Summer, Winter,Apogee,Perigee

We have summer when the north pole is tilted toward the sun, winter when it is tilted away; the opposite holds for the southern hemisphere. At the same time, the earth is passing from apogee to perigee in its orbit around the sun.

So whose summer occurs when we are closer to the sun, the northern hemisphere’s or the southern’s? Or to put it another way, in what months do apogee and perigee occur?

The northern hemisphere is in winter when the earth is closest to the sun.

Apogee and perigee occur once a month regardless of the time of year. Those are the terms which refer to the “highest” and “lowest” points in the Moon’s orbit around the Earth.

Aphelion and perihelion are the equivalent terms for a planet’s orbit around the Sun.

The angle that sunlight strikes the Earth’s surface is a far greater factor in climate than orbital distance. And the Northern hemisphere is tilted away from the Sun, thus having winter, when we are at perihelion. The tilts and eccentricities don’t line up exactly right, so perihelion is about three weeks after the winter solstice. I’d talk to the Management about such sloppy finish work, you would think a Cosmic Designer would keep these things neat. Maybe we’ll have to call in a Celestrial Mechanic to fix it…

Dr. Fidelius, Charlatan
Associate Curator Anomalous Paleontology, Miskatonic University
“You cannot reason a man out of a position that he did not reach through reason.”

I believe the reason that the winter solstice and the perihelion don’t coincide is that the axis of the Earth’s rotation “wobbles” like a slowly spinning top. IIRC, it takes about 22,000 years to make a complete rotation; so in 11,000 years, winter in the N. hemishpere will occur opposite of Earth’s orbit as it does now.


I don’t think I like those figures. About 11 thousand years ago we were just getting over an Ice Age (and killing off most everything big enough to be worth eating). I wonder if there’s a connection, and why no one is planning for the Y13K climatic disaster…

My back of the envelope figures seem to show that the winter solstice aligned with perihelion back in about 730 CE. So we only missed the “neatness” by a few centuries.

Wouldn’t that imply that the northern hemisphere would be more temperate than the southern? (Warmer winters, cooler summers.) In fact, the opposite seems to be true.

Because, even though we are at or near perihelion, the axis is tilted away. That, not-so-incidentally, accounts for those long nights at the North Pole–the axis is tilted far enough away that the sun never clears the horizon.

The Northern Hemisphere has most of the land on this planet. Land heats up and cools down faster than water, so we get more extreme weather (in general) than the Southern Hemisphere. Oceans are good at distributing heat energy, so the hemisphere with more surface water has more temperate climate.

Thanks to Nickrz and DrF. Aphelion. Yeah, I knew that somewhere way in the back…

The “wobble” is known as precession of the equinox, since the cardinal points of the orbit (as well as the seasons) precess gradually through the calendar. It’s just coincidence, therefore, that the Solstices are as close to the perihelion and aphelion as they are (about two weeks apart – we passed aphelion on July 5 at ~6pm CDT).

There are several other orbital parameters that have greater effects on global climate, however. Earth’s tilt varies between ~ 22-26 degrees, with a period of 40 kyrs. There is also a 100 kyr signal in the climate record, that appears to have the greatest amplitude. If it is orbitally induced then the best candidates are teh eccentricity of the orbit (varies by about 3% over 100 kyrs) and the angle of the ecliptic wrt the solar equator. The idea behind the latter is that we receive less solar energy if we spend our entire orbit within the solar equator, where there is more debris, rather than passing through it twice a year as we do now…

That’s like saying that Lincoln didn’t die at midnight because Washington is in the wrong time zone. There’s no earthly reason that perihelion should coincide with any seasonal event in the first place.

Yes, and “about” 100 years ago we were having a Civil War. Gee, does that mean we have a Civil War every 100 years?

John W. Kennedy
“Compact is becoming contract; man only earns and pays.”
– Charles Williams

Dr Fidelius: Granted, my figures may be wrong–jrepka states 40Kyrs as the period of the “wobble” of Earth’s axis, but my point was that the winter solstice and perihelion are independent of each other. As far as the coincidence of them occurring together goes, even a broken watch is correct twice a day. Also, the Ice Age 11,000 years ago lasted through Earth’s passage through the aphelion and perihelion over thousands of years. I think some positive feedback mechanism involving the circulation and cooling of the oceans you mention had more to do with it than whether winter occurred during aphelion or perihelion or anywhere in between. Still, I don’t think anyone has to change their vacation plans anytime soon.

