I intended to write a small post, but given there was so much confusion, I sat researching on the topic for more than one hour, just to understand the concepts myself. It has become longer. If you are interested about what causes season, this is a preliminary note.

TOPIC; The mid-winter occurs at perihelion.

(contrary to my earlier erroneous intuition that winters occur due to farthest distance from sun, perihelion means a point of least distance from sun or any primary gravity source in general, sans; purna or puri-ahna etc)

Also mid-summer occurs at aphelion (farthest distance from sun) contrary to intuition again.

Yes it is indeed winter but you wouldn’t know until the sun sets when suddenly there is a cool feeling. The day weather is still not very cool. The abrasiveness is caused by the direct glares and flares from sun which are instantly cut off when sun sets and the equilibrium conditions are felt. Plus remember that the seasons are not due to the distance of earth from sun, between maximum and minimum, the earth varies its distance only by 3.4%. The season are due to what are called the “obliquity of ecliptic” that is; The inclination of earth in its Orbital Plane of sun (maxm; 23.44 degree).

During a variation of inclination of earth the sun also changes its relative distance from above a point on earth, causing what we call as season. In scientific terms we say seasons are effected due to equinoxes and sub-solar points or solstices. All together there are 4 of these points for a place on earth per year. SO any point on earth will have 2 equinoxes and 2 solstices. Their perihelion and aphelion perhaps brings the central of average equilibrium temperature. Which may be the reason (partly) that Indian winters are not as harsh as say Siberia winters. The perihelion might be adding same amount of solar energy to both places, but due to different location on hemispheres and different elevation profiles the variation in actual temperature are widened. My place would see a minimum winter temperature of 10-15 degree while Siberia would see -40 degree. (Of-course there would be many other reasons for exact changes in temperature)

That inclination of earth is toward sun for 6 months and away from sun for another 6 months, causing seasonal differences, in the respective hemispheres. (S and N) The two times of the year (20 March and 22 September) when the inclination of earth in its orbit of sun, is neutral, that is zero (or 90 degree), when Sun’s light falls on all point of any one of the hemisphere is uniform, for both hemi-spheres are called equinox. (Sans; ekanaxatra = the same amount from the star, the Greek nox for night might have derived sans; naxatra = star, which in turn derives sans; sunyaxatra meaning the region of sun, or space) As a result length of day and night are same.

That is equinox is when the earth’s inclination is perpendicular to the plane in which it orbits sun, It neither tilts away nor it tilts toward sun.

Equinoxes are the time when the major opposite seasons summer and winter begin. Last week I experienced a bit chilled evening, a characteristic of winters. As day temperatures were so higher which is not a property of winter (here in India), and this I have experienced for years now, a sudden drop in temperature when sun sets, I wondered often why. Now I understand, around 5 days ago (when I actually thought of writing “winter’s come”) the equinox arrived, the reason why suddenly when the sun goes off directly away, we experience the fact that winter has begun.

A winter solstice is when a given point suffers maximal tilt “away from” sun and maximal elevation of sun’s center from earth, causes winter. A summer solstice is when a given point (on earth) suffers maximal tilt “towards” sun and a minimal elevation (subsolar point).

So solstice and equinoxes are correlated directly. (as far as I understand now) But perihelion and aphelion points are not correlated directly with the solstices and equinoxes.

The time of the year when these winter and summer point occurs, continue to shift. That means many thousand years from this century, summer will occur in December and Winter in July. A complete shifting of these solstice (and therefore equinox) points occurs in 26000 years due to recession (rotation) of the inclination of earth in its ecliptic (called as said already, “obliquity of ecliptic”, this angle’s axis now continues to span a 360 degree rotation in 26000 years, knows as precession of equinoxes. ).

That is in 26000 years the earth suffers two “upside down” reversals while moving in the orbit around sun. So by half of that time you are reversed once (wrt the orbital plane, if you are seeing the orbit from above, in 13 K years you will be seeing that plane from below, then in another 13 K years you will be seeing it from above again, while everything else is still going on)

Also remember that we measure the global parameters from space itself (that is without any particular special importance to sun or earth itself) This we achieve by fixing our references to very distance stars, where all motion of sun, earth, moon etc are then suitably accounted for.

In that global/universal space frame the axis of the earth remains at 23.44 degree eg (wrt the ecliptic plane, that is the plane that contains earth’s elliptic orbital motion around sun) But wrt sun eg the axis of earth could be many angles, giving relative differences of intensity variations. Hence due to various such variations (eg latitude and longitude) we see seasons, day and time variations, geographic climate conditions, polar days (24 hr sun) and polar nights (24 hr nights), mid night sun etc. [At the poles themselves, the sun rises once and sets only once each year]

Things to understand; earth’s perihelion, aphelion, summer and winter solstice, equinox and their inter-relation.

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