Proximity of Winter Solstice and Perihelion

Earth reaches perihelion the point in its orbit when it’s closest to the Sun between January 2 and 5, depending on the year. That’s about two weeks after the December solstice, December 21 or 22. Thus winter begins in the Northern Hemisphere at about the time that the Earth is nearest the Sun. Is there a reason why the times of solstice and perihelion are so close, or is this a coincidence?

Answer

The proximity of the two dates is an artifact of the particular century we live in. The date of perihelion does not remain fixed, but slowly moves later into the year at the rate of about one full day every 58 years. It turns out that the period from perihelion to perihelion (the anomalistic year) is about 25 minutes longer than the year defined from equinox to equinox (the mean tropical year). The date of perihelion thus moves completely through the tropical year in about 21,000 years. This slow change in the date of perihelion may have a long term effect on Earth’s climate. At this time the temperature extremes are moderated somewhat in the Northern Hemisphere, but that will change as the perihelion shifts in the direction of summer.