Just to keep things straight – the “wobble” we spoke of, known as precession, is the slow rotation of the axis (imagine a top, spinning very rapidly around its axis while the axis slowly rotates around the vertical). Right now the northern axis, or pole of rotation, points almost directly at Polaris. A few thousand years ago it pointed at some other star and a few thousand years hence it will point somewhere else still. It will point at Polaris again (more or less) in 22 kyrs (and the solstices will fall on the same calendar dates that they do now). Thus 22 kyr is the precessional period.

The 40 kyr cycle I referred to is the change in the amount of tilt, which is 23.5 degrees now and varies +/- 3 degrees from that, with a period of 40 kyr. Of the orbital parameters this one should affect climate most significantly – imagine the earth with no axial tilt. All latitudes would receive more or less the same amount of sunlight, at the same angle, throughout the year. The steeper the axial tilt, the more severe the seasons are…

First a nit to pick.

Yes and No. An apogee and perigee occur once a month in reference to the moon, but the terms are not lunar specific, they are generic terms to desribe the actions in any orbit be it microscopic, terrestrial, lunar, solar or totally alien. In short his terminology was correct but not quite as precise as saying Aphelion and perihelion.

About the whole Ice age thing. There are two periodic events which trigger climatic changes and their coincidence can effect the scale of the ice age.

One is the change in the tilt of the Earth on its axis. As someone already stated the Earth’s tilt varies from 22-26 degrees. This angle is the variation of the “true” north from the magnetic north (the magnetic being coincident with the axis drawn normal to the plane of the Earths orbit). This variation is precession (wobble). As the tilt off solar perpendicular (made up term) reaches one extreme (I can’t recall which at the moment) it causes an unstable balance in the global climate triggering an ice age (for very complex reasons its not a gradual progession, it kinda sets off a trigger that then preceeds way of the norm). The graph of this angle can be matched up with graphs of global ice mass and they coincide at the same periods (there is a slight lag of several hundred years). Closer inspection shows that some of the ice ages are much worse than others, this is because of fact two.

Two, the eccentricity (the amount of difference in the Earths orbit from perfectly circular, and by extention the amount of diffence between apogee and perigee) of the Earths orbit changes in a much longer period than that of the presession. If the extrema of the ice mass plot are graphed it coincides with the period of the change in eccentricity.

In short the precession causes ice ages in a regular period, the change in eccentricity alternates the severity of these ice ages in a regular period.

Interestingly, one major fear in the global warming, greenhouse debate is that the change in the composition of the Earths atmosphere could alter that “trigger” I mentioned. So that instead of the next ice age occuring in 3 thousand years (made up number) it could occur out of period in 1000 years.

Kinda. I don’t like the analogy with the top because the top rotates about a stationary point at the bottom. This is not quite precession. Precession is this, you’ll need to think in 3D, the axis of the Earth points close to the position of Polaris, but not at, so that when the Earth rotates (NOT revolves) polaris (to an observer on Earth) traces out a circle around the axis. When the Earth precesses the distance the axis is away from the actual position of polaris changes slightly and the circle it traces out gets bigger or smaller (when viewed from the same place on Earth and same day of the year). The Earth does not rotate about the axis that indicates up relative to the plane of the orbit. This is what the analogy of the top indicates.

Hmm, well that would explain something. My last girlfriend had cats named Apogee and Perigee. She sure was goofy at full moon, too.

It is not lunar-specific, but it is earth-specific. That’s “gee” as in “Gaia”. Anyone using the words “apogee” or “perigee” with respect to an orbit around anything but the Earth is being a verbal slob.

And this is complete nonsense.

John W. Kennedy
“Compact is becoming contract; man only earns and pays.”
– Charles Williams

The Earth’s magnetic field is generated in the outer core, probably due to interactions with the inner core. This geodynamo is metastable and the magnetic poles thus have a tendency to “wander.” The present MNP is northwest of Hudson Bay and is moving in an easterly direction at (as I recall) a few 10s to 100 meters/year. The position of the magnetic poles is unrelated to the “plane of the Earth’s orbit.”

