Is it just coincidence that the 4 season begin and end at same point in the orbit around the sun?
In: Physics
*truly ELi5 answer*
The tilt of the earth causes an effect that makes the sun appear to *move* in the sky through the year and return to the same spot after one orbit of the earth. By counting the number of days it takes for the sun to return to same spot we created a calendar based on that number, which is 365 and a quarter.
Seasons are both due to the tilt of the earth and the fact that the tilt stays constant as it revolves around the sun. If the earth didn’t move around the sun, we also wouldn’t experience seasons. If the Tilt always pointed in the same direction, relative to the sun, we wouldn’t experience seasons. However since the tilt’s orientation is constant and the earth revolves around the sun, the northern hemisphere gets more sun for part of the year, and the southern hemisphere gets more sun for part of the year.
Your question have an incorrect assumption and that is that the year is based on the orbit but is is not. It is based on the axial direction of the earth’s relative sun. They are close but not the same. There is a difference of 20 minutes per year. The obit definition is often used because it is simpler and works well enough in most cases but not in your question.
The first question is what is a year? There are many years:
A sidereal year is when the position of the sun is identical relative to the stars in the sky. This is one orbit around the sun but it is not what our calendar is base on.
A tropical year is based on the direction of the axis of the earth relative to the sun. So on the equinox, the equator is parallel to the orbital plane of the sun. This is what our calendar is based on because it will correspond with the seasons. It is 20 minutes shorter than a sidereal year.
So the calendar matches the season because both is a result of the direction of the earth axis relative to the sun. The calendar is not directly based on an orbit around the sun.
The axis of the Earth is very quite stable. There is [axial_precession](https://en.wikipedia.org/wiki/Axial_precession) like a gyroscope but the period is 25,772 years. It is this change that results in the 20 minutes
So between two years, the position of the stars relative to the sun is quite close on the same day but not identical.
So there is a slow change and the position of the start for the same time of year changes slowly. They will move 1 degree every 71.6 years. So on the equinox, the stars are 1 degree of from where they were in 1949
So in most cases, it is enough just to explain a year with the orbit around the sun but for very exact answer you have to include that axial precession and the difference between a sidereal year and the tropical year.
If you look at the [dates of the Zodiac](https://en.wikipedia.org/wiki/Zodiac#Table_of_dates) you can see that the used dates and the real position of the stars do not match up. The common Ptolemaic tropical zodiac is based on the location of consolation around 2000 years ago and does not match what you observe today.
This is because of the difference between a sidereal year and the tropical year.
A day that in normal use is a solar day is not one rotation of the earth around its axis but the time the sun is in the same sport in the sky. We move around the sun. Even if the earth did not rotate the sun would have move one revolution in the sky. So a day is the combined effect of earth axial rotation and orbit around the sun.
So a sidereal day the is stars return to the same position ie one revolution of earth is 23 hours, 56 minutes, 4.0905 seconds the extra time ~4 minutes is from the orbit around the sun.
One revolution of the sun per year because of the obit around the sun is approximately 24*60/365= 3.94 minutes per day. It and the sidereal day added to result in a solar day,
So a solar day is 24 hours and earth rotates a bit more than one revolution per day.
So it is not a coincidence because the calendar is not based on orbit by axial changes of the earth. The same season will start on the opposite side of the sun after 12886
So like most concepts in science you can get a bit simplified explanation that works most of the time like 1 year = 1 orbit around the sun. It is often good enough, but for more advanced questions like the one you asked you have to look closer with a more exact but complex explanation.
Seasons are due to the tilt of the earth **and** the rotation around the sun.
The earth always tilts in the same direction (at least over the short term). In December the north pole tilts towards the sun, and in June, it tilts away. The direction of the title doesn’t change, only the earth’s position around the sun.
the tilt and the orbit are connected.
better say it’s the tilt of earth and not the distance to the sun.
It’s both the tilt and the orbit. The earth has the same tilt throughout its orbit. During winter on the northern hemisphere it is splinter on the Southern Hemisphere because the tilt is such that the Southern Hemisphere is less angled with respect to the sun. The bigger the angle with respect to the sun, the more the sunlight is spread over the surface of the earth. The more spread, less heat, while more concentrated means more heat. When the earth complete half it’s orbit, such that the tilt makes it so that it is summer in the northern hemisphere and winter in the Southern Hemisphere.
The reason you are probably thinking it is only the tilt is you probably heard someone say the distance to the sun doesn’t matter and in the context of a single planet in an unchanging orbit, the only possible source for a change in distance is due to the orbit (orbits can be elliptical), so the person probably mentioned orbit
We’ve specifically designed the calendar so that the seasons don’t drift.
This actually was a problem in early calendars – people tried to divide the year up into lunar cycles (month comes from moon-th) but this leaves you nearly a week off of the solar cycle at the end and the error rapidly builds.
The Romans first tried a 365-day calendar that corrected this, but even that isn’t quite perfect. Under Caesar they developed the 365.25 day “Julian” calendar that corrects the drift better, and this was further refined in the middle ages into the current 365.2425 day calendar.
It takes 365.2425 days to compete an orbit around the sun and reset the seasons, and the modern “Gregorian” leap year system corrects for those dangling fractions of days.
It’s not coincidence. The tilt of the Earth means that sunlight is distributed differently over the Earth’s surface depending upon where in its orbital path it is. Opposite points in Earth’s orbit for summer and winter (ie. on different sides of the Sun) will have the sunlight more focused upon different hemispheres. This is the exact same reason why summer is at opposite times of the year for each hemisphere, so whilst it’s winter in January for the northern hemisphere, the southern hemisphere will be experiencing summer. Australians celebrate Christmas in the summer heat, often on the beach with barbecues going.
When people say that seasons are a result of Earth’s tilt rather than its orbit, they mean it’s not due to the fact that Earth is farther from the sun at certain points in its orbit than others (because the orbit is not a perfect circle, all orbits are elliptical to some degree). The distance between Earth’s closest and farthest points to the sun is much too small to make a noticeable difference, but it’s a common misconception that this is what causes seasons in people who have not had the real reason explained to them.
On/about December 21st (varies a little with leap years) the North Pole is angles furthest from the sun. This causes the northern hemisphere to get the least hours of sunlight on that day, and the sunlight is weakest as it’s hitting the surface at an angle. This results in winter, going to zero sunlight above the arctic circle. There’s a bit of a lag between this and the temperatures hitting their winter low a few weeks later, so around this time is generally the start of winter.
On/about June 21st (leap years again) the opposite is true. The North Pole is angled as much as possible towards the sun. The northern hemisphere gets its maximum hours of sunlight, and places above the arctic circle experience zero hours of nighttime. In addition the sunlight is at its highest intensity as its hitting the earth straight on (as much as possible). This results in summer, with similar lag in warming so the hottest days are normally a few weeks later.
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