Re: newbie: Q about Sun's orbit
Greg Neill
> newbie - here goes anyway...
>
> Why is it (regarding our Sun), that
> perihelion not= Winter solstice
> and
> aphelion is not= summer soltice?
> in terms that they occur at differnt times & positions?
>
> I understand the Earth's orbit around the Sun is slighty elliptical
> but can't see why the solsticies do not occur exactly at ap(peri)
> helion?
>
> (Is it perhaps due to the fact that the Sun's rotational axis is
> itself "tilted" wrt Earths?
>
> Some Sunlight from this ng appreciated.
> Thanks and HNY to all.
If the projection of the Earth's spin axis on the
plane of the Earth's orbit about the Sun (the
ecliptic plane) coincided with the major axis of
the ellipse, then the solstices would indeed
occur at perihelion and aphelion. Then the
equitorial plane of the Earth would bisect the
orbital ellipse through its center point and
along the line of the minor axis.
But this isn't the case; the Earth's spin axis
is not so fortuitously aligned with the orbital
ellipse. The equitorial plane cuts the ecliptic
plane along a line that doesn't correspond to
either of the axes of the orbital ellipse
geometry. Thus the equinoxes don't occur when
the Earth reaches the ends of the minor axis, and
the solstices don't occur at the ends of the major
axis.
OG
"dave" <da...@127.0.0.1> wrote in message
news:agbpj5hkopt60ptjv...@4ax.com...
> newbie - here goes anyway...
>
> Why is it (regarding our Sun), that
> perihelion not= Winter solstice
> and
> aphelion is not= summer soltice?
> in terms that they occur at differnt times & positions?
>
> I understand the Earth's orbit around the Sun is slighty elliptical
> but can't see why the solsticies do not occur exactly at ap(peri)
> helion?
>
The date of the solstices (and the equinoxes) are due to the polar tilt of
the Earth
The date of the perihelion (and aphelion) are due to the shape of the
Earth's orbit.
There is no direct link between them, and it is only a coincidence that the
date of the winter solstice (in N Hemishpere) is close to the date of
perihelion at the present time.
If you look at the way that both the polar tilt and the orbit change over
time you can see that over periods of millennia, the two sets of dates can
move full circle relative to each other.
http://en.wikipedia.org/wiki/Perihelion_precession
If you look at the way that
oriel36
> newbie - here goes anyway...
>
> Why is it (regarding our Sun), that
> perihelion not= Winter solstice
> and
> aphelion is not= summer soltice?
> in terms that they occur at differnt times & positions?
>
> I understand the Earth's orbit around the Sun is slighty elliptical
> but can't see why the solsticies do not occur exactly at ap(peri)
> helion?
>
> itself "tilted" wrt Earths?
Here is what you do - forget this 'tilt' business,the Earth is
basically a large rotating liquid object covered with a fairly thin
fractured crust,the specifics of that interior act on the surface
crust by either creating it at the Mid Atlantic ridge or at other
times showing up as Earthquakes as the thin crustal plates moves
under,past or sideways to each other.This should help you get rid of
the idea that the Earth is a solid object like a gyroscope and people
who imagine a 'tilt' are just unfamiliar with the internal fluid
dynamics of the planet.
The next thing you do ,and it is the most important,is to recognise
that the Earth's daily rotational and orbital motions are completely
separate to each other so that you can look at both of them
separately ,if you do this you will not have to suffer those
unintelligent explanations based on 'tilt' to the Sun/orbital plane/
ecliptic plane.
The Earth orbits the Sun in a specific way by turning 360 degrees to
the central Sun and taking a whole year to do it,it has nothing to do
with daily rotation to the central Sun and can easily be grasped by an
analogy of using a broom and walking around a central object.Pointing
a broom handle at any object and walk around the central object while
keeping the broom handle pointed in the same direction requires that
you move forwards,then sideways,then backwards and then forwards again
to complete the [orbital] circuit and you can see this happening with
Uranus who has a remarkable set of circumstances (rotational
orientation,distance from Earth,equatorial rings ) which makes it easy
enough for a newbie or anyone else.
http://astro.berkeley.edu/~imke/Infrared/UranusAo/ur_time_2001_2005.jpg
So,drop hemispherical ideas of summer/winter associated with the old
'tilt' explanation for the seasons and focus strictly on the orbital
motion of the Earth,that way you can enjoy the explanation why
aphelion and perihelion events do not require a 'tilt' reference as
'tilt is merely a consequence of the daily rotation of the Earth.
Quadibloc
> Why is it (regarding our Sun), that
> perihelion not= Winter solstice
> and
> aphelion is not= summer soltice?
> in terms that they occur at differnt times & positions?
>
> I understand the Earth's orbit around the Sun is slighty elliptical
> but can't see why the solsticies do not occur exactly at ap(peri)
> helion?
>
> (Is it perhaps due to the fact that the Sun's rotational axis is
> itself "tilted" wrt Earths?
You've already received answers that you were satisfied with,so
perhaps it isn't worth coming here with another explanation.
But it seems to me that a more nontechnical explanation than you were
given might help.
Aphelion and perihelion bring the Earth closer to, and farther away
from, the Sun. So they would have an effect that makes the world
warmer at the time of perihelion, and colder at the time of aphelion.
As it happens, though, the Earth's orbit has a very small eccentricity
(only those of Venus and Neptune are smaller) and so this effect is
very small.
Note, too, that this would be an effect that works the same
everywhere. As you may be aware, people in Australia have winter at
the opposite time of the year to people in the Northern Hemisphere.
The orientation of the Sun's rotational axis does not enter into it.
Different parts of the Sun's surface still shine as brightly.
What would have the opposite effect in the Northern and Southern
hemispheres?
As it happens, the Earth's rotational axis is tilted - or inclined, if
you (or, rather Oriel) would prefer - with respect to the Earth's
orbit around the Sun.
Why does this matter?
If one looks at things from the Earth's perspective, at the time of
the Winter solstice, the Sun is 23 1/2 degrees below the Equator, and
at the time of the Summer solstice, the Sun is 23 1/2 degrees above
the equator. The Earth's axis always points in the same direction
(towards Polaris) but when the Earth moves to the opposite side of the
Sun in its orbit, then the direction in which the Earth leans relative
to the Ecliptic changes from being towards the Sun to being away from
the Sun.
When the Northern Hemisphere is facing towards the Sun, places there
get more hours of daylight, and, because the Sun shines on them at a
less oblique angle, more energy from the Sun hits every square foot of
the ground.
John Savard
palsing
.. A whole bunch of irrational drivel...
"To realize that you do not understand is a virtue; Not to realize
that you do not understand is a defect."
- Lao Tzu
dke...@hotmail.com
Hi
A couple of interesting places to look for the effects of these two
factors
are first, look at an anlema. Note the unequal loops. This in
dominated by
the orbit. Now look at a good plot of the equation of time. You'll see
some
notches in the curve. These are caused by the interaction to the two
super imposed on each other, tilt and orbit.
Dwight
Quadibloc
> A couple of interesting places to look for the effects of these two
> factors
> are first, look at an anlema. Note the unequal loops. This in
> dominated by
> the orbit. Now look at a good plot of the equation of time. You'll see
> some
> notches in the curve. These are caused by the interaction to the two
> super imposed on each other, tilt and orbit.
Yes, that is true.
http://www.quadibloc.com/science/eot.htm
John Savard