Magnetic field has absolutely nothing to do with anything. The 23.5 degree tilt of the Earth’s rotational axis is WRT the Earth’s axis of revolution. I failed to make this clear in my previous post, so I will say it again: Change in tilt is not “wobble” (precession). Think back to the top analogy – the spinning top can precess without changing the angle between its axis and the vertical.

A precessional cycle takes 22 kyrs, while the angle of tilt cycles with a period of 40 kyrs. Obviously the two are related (there could be no precession if there was no tilt) but they are independent factors in determining the incident solar radiation.

This is exactly precession. Remember there are no privledged frames of reference. Think of the top rotating about its center of mass rather than a fixed point on a table and it is precisely analogous to the Earth’s precession.

It is much more complex than this. First, orbital variation occurs in cycles not “periodic events.” The latter implies that the tilt or eccentricity remains the same for awhile and then suddenly changes, then changes back after a period of time. I know this is not what you meant, but in the absense of graphics we need to be careful what pictures we put into people’s heads.

Also, Earth’s climate is controlled by the interactions of many more than two factors, as plots of global ice volume and eustatic sea level variations indicate. Once again, I know you didn’t mean to imply that only two things were important, but your statement implies a certainty of understanding rivaled only by radio talk show hosts. :slight_smile:

Eccentricity varies with a period of 100 kyr. While spectral analysis of the Oxygen isotope plot indicates that the strongest signal is at 100 kyr (for the last half-million years – previous to that the strongest signal was 40 kyr), there are strong arguments against eccentricity as the forcing mechanism: (1) Why is it the strong signal only for the past half-million years? (2) Eccentricity varies by only ~ 3% – the increase or decrease in solar radiation due to this should minor compared with both tilt and precession.

It is likely, or at least possible, that there is another forcing mechanism that accounts for the 100 kyr cycle. For instance, we know that the sun has both 11 year and 150 year sunspot cycles. It is not outside the realm of possibility that solar output is a forcing mechanism. Another possibility is the angle of our orbit WRT the solar equator. Currently we are tilted ~3 degrees, and we pass through the equatorial plane twice a year (August and February, I believe). Since the majority of the interplanetary dust and debris lies in or close to this plane, the more time we spend here the less incident solar radiation we are likely to receive.

The strongest correlation between ice volume (O18 maximum) and orbital forcing occurs when we calculate incident solar radiation at 60 degrees latitude. As anyone who has lived at this latitude can tell you, getting snow to fall during the winter is no problem. The present interglacial is defined by the fact that all of the annual snowfall melts during the summer. This suggests that milder summers at high latitudes may be one cause of glacial advance. This could be due to precession (Earth-Sun distance at the solstice is medial rather than extreme).

While orbital forcing mechanisms are easy to correlate to ice age maxima and minima, the fact that changes seem to occur rather quickly indicates that internal mechanisms play a big role. Greenhouse gases are a possibility but they are somewhat buffered by rock weathering and ocean absorption. Ocean circulation is a good possibility – if the heat-exchange between tropical and polar regions becomes less efficient, high latitudes become significantly cooler.

The Merriam-Webster Dictionary precisely reads:

Apogee: the point at which an orbiting object is farthest from the body being orbited.

Perigee: the point at which an orbiting object is nearest to the body being orbited.

I have no doubt in the etymology of the word, but that doesn’t realy make for a defintion. I’d do a little research first, and maybe trust that there are practical defintions beyond that of their latin roots. I’ll trust my text books. Just goes to show that a little knowledge is a dangerous thing.

jreka, About the Magnetic axis thing, you are absolutely right. I mistakenly used the terminology wrong and if you ignore the word magnetic and use my description based on the axis of revolution instead I stick by my explanation.

Also, I was wrong in my defintion of precesion, but my description in the changing tilt and its importance to ice ages is accurate. Precession on the other hand has no influence on the amount of solar energy that reaches the Earth.

M-W definition of cycle is: “one complete occurance of a periodic process”, I think that my shortening of it to a periodic event is accurate, and in the context is correct, while I conceed that under other context your suggestion of the implications also apply. I don’t see this as any point to quibble about, but my words are right also.

