Milankovitch Cycles and change of seasons


by arunshanker
Tags: cycles, milankovitch, seasons
arunshanker
arunshanker is offline
#1
Dec6-13, 04:28 AM
P: 7
I read of the Milankovitch Cycles which explains the collective effects of changes in the Earth's movements upon its climate or in other words long term climate change. There is a flash animation here
http://www.wwnorton.com/college/geo/...tions/18_2.htm
I have also read that
There are eccentricity, obliquity and precession involved here
What I want to know is will there be a time when Northern Hemisphere will experience summer in December and winter in June because of this.

There are some sites that say this will happen ( Northern Hemisphere will experience summer in December) but wiki says that
The hemisphere which is in summer at perihelion will get increase in solar radiation, but that same hemisphere will be in winter at aphelion and have a colder winter. The other hemisphere will have a relatively warmer winter and cooler summer.
At present, perihelion occurs during the Southern Hemisphere's summer, and aphelion is reached during the southern winter. Thus the Southern Hemisphere seasons are somewhat more extreme than the Northern Hemisphere seasons, when other factors are equal.
Can anybody explain
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Bandersnatch
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#2
Dec6-13, 06:05 AM
P: 549
It won't happen, even though in a sense it will.

The seasons are determined solely by the direction of the axial tilt. If the axis points towards the Sun, it's summer. If away, it's winter. If exactly sideways, it's either spring or autumn.

Furthermore, our calendar is set to have certain dates in it coincide with solstices and equinoxes. It's always 22nd of December when the winter solstice happens in the nothern hemisphere, always 21st of March during spring equinox, and so on.

So while due to the precession the direction the axis of Earth's rotation points to changes over the Great Year(~25000 years), and in exactly half a Great Year it will point exactly opposite to where it points now(so it'll be almost summer in the northern hemispere instead of almost winter), the calendar will have followed the gradual changes, and it'll be June instead of December.


Changes in perihelion position, precise angle of axis inclination, and orbit eccentricity don't matter here, as they can't change the direction the axis is pointing to. Even though they do affect the overall climate.


As you can see, even though in about 12500 years at this time of year it'll be late spring up north rather than late autumn, you can't ever have summer in December(N hem.). Unless you change the calendar so that it is no longer tied to the seasons. But then, in 12500 years it seems unlikely that whomever is around will be still using ours.
AlephZero
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#3
Dec6-13, 08:59 AM
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Historically the calendar has been tied to the seasons. There is one change that is observable, and that is the gradual shift in the stars visible at different seasons. In 12500 years, the stars that are currently visible at midsummer will be visible at midwinter, and vice versa.

That was re-discovered in about 1500AD when the ancient Greek writings on astronomy were re-discovered, and it was realized that all the Greek records were systematically "wrong" by a large amount - i.e. about 2000 years worth of the 25,000 year cycle.

"Re-discovered," because the ancient Greeks already knew about this, comparing their own observations with ancient Egyptian data (some of which is accurately defined by the alignment of the pyramids, etc) from a few thousand years earlier still.

This also makes a nonsense of systems of astrology which are based sky charts when they were first invented but have ignored the effect. The constellations of the zodiac today are nowhere near the positions that popular astrology (in newspapers etc) say they are!

tadchem
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#4
Dec12-13, 12:15 PM
P: 61

Milankovitch Cycles and change of seasons


There are several 'years' of astronomical importance.
The sidereal year (avg 365.256363004 mean solar days) is the time it takes for the sun to apparently move 360° relative to the (so-called 'fixed') stars.
The tropical year (avg 365.24219 days) is the time it takes the sun to to apparently move 360° relative to the equinoxes (which define the seasons).
The anomalistic year (avg 365.259636 days) is the time taken for the Earth to complete one revolution with respect to its apsides (perihelion or aphelion).
The 'common year' is the tropical year, so the winter solstice will *always* be in December for the northern hemisphere and June for the southern hemisphere.
The stars behind the sun at the equinoxes and solstices will gradually change (by 0.014173 days each year), so when the sun reaches the NH spring equinox (astrologically entering the Sign of Aries) it is actually "in" the Constellation of Pisces. The drift of the equinoxes relative to the stars takes about 25,800 years to complete a cycle. This is because of the precession of the earth's axis. In about 2400 CE the sun will be in the constellation of Aquarius on the first day of Spring - according to some this will be the beginning of 'the Age of Aquarius.'
The Milankovich cycles are more concerned with the difference between the date of the winter solstice and the date of perihelion. The 'drift' of the perihelion relative to the solstices is found by the difference in rotation rates: 0.017446 days per year, or about 20,950 years per cycle.
So 10,500 years ago (about when the Younger Dryas ended) the relative positions of the equinoxes and apsides were 180° opposite - the earth was closest to the sun during the northern hemisphere winter.
klimatos
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#5
Dec12-13, 06:49 PM
P: 403
Quote Quote by Bandersnatch View Post
Furthermore, our calendar is set to have certain dates in it coincide with solstices and equinoxes. It's always 22nd of December when the winter solstice happens in the nothern hemisphere, always 21st of March during spring equinox, and so on.
Not quite. Actually, both the equinoxes and the solstices occur at some particular instant in time. At that instant, it will always be two different dates over the face of the Earth. Therefore, the date of the equinox or solstice will depend upon which particular time zone you happen to be in at that instant. I seem to recall that there are 29 different time zones on Earth (some zone are on the half hour compared to their neighbors).

