Sun, Moon and Earth
A year is the Earth's journey around the Sun.
A month is the Moon's journey round the Earth
A day is the Earth's rotation on its own axis
A week is a subdivision of the Moon's journey round the Earth
The Earth goes round the Sun in an anti-clockwise direction- looked at from the North Pole - the Sun always rising in the East.
The seasons are the Earth tipping on its own axis. The maximum extent of this tipping is marked by the imaginary lines of the Tropics of Capricorn and Cancer.
The base distribution of cold and heat on the Earth is determined by a number of things: length of day and position of the Sun.
The diameter of the Earth is 12,742 km (7,900 miles). This means the Equator is approximately 6000km closer to the Sun than the Poles. This is a negligible difference compared to the distance of the Earth from the Sun - 152-147 million km.
The angle of the sun is crucial for two reasons. When overhead it is focussed on a relatively small area and comes directly through the Earth's atmosphere. As the angle of the sun increases away from the Equator it is diffused over a wider area and travels through more of the Earth's atmosphere.
The Earth's equator is about 40,075 kilometres (24,901 mi) long; 78.7% is across water and 21.3% is over land.
The Equinox
'In its seasonal apparent movement across the sky the sun passes over the Equator twice each year, at the March and September equinoxes. At the moment of the equinox, light rays from the center of the sun are perpendicular to the surface of the earth at the point on the Equator experiencing solar noon.'
'For approximately half of the year (from around March 20 to around September 22), the northern hemisphere tips toward the Sun, with the maximum amount occurring on about June 21'.
Seasons
Seasons are marked by changes in the intensity and duration (daylight hours) of sunlight that reaches the Earth's surface
'The seasons result from the Earth's axis of rotation being tilted with respect to its orbital plane by an angle of approximately 23.5 degrees'.
Axial Tilt or Obliquity
'The Earth's axis remains tilted in the same direction with reference to the background stars throughout a year (throughout its entire orbit). This means that one pole (and the associated hemisphere of the Earth) will be directed away from the Sun at one side of the orbit, and half an orbit later (half a year later) this pole will be directed towards the Sun. This is the cause of the Earth's seasons.'
From Wiki Axial Tilt
My explanation:
The Earth's tilt is constant relative to the background stars rather than to the Sun. As the Earth orbits the Sun through a year this means that the relationship of the Northern and Southern Hemisphere's relative to the Sun changes.
At the 21 June Northern Hemisphere summer solstice the top of the earth is tilted in towards the Sun. On the 21 December Northern Hemisphere winter solstice the top of the Earth is tipped away from the Sun whislt the bottom is tipped in towards the Sun.
I find it very difficult to grasp this very basic fact of seasonal change. I think my problem has been that I thought the Earth's tilt was relative to the Sun. If this were the case the Sun would always shine on that unchanging part of the Earth tilted closest to the Sun and there would be no seasons.
However, the Earth's tilt is independent of the Sun and not caused by the Sun.
Axial Precession
'In astronomy, axial precession is a gravity-induced, slow, and continuous change in the orientation of an astronomical body's rotational axis. In particular, it refers to the gradual shift in the orientation of Earth's axis of rotation, which, similar to a wobbling top, traces out a pair of cones joined at their apices in a cycle of approximately 26,000 years.'
From http://en.wikipedia.org/wiki/Axial_precession
Presumably precession differs from tilt in that the angle of tilt stays the same as the Earth wobbles around its axis?
The Earth's Eliptical Orbit
The Earth's orbit of the Sun is eliptical, This is due to gravitational interfence from other bodies oorbiting the Sun, in particular Jupiter and Saturn. The eccentricity of the orbit is cyclical - getting greater and smaller. The cycle of eccentricity is 413,000 years.
The Earth is actually closest (147m km/91m miles) to the Sun on January 3rd (Periapsis/Perhelion) and furthest (152m km/95m miles) on 3rd July (Apoapsis/Aphelion).
Variations in Orbital Eccentricity
With the variation of orbital eccentricity these maximum and minimum distances change.
Currently difference between perihelion and aphelion is only 3.4% (5.1 million km)
and accounts for a variation in incoming solar radiation of about 6.8%.
