Quick Question about Solstice and Perihelion and the 4 Seasons. How does it work?

Follow up question:

Why don't the orbit of the planets degrade over time and eventually fall into the sun?

Is it because the centrifugal force of the orbiting mass of the planet has perfectly balanced out with the gravitational attraction of the mass of the sun? It must be that.

Here's a little tidbit to help confuse the issue. Astrology (yea!), sort of.

Currently, on the day of the winter solstice (northern hemisphere), the Sun rises in the constellation Sagittarius. Just barely. A thousand years ago, it was pretty much smack dab in the middle of the constellation but in another thousand years it's going to be between somewhere between Scorpius and Ophiuchus.

How come? It's because of precession of the equinoxes. Now, that word is tossed around a lot in various contexts but its effects are often misunderstood. Precession is caused by the "wobble" of Earth's axis. It is a very slow wobble, taking about 26,000 years to complete its cycle.

There is a common misconception about precession, that it is a change in the tilt of the axis. If this were true precession would have an impact on length the day/night cycle. But it isn't true. It is not the tilt of the axis that changes, but a change in where the axis "points" to, in the sky.

Now, here's the fun part. Currently, on December 21 (the solstice), the Earth is pretty much at perihelion (its closest distance to the Sun). But, in 13,000 years the solstice will occur when the Sun is at aphelion. Will that cause a change in seasons or climate? Not in seasons (winter will still be winter and occur in the same months) but because the Earth will be about 3% farther from the Sun than it is now there will be a bit less solar radiation. So, in the northern hemisphere winters will be a bit colder and summers will be a bit warmer. And vice versa for the southern hemisphere.

Oh, and the Sun will rise in Gemini.

a reply to: Phage

Thanks Phage, that's interesting. I thought it was due to the movement of the solar system up and down above and below the galactic plane that was responsible for precession.

Can you speak to the follow up question I've posed top of page 2 here regarding the long term stability of planetary orbit around the sun?

Thanks again.

a reply to: AnkhMorpork

You have it right, pretty much. But that balance is a result, not a cause, and "centrifugal force" really is not a thing. The correct term is angular momentum. Essentially, the Earth is being pulled toward the Sun. It's falling. But because it isn't aimed at the Sun, it misses it.

There is a "vertical" motion of the Solar System within the Milky Way. But that cycle takes about 60 million years.

a reply to: AnkhMorpork


Also, when something like that Chinese space station falls back to earth it is due to the fact that the atmosphere is causing drag upon it and slowing it down. That slow down is enough for the orbit to degrade causing gravity to take over and it to fall back to earth.

In space, the particles are so far apart that no degradation can take place upon a planet so they do not slow down and crash into the sun like a satellite does to earth.

a reply to: TEOTWAWKIAIFF

Yes, but without the circling motion, the Earth would fall into the sun due to the gravity well/pull of the mass of the sun.

I assume that Mercury and Venus, depending on their mass have much faster orbits around the sun.

Regarding the Earth's rate of rotation. It's my understanding that for an inner planet, that the tendency is towards a slowing down toward a tidal locking scenario, and yet the earth rotates at 365 times per orbit around the sun, which considered rather fast compared to say Mercury and Venus. I've heard that this is due to the 3 body problem with the addition of our single, giant moon, which keeps the Earth spinning fast for a long longgggg time.

Edit: if this analysis is valid, it would mean that the length of the day/night cycle would be slowing at a very very slow rate, which will slow even more as the moon ever so slowly pulls away from the Earth and that in future eons, the Earth will eventually tidal lock with the sun and stop rotating, with the same face facing the sun in the same way that we only see the one side of the moon.

Be blessed (while the blessings last),

Ankh

It IS due to the tilt. But it doesn't tilt back and forth (per se), but rather the tilt relative to the Sun varies depending on where the Earth is during it's one-year orbit.

The Northern Hemisphere is tilted away from the Sun in December through March, so the sunlight that hits the Northern Hemisphere during that time is less direct. Therefore it is cooler/winter.

At that same time, the Southern Hemisphere is pointed more toward the Sun, which means the radiation from the Sun hitting the Southern Hemisphere is more direct. Therefore, they get summer from December through March.

And then the opposite is true for June through September,when the Northern hemisphere is tilted toward the Sun and the Southern Hemisphere is tilted away.



By the way, the Earth is closest to the Sun (along that elliptical orbital path) in January, which seems counterintuitive for the Northern Hemisphere -- until you realize that the distance from the Sun is not nearly as important to the seasons as the tilt of the Earth/directness of the sunlight.

Image sources:
kdhs.wikispaces.com...
www.ingridscience.ca...

Is this thread proof that science class was banned from the education system a decade or two back.

a reply to: roadgravel

No.

originally posted by: AnkhMorpork
a reply to: roadgravel

No.

Then it must be lack of attention in class.

a reply to: AnkhMorpork

I assume that Mercury and Venus, depending on their mass have much faster orbits around the sun.

It doesn't much matter what their mass is. Orbital mechanics dictate that an object in a lower orbit (around any object) will have a higher radial velocity than one in a higher orbit.

a reply to: AnkhMorpork

Yes. But the Sun will become a red giant (and probably consume the Earth) before that occurs.

a reply to: Phage

I guess it would have to, to remain in orbit. One would think that mass would play a role ie: higher angular momentum or inertia and thus, not need to remain quite as fast to fall into that equilibrium between the sun's gravity well and what I referred to as centrifugal force.

a reply to: roadgravel

Funny because I asked a colleague of mine about this earlier today, and he has a university degree in biology and is a bit of a geek and he thought that the seasons were due to a back and forth wobble on the axis. I just held up a pen on an angle and showed him what that means as it goes around the sun, and he went "duh, yeah".

The asking of a question, posed honestly and authentically, no matter how stupid or ignorant it might appear, should not be the basis of ridicule in matters of scientific inquiry, imho.

Even Phage can make mistakes from time to time.

a reply to: Phage

This question of mine from page 1 has been overlooked. Can you help answer this? Thanks

However I am having trouble seeing how it makes sense when the globe spins every 24 hours. Should the opposite hemisphere have a larger angle of solar dispersion and therefore colder, and repeat the same the next day

Another way of asking is if it spins every 24 hours the northern hemisphere would end up being the southern hemisphere (and vice versa) and therefore receive the suns rays at a different angle and yet we have opposing seasons every 4-6 months not daily.

a reply to: Phage


So precession has nothing at all to do with the whole solar system's movement relative to the galactic plane? Just a slow wobble of the Earth's axis?

Do you know what causes that or what makes that the result?

a reply to: TheConstruKctionofLight


The earth spins at a 90% angle to it's axis, right?

a reply to: AnkhMorpork

Think about that "balance" you were talking about. Greater mass also means a greater force of attraction between the objects.

Mass does have an effect on orbital velocity. But the Sun is so massive it pretty much cancels out the difference between planets.

For example:
At 1 au (Earth's orbital distance) it takes 365.194 days for the Earth to complete an orbit. If Earth had the same mass as Jupiter it would take 365.020 days. Jupiter is 318 times the mass of Earth but the orbital velocity changes by 0.05%.
www.calctool.org...

a reply to: TheConstruKctionofLight

if it spins every 24 hours the northern hemisphere would end up being the southern hemisphere (and vice versa) and therefore receive the suns rays at a different angle

Half of the 24 hour day the Earth is not facing the sun.

eta:

roughly half the day