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.”
The Earth’s orbit around the sun. Can you tell that it is not exactly a circle? Image from University of Nebraska-Lincoln.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).
The apparent size of the sun as seen from Earth at perihelion (left) and six months later, at “aphelion” (right). Images created with Stellarium.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.
The Earth orbits the sun because it is essentially falling toward the sun by reason of the sun’s gravity. Here the elliptical nature of Earth’s orbit is shown greatly exaggerated. As the Earth travels from P, the perihelion point, toward A, the point of greatest distance from the sun (the “aphelion” point), it is moving against the pull of the sun’s gravity, and so it loses speed—just like a ball that is thrown upward loses speed as it moves against the pull of Earth’s gravity. Then as the Earth moves from A toward P, it moves with the pull of the sun, and gains speed—just like the ball gains speed as it falls back to earth. Image from University of Nebraska-Lincoln.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.
The time for a person at A to be carried around the Earth by its rotation one time with respect to a star is 23 hours, 56 minutes.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).
In this diagram the Earth’s rotational motion on its own axis is indicated by the red arrow, while its orbital revolution about the sun is indicated by the green arrow.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.
Graph of the changing time of mid-day for Charleston, South Carolina (USA) and Santiago, Chile. Note how, in both cities, the time of mid-day drifts backwards against the clock by about 25 minutes during the period from mid-November to mid-January. If the day length were unchanging, then this perihelion drift would cause the sun to set 25 minutes later during this period; it would also cause the sun to rise 25 minutes later. The combination of this perihelion drift and the changing length of daylight near the solstice is what causes the complex effects involving the times of sunrise and sunset at this time of year. Data from the US Naval Observatory.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”!
