On 4 January, the Earth is due to pass perihelion, the closest point in its orbit to the Sun. This may seem strange because it happens right in the middle of winter. But in fact, it has nothing to do with the length of the daylight hours or the temperature outside.
Perihelion
When one speaks of the Earth’s orbit, it is difficult to refrain from distorting the facts. Because if we say that it is a circle, it would not be true, because it is an ellipse, i.e. a closed trajectory with two focal points inside, one of which is the Sun. So the distance to our luminary is changing all the time.
The closest point is called perihelion and the farthest point is called aphelion. But if we depict the Earth’s orbit as a perceptibly elongated ellipse, it would also be misleading. After all, in perihelion the distance from the Earth to the Sun is 147 million kilometers, and during aphelion – 152 million kilometers. So the difference between the two is about 3%. If we were to depict an ellipse, which is only 3% different from a circle, it would be quite difficult to notice the difference.
Nevertheless, as early as 3:28 p.m. UTC (10:28 a.m. UTC-5) on 4 January, the Earth will be at the closest point of its orbit to the Sun. Visually it will not be marked in any way, because the difference between perihelion and aphelion, as already mentioned, is small. And yet the question arises why at this moment we do not feel additional heat, but, rather, on the contrary.
Why does the perihelion occur in winter?
To understand why perihelion occurs exactly in winter, it is necessary to remember what causes the change of seasons on the Earth. The primary cause is the tilt of the axis of its rotation relative to the perpendicular to the plane of its orbit. If this deviation did not exist, the only reason for seasonal changes would be the ellipticity of the orbit, and we would not feel these changes practically.
However, the Earth’s axis of rotation is tilted, and this tilt is maintained throughout the year. As a consequence, our planet turns to the Sun from the North Pole to the South Pole, and we experience this as a change in the length of the daylight part of the day. For example, a fortnight ago our planet passed the point at which its North Pole was maximally deflected in the direction opposite to the Sun.
We experienced it as the winter solstice. In the Northern Hemisphere, it was the longest night of the year. And in the Southern Hemisphere, on the contrary, that day was the longest, and summer is raging there now. And in general, we should remember that winter and summer come in turn in the two hemispheres of our planet.
Changes in the length of daylight hours and the angle of incidence of the Sun’s rays on the Earth’s surface affect the amount of solar energy received much more strongly than the ellipticity of its orbit. Although it still has its influence: in perihelion our planet receives 6.9% more energy than in aphelion. However, this energy is mostly received by the southern hemisphere.
Does perihelion occur on the same day every year?
One of the most difficult things to understand about the perihelion of the Earth’s orbit is that every year the Earth passes it at about the same time, between the 2nd and 5th of January, but the dates and the specific moment are different each time. There are several reasons for this. One is the divergence of the tropical year from the calendar year by a few hours, which we need leap years to overcome.
The second reason is fluctuations in the Earth’s orbit itself and changes in its eccentricity. They are caused by the gravitational influence of other planets in the solar system. Usually, they are insensible for our daily life, but they do affect the aphelion and perihelion points.
Finally, we should not forget about such a phenomenon as the precession of the Earth’s axis. This is the same phenomenon that makes the point of the vernal equinox shift all the time. Because the same perihelion is very slowly moving away from the point of the winter solstice and in about 13 thousand years will coincide with the summer solstice.
