Astronomy Essentials

Are the December solstice and January perihelion related?

December solstice and January perihelion: A view of Earth from orbit, with the sun in the distance.
Are the December solstice and January perihelion related? They’re both about our world’s relationship to the sun that binds it in orbit. Image is the Earth and sun, acquired on International Space Station Expedition 13. It’s via NASA.

December solstice, January perihelion

Earth will come closest to the sun on January 4, 2022. This event is called Earth’s perihelion, from Greek roots peri meaning near, and helios meaning sun. Meanwhile, the December solstice takes place today, December 21, 2021. At perihelion in January, Earth will swing to within about 91 million miles (147 million km) of the sun. That’s in contrast to Earth’s distance from the sun at its aphelion – most distant point – in July, when the sun and Earth will be separated about 94 million miles (152 million km). The December solstice and January perihelion come close together in time. Are they related? No. It’s just a coincidence that they fall within about a two-week period each year.

The December solstice marks the sun’s southernmost path across our sky. At this solstice, Earth’s Southern Hemisphere is tilted most toward the sun. And so it’s summer in that hemisphere. Meanwhile, the northern half of the globe is tilted most away from the sun. Winter begins on our half of the globe.

Perihelion isn’t about Earth’s tilt. It’s about how near or far we are from the sun in orbit. The date of Earth’s perihelion drifts as the centuries pass. These two astronomical events are separated by about two weeks for us now. But they were closer a few centuries ago. In fact, in the year 1246 A.D., they happened on the same day.

Date of perihelion drifts

But, like all things in nature, the shape of Earth’s orbit isn’t static. Writing at TimeandDate.com, Aparna Kher explained:

Due to variations in the eccentricity of the Earth’s orbit, the dates when the Earth reaches its perihelion or aphelion are not fixed. Since 1246 A.D., the perihelion and aphelion dates have drifted by a day every 58 years. In the short-term, the dates can vary up to two days from one year to another.

So, the centuries continue to pass, the December solstice and January perihelion will drift even farther apart. Kher continued:

On the average, one revolution of the Earth relative to perihelion is about 25 minutes longer than one revolution relative to the December solstice. Perihelion advances one full calendar date about every 58 years.

Mathematicians and astronomers estimate that in 6430, over 4000 years from now, the perihelion will coincide with the March equinox.

Graphic showing distance differences between aphelion and perihelion.
Earth-sun distance differences between the January perihelion (closest point) and July aphelion (farthest point). Earth is farthest from the sun when it is summer in the Northern Hemisphere. Illustration by Timeanddate.com.

Bottom line: Earth’s December solstice and January perihelion are not related.

Earth closest to the sun on January 2-3, 2019

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Posted 
December 21, 2021
 in 
Astronomy Essentials

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