Bandersnatch is correct in attributing seasonality to the axial tilt. Right now, the northern hemisphere gets its maximum daily insolation on the June solstice. However, the Earth is closest to the Sun about January 4th. Over time, the precession of the Equinox will cause perihelion and the June Solstice to occur on the same date. This is a major part of the Milankovitch theory of continental glaciation advance and retreat.
pikpobedy
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#6
Jan19-14, 05:49 PM
P: 39
Quote Quote by arunshanker View Post
At present, perihelion occurs during the Southern Hemisphere's summer, and aphelion is reached during the southern winter. Thus the Southern Hemisphere seasons are somewhat more extreme than the Northern Hemisphere seasons, when other factors are equal.
Can anybody explain
That is a coincidence that will gradually be reversed over tens of thousands of years. AKA, the precession of earth's orbital ellipse.
klimatos
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#7
Jan19-14, 10:40 PM
P: 403
Quote Quote by arunshanker View Post
At present, perihelion occurs during the Southern Hemisphere's summer, and aphelion is reached during the southern winter. Thus the Southern Hemisphere seasons are somewhat more extreme than the Northern Hemisphere seasons, when other factors are equal.
Can anybody explain
The Earth’s annual orbit around the Sun is elliptical. That point on the orbit that is closest to the Sun (perihelion) occurs around January 4th (the date varies somewhat depending upon what time zone you are in and how close the current year is to leap year and some other factors). That point on the orbit that is farthest from the Sun (aphelion) occurs around July 4th.

This variation affects the intensity of solar radiation at the outside of the Earth’s atmosphere by 23 Wm-2, or some 6.73%. This would make southern hemisphere summer hotter than northern hemisphere summers, and winter colder if other things were equal. Other things are not equal. The southern hemisphere has most of its surface area in water. This moderates both summer and winter temperatures from what they might otherwise be.

The Precession of the Equinox makes these dates progress through the calendar over a 26,000 year cycle. About 13,000 years from now these dates will be reversed and the northern hemisphere will get increased insolation in summer and less in winter. This cycle is one of the three orbital cycles that make up the Milankovitch Theory (orbital forcing). This theory explains the systematic re-occurrence of Ice Ages at approximately 100,000 year intervals.
seany
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#8
Feb19-14, 03:15 PM
P: 6
What I want to know is will there be a time when Northern Hemisphere will experience summer in December and winter in June because of this.

Short answer: In earth no.

Long answer : there are planets where something else plays a role : the change on axial tilt. E.g mars. In such cases, one hemisphere may experience extremely long summer for more than 6 months, due extremely long lengths of days. and the other hemisphere, exetremely long winter.But Milankovich cycle ALONE wont change the summer and winter marked by when the temperature extrremes are occuring, but may very well change the lengths of them.
Eagle9
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#9
Feb21-14, 12:49 PM
P: 129
I have heard that the precession is caused by two circumstances:
1) Because of Earth’s obliqueness the equatorial places on the Earth have got more mass.
2) Because of lunar (solar also) gravity.
These two factors are interconnected as I know.
If this is true then:
1) If the Earth did not revolve around its axis (that is if its sidereal period was equal to zero) there would be no precession.
2) If Earth had no Moon there would be no precession.
According to this logic the Venus should have no precession since it does not have its Moon and also it revolves around its axis very slowly and its obliqueness is much less than Earth’s one.
Did I write everything correctly?
seany
seany is offline
#10
Feb22-14, 05:55 PM
P: 6
1. yes. Precession is what happens to a spinning top. No matter how slow, it has some components of precession. If you absolutely stop it, then there is no precession

2. The moon actually stabilizes a planets precession. Lack of moon, or less massive moon (in case of mars) messes it up. The fact that there is more mass near the equator contributes to the generation of precession, but not only a moon, but all other planets, especially jupiter, nad sun itself too, contributes to it.
seany
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#11
Feb22-14, 05:56 PM
P: 6
1. yes. Precession is what happens to a spinning top. No matter how slow, it has some components of precession. If you absolutely stop it, then there is no precession

2. The moon actually stabilizes a planets precession. Lack of moon, or less massive moon (in case of mars) messes it up. The fact that there is more mass near the equator contributes to the generation of precession, but not only a moon, but all other planets, especially jupiter, nad sun itself too, contributes to it.
Eagle9
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#12
Feb23-14, 08:01 AM
P: 129
seany
The moon actually stabilizes a planets precession. Lack of moon, or less massive moon (in case of mars) messes it up. The fact that there is more mass near the equator contributes to the generation of precession, but not only a moon, but all other planets, especially jupiter, nad sun itself too, contributes to it.
So, if I understood correctly if there was no Moon then Earth’s axis of rotation would somersault, would not be stabilized and aimed to Polaris star, right?


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