Explaining The Solstices

This week is the Northern Hemisphere Winter Solstice. What that means is that we will have the shortest day and longest night this week, then the days will start getting longer. The reason for this change is that the earth is tilted on it's axis, relative to the plane of it's orbit around the sun, by 23 1/2 degrees. While the days are always 24 hours, because the earth's rate of spin is constant, the amounts of daylight and night in each day varies. 

The sun is directly overhead, at some point during the year, for latitudes up to 23 1/2 degrees each side of the Equator. These latitudes are known as the Tropics. The northernmost latitude that the sun is ever directly overhead is the Tropic of Cancer. The southernmost latitude that the sun is ever directly overhead is the Tropic of Capricorn.

On most of the earth's surface, the sun always rises and sets once every 24 hours. But because of this tilt, the areas within 23 1/2 degrees of the north and south poles are an exception to this rule. These areas are called the Arctic and the Antarctic. So the Tropics are 23 1/2 degrees on either side of the Equator and the Arctic and Antarctic are 23 1/2 degrees from each pole.

Aside from the two Solstices every year, there are also two Equinoxes. As the name implies, this is when day and night are equal. The Equinoxes occur when the sun is directly overhead at the Equator. The Northern Hemisphere Winter Solstice occurs when the sun is directly overhead at the Tropic of Capricorn and Summer Solstice occurs when the sun is directly overhead at the Tropic of Cancer. 

These two Solstice points represent the beginning of winter and summer in the Northern Hemisphere. When the point with the sun directly overhead is moving northward, the Equinox is the Vernal Equinox and represents the beginning of Spring. When it is moving southward it is the Autumnal Equinox and the beginning of Autumn.

The orbit of the earth around the sun is not perfectly circular. The earth is closest to the sun on January 4, known as Perihelion, and furthest on July 4, known as Aphelion. What I found interesting is that there is a two-week gap between the Solstices and Aphelion or Perihelion. I referred to this here as the "Solstice Gap".

The Northern and Southern Hemispheres are not of equal mass. Most of the earth's land is in the Northern Hemisphere and, since rock is heavier than water, that makes the Northern Hemisphere heavier. Since the earth's axis is tilted the 23 1/2 degrees, this makes it a lower energy state for the heavier Northern Hemisphere to tilt away when the earth is closest to the sun, and toward the sun when it is furthest. This results in a lower total distribution of mass. This tilt is what causes the seasons as it is much more important in the distribution of sunlight than the variation in the distance to the sun.

The following diagram is nowhere near to scale. The earth, represented by the blue circles, is closest to the sun in Position A, which represents January 4, and furthest from the sun in Position B, which represents July 4. The earth's tilt on it's axis is shown. It is a lower energy state, in terms of planetary mechanics, for the heavier Northern Hemisphere to tilt toward the sun when the earth is furthest from the sun, because it represents a more balanced total distribution of mass.

It would seem that it would be the lowest energy state, and thus preferred, for the Aphelion and Perihelion to coincide with the Solstices. But yet it doesn't, there is the two-week gap. Aphelion and Perihelion come two weeks after the Solstices. This is the "Solstice Gap".

The Solstice Gap is explained by the fact that the earth moves faster in it's orbit when it is closer to the sun. Remember Kepler's Law stating that, in the orbital path, a line from the planet to the sun must sweep over equal areas of space in equal periods of time. This basically means that the planet moves faster when closer to the sun. So the rotation of the earth, producing day and night, is constant, the path around the sun, producing the year, is constant but it's velocity along that path isn't constant.

This shows up as the two-week gap between the Solstices and Aphelion and Perihelion.

Here is something interesting that I noticed. It isn't anything breakthrough but I haven't seen it pointed out before. The Aphelion distance of the earth from the sun is 94.5 million miles, or 152.1 m km. The Perihelion distance from the sun is 91.4 million miles, or 147.1 m km.

There are 52 weeks in a year and the difference between the Solstices and Aphelion and Perihelion is 2 weeks. Keeping in mind that these are all round figures, the proportion of both is roughly the same, between 3 and 4 percent. This shows that the reason for the Solstice Gap is that the earth is moving faster in it's orbit when it is closest to the sun. The rotation and the orbital path of the earth is constant but it's velocity along the orbital path isn't constant.

There are two compound postings about the Solar System. "Our Solar System", October 2022 and "The Configuration Of The Solar System Made Really Simple", March 2017.