The Earth's orbit around the Sun is determined by not only the gravitational pull of the Sun, but also the weaker influences of the moon and the surrounding planets.
These forces cause the Earth to have a slightly irregular orbit (orbital eccentricity). Currently, there is about a 3% difference in the distance between the Earth's closest point to the Sun in its orbit (perihelion), and its furthest point from the Sun (aphelion). In terms of climate, this means that there is a 6% difference in incoming solar radiation between January 3 (perihelion) and July 4 (aphelion).
The shape of the Earth's orbit cycles from elliptical to almost circular every 90,000-100,000 years because of the gravitational pull of other nearby celestial bodies. When the orbit is highly elliptical, the amount of solar energy reaching the Earth is 20-30 times more when the Earth is closest to the Sun, than when it is at its furthest point away from the Sun. Examine the images at "Orbital Variations" for illustrations of orbital variations.
Data Activity: Orbital Forcing and Climate Cycles
These data activities will enable you to analyze the astronomical/orbital forces on the Earth's climate operating on geological timescales.
Lake Vostok is a large lake buried under 4 km of ice in Antarctica. An ice core was recovered from the ice above the lake, which yielded a paleoclimate record of 400,000 years.
Activity 1: Examining Earth's Irregular Orbit—Eccentricity
Learn more about the Earth's irregular, or eccentric orbit (eccentricity), with the "Vostok Core & Milankovitch Cycles Climate Applet".
On the right side of the applet is the temperature record reconstructed for the past 400,000 years from the Vostok (Antarctica) ice core. For now, review just this temperature record and look for any cycles in the data.
Imagine that you are a scientist seeing this data for the first time. Describe any patterns you see in the temperature record from the Vostok Ice Core.Answer
Over 400,000 years, there is evidence of four warm periods, including the one we are in today, and four cold periods. There appears to be a cycle of approximately 90,000-100,000 years between each warm period.
Now go back to the "Vostok Core & Milankovitch Cycles Climate Applet" and click on the gray, rectangular button on the left so it displays "Show Top View." This will give you a better idea of the true geometry of the Earth's orbit. Today, there is only a difference of 3%, or 5 million kilometers, between the closest (perihelion) and furthest (aphelion) distances of the Earth from the Sun during its orbit.
Drawings of the Earth’s orbit around the Sun cannot be drawn to scale on a page or screen. However, it is precisely this inaccurate perspective drawing, reproduced in countless textbooks, that is responsible for the broadly held misconception among students that the Earth has an oblique orbit. This misunderstanding is responsible for yet another misconception—that summer is when the Earth is closest to the Sun, and winter is when the Earth is farther away. When viewed from the side, the Vostok Core & Milankovitch Cycles Climate Applet shows the mistakenly highly elliptical orbit that adds to students’ confusion over the shape of the Earth’s orbit around the Sun.
Describe the orbit that you see when the Earth and Sun system are viewed from the top.Answer
The orbit is nearly circular when viewed from the top.
Next, check the "Eccentricity" field to add eccentricity curves to the record. You will notice magenta curves over the temperature graph that represent the calculated eccentricity of the Earth's orbit over the past 400,000 years.
What is the relationship between the Vostok temperature graph and the eccentricity plot?Answer
There is not an exact correlation between the Vostok temperature graph and the eccentricity plot, but there is correspondence between the increased eccentricity of orbit and periods of colder temperatures.
Play with the slider on the left of the graph to see the very slight changes in the Earth's orbit that result from eccentricity.
Do you think there is any difference in the total amount of radiation over the course of a year during the time when the orbital is most eccentric?Answer
No, there is no change in total solar energy, but there would be a larger difference in insolation of the planet between when the Earth is at its furthest point in the orbit (perihelion), versus when it is at its closest approach to the Sun (aphelion).
Activity 2: Exploring Earth's Axial Tilt—Obliquity
Eccentricity is only part of the puzzle. Since it does not change the amount of total solar energy, it cannot be the factor that caused the natural climate cycles we see during the Ice Ages. Now look at another astronomical factor: the Earth's tilt on its rotational axis, called Obliquity. Examine the images at "Orbital Variations" for illustrations of axial tilt (obliquity) by scrolling down towards the middle of the page.
It may come as a surprise to learn that the Earth's tilt on its axis has changed over time. On the other hand, you may have heard that the Earth's axis tilt increased slightly (25 cm) because of the 2011 9.0 magnitude earthquake in Japan. Today, the Earth's axis is tilted at 23.5 degrees from the plane of the Earth's orbit. Return to the "Vostok Core & Milankovitch Cycles Climate Applet" to learn more about the Earth's axis tilt and its relationship with climate.
Uncheck the "Eccentricity" field and check the "Tilt" field. Look back at the temperature graph to see a magenta line that shows the calculations for changes in tilt over time.
Describe the curve and its relationship to the Vostok temperature data.Answer
There are 10 peaks in the curve over 400,000 years, so there appears to be a periodicity of approximately 40,000 years where the Earth's axis of rotation tilts back and forth. There does not appear to be a clear relationship to the temperature curve.
Now check the "Eccentricity" field again while leaving the “Tilt” field checked and review the magenta line on the temperature graph, which shows the spectra combining the curves from eccentricity and tilt.
How does this combined curve fit with the temperature graph?Answer
The combined eccentricity-tilt curve has a better fit with the temperature data. It is not perfect, but the correspondence looks too close to be random.
The Earth's tilt/axial obliquity on its rotational axis is the "reason for the seasons." More tilt creates exaggerated seasons and less tilt creates milder seasons.
Activity 3: Precession of the Seasons
Changes in axial tilt also play a role in the timing of when a hemisphere is at its closest point in its orbit round the Sun. When a hemisphere is at its closest point (perihelion), it receives more solar energy. When the hemisphere is at the furthest point from the Sun in its orbit (aphelion), it receives less. Examine the images at "Orbital Variations" for illustrations of precession which refers to a change in the orientation of the rotation axis of a rotating body.
Return to the "Vostok Core & Milankovitch Cycles Climate Applet" to learn more about precession.
Uncheck any selected fields and then check the "Precession" field. Look back at the temperature graph to see a magenta line that shows the calculations for changes in precession over time.
Describe the curve and its relationship to the Vostok temperature.Answer
There are about 20 wiggles in the curve, approximately one every 20,000 years. It is unclear by looking on screen just by eye and it is not possible to determine if there is a good correlation to the temperature data.
Explore precession of the equinoxes by moving the slider up and down the graph. Select three or four places on the curve, and note where summer solstice takes place with respect to the perihelion and the aphelion. Notice how precession causes the seasons to move around the orbit: the summer solstice, which occurs today at the aphelion used to take place at the perihelion 400,000 years ago.
Now check all three parameters: "Eccentricity," "Precession," and "Tilt" and observe the results.
What do you notice about the alignment between the temperature curves and the orbital parameters?Answer
With all three parameters, there is the best alignment between the temperature curves and the orbital parameters
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