Earth is closest to the Sun in early January every year, and is said to be at perihelion, which comes from the Greek words ‘peri’, meaning ‘near’ and ‘helios’, meaning ‘Sun’. The perihelion is the closest distance between Earth and the Sun. The reason why Earth is not at a uniform distance from the Sun at all times is that the blue planet’s orbit is elliptical.
What happens when Earth is closest to the Sun?
In 2023, Earth will be closest to the Sun on January 4 at 11:17 am EST (9:47 pm IST). In early January, Earth is about five million kilometres closer to the Sun than the planet is during its aphelion, which refers to the point on Earth’s orbit farthest from the Sun. According to Earth Sky, our planet is about three per cent closer to the Sun than it is during the aphelion in early July.
The average Earth-Sun distance is 159 million kilometres.
While Earth is travelling at a speed of about 1.6 million miles (2.575 million kilometres) in its orbit around the Sun on January 4, the solar system as whole will move about 32 million miles (51.5 million kilometres) around the centre of the Milky Way, according to an article by Forbes.
When the Earth is closest to the Sun in early January, the Sun appears larger than it does in the entire year.
What will be the Earth-Sun distance at this year’s perihelion?
In early January, when it is winter in the Northern Hemisphere, Earth is closest to the Sun. In early July, when it is summer in the Northern Hemisphere, Earth is farthest from the Sun.
According to the Forbes article, the centre of Earth will be at a distance of 14,70,98,925 kilometres from the centre of the Sun at 9:47 pm IST. This distance is equivalent to 0.9833 astronomical units (au). The average distance of the Earth to the Sun is called au.
Why is Earth at its coolest during perihelion despite being closest to the Sun?
Although Earth receives the most radiation from the Sun at perihelion, the planet is at its coolest during perihelion. This happens due to Earth’s tilt.
For the unversed, Earth’s orbit does not cause the seasons, but the planet’s 23.44 degree tilt does. This is because Earth’s tilt affects how much sunlight each hemisphere of Earth receives at different times of the year. The tilt determines how long the days will be and at what point in the sky the Sun will be present.
The December solstice marks the beginning of the astronomical winter in the northern hemisphere. It marks the shortest day of the year in the Northern Hemisphere, and the longest day of the year in the Southern Hemisphere. Therefore, all regions in the Northern Hemisphere see day lengths shorter than 12 hours, and all regions in the Southern Hemisphere see day lengths longer than 12 hours.
On the day of the December solstice, the Northern Hemisphere receives the most indirect sunlight, due to Earth’s tilt on its axis. For the same reason, the Southern Hemisphere receives the most direct sunlight.
The December solstice is the shortest year in the Northern Hemisphere because the Sun shines indirectly on the northern half of Earth, and the longest day in the Southern Hemisphere because the Sun shines directly on the southern half of the planet.
Despite the fact that Earth receives the most radiation from the Sun at perihelion, the planet remains cool because the Northern Hemisphere is tilted away from the Sun, and the ocean in the Southern Hemisphere, which is tilted towards the Sun, soaks up the extra heat and negates the impact of the perihelion.
This year, Earth’s aphelion will be on July 6.
How Earth’s perihelion and aphelion affect seasonal lengths
Earth’s perihelion and aphelion, or the closest and farthest points to the Sun respectively, are not responsible for the seasons, but affect seasonal lengths. According to Earth Sky, Earth is moving fastest in its orbit during perihelion. Currently, Earth is revolving at a speed of almost 30.3 kilometres per second. This is about one kilometre per second faster than when Earth is farthest from the Sun in early July, or during aphelion.
Why winter in Northern Hemisphere is shorter than summer in that half
Since Earth is revolving the fastest during perihelion, the planet rushes from the December solstice to the March equinox, causing winter in the Northern Hemisphere and summer in the Southern Hemisphere to be the shortest seasons.
Summer in the Northern Hemisphere, which lasts from the June solstice to the September equinox, is nearly five days longer than winter in the same hemisphere. It is winter in the Southern Hemisphere when it is summer in the Northern Hemisphere, and summer in the Southern Hemisphere when it is winter in the Northern Hemisphere. Since the Northern Hemisphere summer is nearly five days longer than the winter in that hemisphere, the Southern Hemisphere winter is nearly five days longer than the summer in that hemisphere.
Due to Kepler’s second law of planetary motion, which states that planets do not move with constant speed along their orbits, Earth speeds up around the perihelion and slows down at aphelion, resulting in shorter winters and longer summers in the Northern Hemisphere, and shorter summers and longer winters in the Southern Hemisphere.
According to Earth Sky, the Northern Hemisphere winter this year started on December 21, 2022, and will last till March 20, 2023. Also, the Northern Hemisphere winter period is shorter by almost five days compared to the summer in the hemisphere, because Earth moves faster the closer it is to the Sun, and slower the farther it is from the Sun.