point of view of any human observer), causing the Sun to appear to rise higher and lower in the skies of each hemisphere as the year goes by. 

In the northern winter the Southern Hemisphere is tilted towards the Sun, so that the days are longer there (and in around the Southern Solstice in December, permanently above the horizon at the South Pole). In addition, the Sun being directly overhead means that the energy from the Sun has to pass through less of the atmosphere before it reaches the surface of the Earth, so that less energy is lost to the atmosphere, causing greater surface warming in the hemisphere pointed towards the Sun.

How the tilt of the Earth relative to its plane of orbit causes the seasons. ESA.

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The Southern Solstice.

The Southern (or December) Solstice this year falls on Sunday 21 December, with the Sun reaching its southernmost point in the sky at 3.04 pm GMT. This is the shortest day of the year in the Northern Hemisphere, where it is known as the Winter Solstice and the longest day in the Southern Hemisphere, where it is known as the Summer Solstice. At very high latitudes the sun may not rise (Northern Hemisphere) or set (Southern Hemisphere) for several weeks on either side of the Southern Solstice.

The solstices are entirely a product of variation in the Earth's rotation on its axis, which is at an angle of 23.5° to the plain of the Earth's orbit about the Sun. This means that in December the Earth's Southern Pole is tilted towards the Sun, while the Northern Pole is tilted away from it. This means that around the Southern Solstice the Southern Hemisphere is receiving radiation from the Sun over a longer part of the than the Northern, and at a steeper angle (so that it to pass through less atmosphere to reach the planet), creating the southern summer and northern winter.

The tilt of the Earth during the December Solstice. Wikimedia Commons.

The solstices are fairly noticeable astronomical events, and tied to the seasons which govern the life cycles of life on Earth, and they have been celebrated under different names by cultures across the globe, but most notably by those at higher latitudes, who are more profoundly affected by the changes of the seasons.

See also...

The September Equinox

The September Equinox falls on Monday 22 September this year (2025), when the day and night will be of equal length in both of the Earth's hemispheres. The Earth spins on its axis at an angle to the plain of the Solar System. This means that the poles of the Earth do not remain at 90° to the Sun, but rather the northern pole is tilted towards the Sun for six months of the year (the northern summer), and the southern pole for the other six months (the southern summer). This means that twice a year neither pole is inclined towards the Sun, on days known as the equinoxes.

Simplified diagram showing the tilt of the Earth throughout the year. Not to scale. The Human Adventures in Space Exploration.

The equinoxes fall each year in March and September, with the March Equinox being the Spring (or Vernal) Equinox in the Northern Hemisphere and the Autumn Equinox in the Southern Hemisphere, while the September Equinox is the Autumn Equinox in the Northern Hemisphere and the Spring Equinox in the Southern Hemisphere. On these two days the day and night are both exactly twelve hours long at every point on the planet, the only days on which this happens.

How the combination of the Earth's equatorial plane (horizontal circle labelled as Celestial equator) and the plane of the Earth's orbit (diagonal circle labelled as Path of the Sun) creates the Solstices and Equinoxes that we observe. Christopher Crockett.

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The March Equinox.

The March Equinox will fall at 9.01 am GMT on Thursday 20 March this year. The Earth spins on its axis at an angle to the plain of the Solar System. This means that the poles of the Earth do not remain at 90° to the Sun, but rather the northern pole is tilted towards the Sun for six months of the year (the northern summer), and the southern pole for the other six months (the southern summer). Thus, twice a year neither pole is inclined towards the Sun, on days known as the equinoxes.

The tilt of the Earth relative to the incoming light of the Sun at the March Equinox. Przemyslaw Idzkiewicz/Wikimedia Commons.

The equinoxes fall each year in March and September, with the March Equinox being the Spring (or Vernal) Equinox in the Northern Hemisphere and the Autumn Equinox in the Southern Hemisphere, while the September Equinox is the Autumn Equinox in the Northern Hemisphere and the Spring Equinox in the Southern Hemisphere. On these two days the day and night are both exactly twelve hours long at every point on the planet, the only days on which this happens.

The tilt of the Earth relative to the Sun at the planet's equinoxes and solstices. Astronomy Group/University of St Andrews.

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The September Equinox.

The September Equinox falls on Sunday 22 September this year (2024), when the day and night will be of equal length in both of the Earth's hemispheres. The Earth spins on its axis at an angle to the plain of the Solar System. This means that the poles of the Earth do not remain at 90° to the Sun, but rather the northern pole is tilted towards the Sun for six months of the year (the northern summer), and the southern pole for the other six months (the southern summer). This means that twice a year neither pole is inclined towards the Sun, on days known as the equinoxes. 

Simplified diagram showing the tilt of the Earth throughout the year. Not to scale. The Human Adventures in Space Exploration.

The equinoxes fall each year in March and September, with the March Equinox being the Spring (or Vernal) Equinox in the Northern Hemisphere and the Autumn Equinox in the Southern Hemisphere, while the September Equinox is the Autumn Equinox in the Northern Hemisphere and the Spring Equinox in the Southern Hemisphere. On these two days the day and night are both exactly twelve hours long at every point on the planet, the only days on which this happens.

How the combination of the Earth's equatorial plane (horizontal circle labelled as Celestial equator) and the plane of the Earth's orbit (diagonal circle labelled as Path of the Sun) creates the Solstices and Equinoxes that we observe. Christopher Crockett.

See also...