About my apparent level of certainty, well the debate I refered to include the debate of how significant other factor are on climate change. Several scientists think that the climatic change is as simple as orbital influences, other argue that its effects are nonsense. Anyone who claims that only one influence is important is a moron, but I’d lean toward a higher level influence on the orbital changes than the atmospheric based on historic data.

About the other influences you stated. The tilt of our orbit may apply, but I wonder how that could change the amount heat energy we receive, is this to imply that he sun shines “brighter” when your on its equator? This is beyond my knowledge but i can’t intutively think of any reason this would be. As for sun spot cycles, well to my knowledge most of the emmited radiation is in the form of gamma raiation and not light energy that is easily transfered into heat. Now I could be missing a step here, but what in the sun spot cycle could change the heat energy absorbed by the earth. Sun spots are to erratic and don’t provide the cumulative influence that a Earth orbit change applies. Atmospheric issues are important in the results of an ice age and IMHO are influenced by orbital dynamics and in a sense are the medium of the change, but not the cause of change. They most definately are why the ice ages act quickly in a “periodic event ;)” once that the “trigger” is reached by the orbital change, but they are resultant not causal.

How would precession (your definition) change the incident radiation at a latitude? Being that the angle of tilt stays the same, so does the radiation at latitude, all that changes is the timing of equinox and solstice. The amount of heat absorbed is the same if solstice occurs in July or in February.

The facts expressed here belong to everybody, the opinions to me. The distinction is
yours to draw…

Omniscient; BAG

Giving the good Doctor the benefit of the doubt, I’ll assume he’s talking about a sidereal month, which is 27.3 days, the time it takes the moon to complete one full orbit of the earth.

If you started a sidereal month with the moon at apogee, that condition would exist twice in that month and perigee once, or vice versa.

An interesting sidelight - try visualizing (or tracing with your finger) the path the moon decribes as it follows the earth around the sun. Most people will trace a series of corkscrews, but that’s not correct. Such a condition would require the moon to become retrograde in its own orbit about the sun, violating the laws of planetary motion. The true path is a sinuous line snaking to the inside and outside of an ellipse describing the earth’ orbit of the sun. (I failed that test when it was first posed me).

Precession is not as important as tilt, but is more significant than eccentricity. Precession affects when cardinal points (esp. solstices) occur, relative to the Earth-Sun distance. Right now, perihelion and aphelion occur within a few weeks of the solstices. When the these orbital extrema coincide, one hemisphere has more intense seasons, the other has less. When solstices are out-of-phase with the perihelion and aphelion the seasons in both hemispheres are equivalent and mild. Milder summers lead to less melting of the winter snowfall.

Much of the solar systems debris orbits in the sun’s equatorial plane. I’m suggesting (speculation ahead!!!) that when Earth’s plane of revolution lies within the sun’s equatorial plane, a not insignificant amount of radiation is scattered before it reaches Earth. By the way this occurs every 100 kyrs.

Where to start? (1) Sun spots are not erratic, they are cyclic, reaching a maximum every 11 years. (2) Very little gamma radiation escapes the sun – for all intents and purposes all radiation incident on the Earth is in the visible spectrum. (3) As sunspots are cooler by several thousand degrees they emit much less radiation than the rest of the sun, and therefore the Earth receives less solar radiation during sunspot maxima.

Certainly an 11 year cycle does not affect climate. I bring it up only to illustrate that the sun does have cyclic variations in its output. Within the past decade a 150 year secondary cycle has been recognized, and (speculation ahead!!!) it’s not unreasonable to expect that the sun goes through longer period variations in output.

Orbital parameters are clearly important forcing mechanisms for climate change over the past few million years. Evidence indicates that the Earth experiences climate variations that, if they are cyclic, suggest other influences which vary over much longer time scales. O18 and sea surface elevation plots, while they correlate well with known orbital parameters, are not explained completely by them.

On a simpler note, despite what MW says, common usage is aphelion/perihelion for objects orbiting the sun, and apogee/perigee for objects orbiting Earth. The -gee indeed refers to Gaia, as the -helion refers to Helios the sun god.