Astronomical Control of Solar Radiation

Understand key aspects of the Earth’s movements:
- Rotation
- Orbit
- Equinox
- Solstice
Name and explain the three orbital cycles:
- Obliquity
- Precession
- Eccentricity
Understand how the orbital cycles impact climate and their effects on different latitudes.

Transition from tectonic-scale change (tens to hundreds of millions of years) to orbital scale change (from the last few million years).
- Focus on the last ~3 million years due to availability of numerous proxy records that provide insights into how climate responded to these changes.
- Importance of the record of forcing and responses, examining dynamic climate interactions.

Seasons result from Earth's tilt combined with its changing position around the Sun.
Solstices:
- June 21: Northern Hemisphere summer, Southern Hemisphere winter.
- December 21: Northern Hemisphere winter, Southern Hemisphere summer.
Equinoxes: (referred to as "equal night")
- March 20: Northern Hemisphere spring, Southern Hemisphere autumn.
- September 22: Northern Hemisphere autumn, Southern Hemisphere spring.

The axial tilt does not remain constant; it varies approximately on a 41,000-year cycle.
- A larger tilt leads to more extreme seasons (hotter summers and colder winters).

Perihelion: Closest distance to the Sun.
- Current distance: 153 million km.
- Occurs around January 3rd.
Aphelion: Farthest distance from the Sun.
- Current distance: 158 million km.
- Occurs around July 4th.

Eccentricity varies in shape from near circular to elliptical.
The difference in the axes of the elliptical orbit increases over time.
- Current trajectory is trending towards a more circular orbit.
- Cycle of approximately 100,000 years.

Precession refers to the long-term wobbling motion of Earth's axis.
- Changes the spatial position of the solstices and equinoxes over time.
- Analogous to a spinning top, where Earth slowly wobbles and alters its tilt direction across epochs.

Precession driven by two components:
- Precession of the axis: The direction that Earth's axial tilt points rotates approximately once every 25,700 years.
- Precession of the ellipse: The entire elliptical orbit of Earth rotates, contributing to the movement known as the precession of the equinoxes (~23,000 years).

Positions of solstices and equinoxes change gradually over time, affecting solar radiation received by Earth.
- Figures illustrate the connections between maximum and minimum distances (aphelion and perihelion) and their associated seasonal effects at predicted points in the future, such as:
- 5,750 years from now
- 11,500 years from now
The environmental and climatic implications from altered distributions of solar energy must be closely monitored.

Definition: A measure of combined effects of eccentricity and precession on solar radiation received.
Variability in eccentricity modulates the changes in Earth-Sun distance aligned with the 23,000-year precessional cycle.
- High eccentricity results in large fluctuations in Earth-Sun distances.
- Low eccentricity results in minimal differences in Earth-Sun distances.

June and December insolation curves exhibit dominance of the 23,000-year cycle at lower and mid-latitudes, and explicitly at high latitudes during summer.
- No significant presence of the 41,000-year cycle (obliquity/tilt) evident at low latitudes.
- Questions arise regarding the influence of eccentricity during these cycles.

Various analytical methods, including:
- Time Series Analysis
- Spectral Analysis
- Utilizes filters to identify cycle amplitudes of significant periodicities (e.g., 100,000 years, 41,000 years, 23,000 years).
- Typical spectral analysis demonstrates the presence of cycles in complex climate signals, as shown in graphical representations, indicating how real-world climate records reflect multi-periodic oscillations.

Reiterate the understanding of key Earth movements.
Review of orbital cycles: obliquity, precession, and eccentricity, and their climate impacts across latitudes and over geological time scales.