Today is Perihelion Day! If you have a 2017 Vatican Observatory calendar you will see this marked on that calendar for January 4. The Earth journeys around the sun in an orbit that is ever-so-slightly elliptical (the elliptical nature of the orbit is so mild that the orbit basically looks like a circle that is slightly off-center from the sun). This means that the distance between the sun and the Earth varies over the course of a year. Today is the day on which that distance is a minimum, and the technical term for the point of minimum distance between the Earth and sun is “perihelion.”
Today is also the day of the “Super Sun” (to borrow the language of the “Super Moon” hoopla of this past fall) because since the distance to the sun is a minimum then the apparent size of the sun in the sky is a maximum (of course, as with the “Super Moon,” this effect is not very great).
When the distance between Earth and the sun is a minimum, the Earth will be moving the fastest in its orbit. This is because an orbit is essentially a fall; the Earth is falling toward the sun on account of gravity. This faster motion at perihelion turns out to have an effect on daylight.
We have daylight and darkness, day and night, because Earth rotates on its own axis. We think of that rotation as being one “day”—24 hours—but in fact the time of rotation is 23 hours and 56 minutes. That is, if you observe a star that is directly overhead one night, you will find the same star to be overhead again 23 hours and 56 minutes later.
So what about the 24 hour day, you ask? Imagine that the Earth journeyed around the sun in a perfectly circular orbit, always moving at the same speed along that orbit. Now imagine two people on opposite sides of the earth. Person A observes a certain star being directly overhead. At the same time, person B observes the sun being directly overhead (person B would be experiencing the “mid-day” point, halfway between sunrise and sunset).
Now we wait 23 hours and 56 minutes until the star is overhead again for A. But during that time the Earth moves along its orbit, from position 1 to position 2 as shown below. The star is so far away that this motion does not matter. But the motion does matter in the case of the sun. Therefore, as seen below, when the Earth is at position 2 and the star is overhead again for person A, the sun is not yet overhead for person B. The earth has to turn a little more—four minutes more, so that B is at B’—in order for the sun to be overhead for B. Thus a “day” measured by the sun is 24 hours while a “day” measured by the stars is 23 hours and 56 minutes.
Now remember that the Earth’s orbit is not a perfect circle; the Earth does not always move at the same speed along its orbit. When the Earth is moving faster (like when near perihelion) it will move farther along its orbit during a given period of time. The distance between 1 and 2 in the above diagram will be greater. Therefore, the Earth will have to turn still more to reach point B’ so that the sun is overhead for B. And therefore mid-day will arrive a little late.
This is why in the northern hemisphere the darkest evening of the year of was in early December, even though the shortest day was at the solstice on December 21 (and in the southern hemisphere the brightest morning was in early December even though the longest day was at the solstice). As the Earth approached perihelion and moved faster in its orbit, mid-day began to arrive late at a rate that exceeded the rate at which the days were changing in length. Because of the perihelion’s effect on mid-day, the period of daylight was drifting backwards against the clock, making the evenings brighter (and the mornings darker) than they would be if Earth were in a perfectly circular orbit. The perihelion daylight drift occurs in both hemispheres—both in Charleston and in Santiago.
So it is perihelion that is behind the strange business of daylight and the solstice! Of course we have one more point to cover in all this—the date of the northern hemisphere’s darkest morning and the southern hemisphere’s brightest evening. We have not reached that yet, but when we do, there will be a post here about it.
A final point regarding perihelion: it is a true Global Event. Regardless of creed, calendar, or hemisphere, everyone everywhere on Earth experiences perihelion today. So today, wish everyone you meet, and even all your social media friends from around the world, a “Pleasant Perihelion”!