When the Earth's orbit is at its most elliptical the variation between perihelion and aphelion is
and equates to a change of solar radiation between aphelion and perihelion of 23%.
Changes in the orbital location of the Earth and solstices
From http://www.global-climate-change.org.uk/2-5-2-3.php
Like obliquity, precession does not affect the total amount of solar energy received by the Earth, but only its hemispheric distribution over time. If the perihelion occurs in mid-June i.e. when the Northern Hemisphere is tilted toward the Sun, then the receipt of summer solar radiation in Northern Hemisphere will increase. Conversely, if the perihelion occurs in December, the Northern Hemisphere will receive more solar radiation in winter (see Figure 2.1). It should be clear that the direction of changes in solar radiation receipt at the Earth's surface is opposite in each hemisphere.
From http://www.ncdc.noaa.gov/paleo/slides/slideset/11/11_185_slide.html
As this graphic illustrates, the shape of the earth's orbit varies from nearly circular (eccentricity approaching 0.00) to more elliptical (eccentricity=0.06). These variations occur at a frequency of 100,000 years (which Leverrier demonstrated) and 400,000 years (which later scientists discovered). Croll understood that variations in orbital eccentricity had a small impact on the total amount of radiation received at the top of earth's atmosphere (on the order of 0.1%), but that the eccentricity cycle modulated the amplitude of the precession cycle. During periods of high eccentricity(a more elliptical orbit), the effect of precession on the seasonal cycle is strong. When eccentricity is low (more circular), the position along the orbit at which the equinoxes occur is irrelevant because all points on the orbit become, in effect, perihelia.
As Croll's research indicated, variations in the precession and eccentricity cycles altered the amount of insolation received at different times of the year, creating variations in the distribution of solar energy received from season to season. But other scientists argued that these variations were too minor to explain the tremendous climatic oscillations that occurred as the earth alternated between glacial and interglacial periods. Croll's response to these scientists represented his third major innovation: the concept of climatic feedback mechanisms in the earth's climate system.
Croll believed that the variation of winter insolation was the critical variable explaining climate change. When the precessional cycle placed the earth at its aphelion during the Northern Hemisphere's winter, the northern winters would be significantly colder if this occurrence was coupled with a period of high eccentricity. Croll reasoned that snow would then begin to accumulate to a greater degree, eventually creating large snowfields and glaciers. As reflective snow and ice covered more of the Northern Hemisphere's land area, the earth would absorb less solar radiation. The climate would cool further as glaciers and ice sheets reflected a great deal of solar energy back into space. Croll also speculated about another feedback effect, this one involving the position of warm currents in the Atlantic Ocean. As the northern latitudes cooled, the strength of the trade winds would increase, drawing them southward towards the equator. This in turn reduced the strength of the Gulf Stream as warm currents turned south rather than north as they flowed towards the bulge of Brazil.
Etymology of Eccentric
Ekkentros - (ex-centre)Eccentric first appeared in English in 1551, with the definition "a circle in which the earth, sun. etc. deviates from its center." Five years later, in 1556, an adjective form of the word was added.
Milankovitch cycles
The Earth's axial tilt fluctuates between 22.1 and 24.5 degrees over time - on a 41,000 year cycle (Milankovitch cycles) thousands of years.
'The Earth's axial tilt varies from 24.5 degrees to 22.1 degrees over the course of a 41,000-year cycle. Changes in axial tilt affect the distribution of solar radiation received at the earth's surface. When the angle of tilt is low, polar regions receive less insolation. When the tilt is greater, the polar regions receive more insolation during the course of a year. Like precession and eccentricity, changes in tilt thus influence the relative strength of the seasons, but the effects of the tilt cycle are particularly pronounced in the high latitudes where the great ice ages began'.
From http://www.ncdc.noaa.gov/paleo/slides/slideset/11/11_187_slide.html
NB In diagram above 'distance' refers not to differential distance from the Sun but to the longer distance sunlight travels through the Earth's atmosphere at the
poles
Why the polar regions are colder: Effect of the Earth's shape and atmosphere on incoming solar radiation.
Compared to equatorial regions (b), incoming solar radiation of the polar regions (a) is less intense for two reasons:
the solar radiation arrives at an oblique angle nearer the poles, so that the energy spreads over a larger surface area, lessening its intensity.
The radiation travels a longer distance through the atmosphere, which absorbs, scatters and reflects the solar radiation.
Tropical areas (i.e. lower latitudes, nearer the equator) receive solar radiation which is closer to vertical.
The angle of incidence of the rays, combined with the albedo of the surface has also a strong influence on the amount of energy being absorbed (or reflected) at the surface. In the ice-covered polar zones, almost all direct energy from the sun is reflected because it is white and the angle is small. In short, the angle of incidence affects the heating of the surface in 3 different ways: length of atmospheric track, variable flux and variable reflection
For simplicity, the diagram ignores the axial tilt of the Earth, which causes each pole to slip into darkness for around 6 months of the year, and means the equator's ground is generally not perpendicular with the sun's light.
From Wiki Effect of the Sun's Angle on Climate
Ice Ages and Insolation
From http://www.ncdc.noaa.gov/paleo/slides/slideset/11/11_188_slide.html
This graph shows how insolation has varied at 65 degrees N over the past 400,000 years. While subsequent refinements in astronomy are reflected in this curve, it is quite similar to the one calculated by Milankovitch. By the 1920s, Milankovitch's theory of orbital forcing was practically complete, though some of his calculations would take another two decades to finish. Once again, scientists used field studies to determine how well the theory of orbital forcing explained past climatic variations. Comparisons were initially positive. Sequences of glacial deposits in North America and Europe seemed to support Milankovitch's theory. But these deposits were very difficult to date accurately, and so any correspondence between the timing of glaciation and Milankovitch's insolation curves remained speculative. Nevertheless, most scientists supported the theory until the 1950s, when new developments in dating technology raised doubts about Milankovitch's ideas.
One of the unintended results of the atomic age was the discovery of radiocarbon dating methods. Over a century after Louis Agassiz first proposed his glacial theory, geologists finally possessed a potent tool for determining the age of glacial deposits. They compared these dates with the insolation curves calculated by Milankovitch, and found that there were more glacial advances in the last 80,000 years than Milankovitch's theory could account for. By 1965, the theory of orbital forcing lay once again in disrepute.
From http://www.ncdc.noaa.gov/paleo/slides/slideset/11/11_189_slide.html
Milankovitch's theory would be resurrected once again, however, this time from the depths of the world's oceans. Beginning in the 1960s and continuing to the present day, scientists have found overwhelming support for the Milankovitch theory in long cores such as this one extracted from the ocean floor. These oceanographic studies have two major advantages over the studies of terrestrial glacial deposits done in the 1950s that seemed to refute the Milankovitch theory. First of all, ocean cores provide a much more continuous record of glaciation than terrestrial deposits do. The reason for this being that subsequent waves of glaciation often erased or altered traces of earlier glacial and interglacial periods. Secondly, by the late 1960s, sediment cores pulled from the oceans could be dated with relative confidence as far back as 650,000 yr. BP. In contrast, radiocarbon dating becomes much less accurate on materials over 40,000 yr. old. Thus, the initial land-based studies that claimed to provide chronologies for glaciation over the past 80,000 years relied heavily on some very tenuous dates.
Insolation and global ice volume fluctuations.
From http://www.ncdc.noaa.gov/paleo/slides/slideset/11/11_194_slide.html
When the data are filtered for solar insolation and global ice volume over the past 400,000 years, we see that insolation and global ice volume fluctuated at the same major frequencies: the precession cycle of 23,000 years and 19,000 years, the obliquity cycle of 41,000 years, and the eccentricity cycle of 100,000 years (n.b.: the data do not extend far enough back to test the 400,000 yr. eccentricity cycle). The curves are thus much more similar than they first appear. So what makes them look so different? The biggest difference between the insolation and global ice volume curves is the surprising amplitude of the 100,000-yr. eccentricity cycle in the ice-volume records. The Milankovitch theory predicts that changes in eccentricity have a smaller effect on climate than variations in precession and obliquity. But climatic records from across the globe suggest that the great ice sheets have advanced and retreated to a 100,000-yr. beat. Why this is so is one of the many questions that remain to be answered about ice ages and the forces that drive them. ?