All bodies with mass gravitationally attract other bodies with mass. The amount of attraction depends on the mass of the two objects and the distance between them. The more mass and/or the closer they are, the more attraction between them. So orbiting planets and their star are continually pulling on each other, and the amount and direction of pull changes where each are in their orbit around the star. These changing amounts and directions of pull create cyclical changes in orbital parameters: obliquity, eccentricity, and precession.
Obliquity is the angle between the planes of the planet’s equator and its orbit around its star. Another way to think about it is the angle of the planet’s axis of rotation from the perpendicular to the plane of orbit.
Eccentricity is the deviation of planet’s orbit from circularity. The greater the eccentricity, the greater the elliptical nature of the orbit.
Precession is the gradual change in the orientation of the rotational axis of a rotating body. Think of the wobbling axis of a spinning top.
Earth's orbit changes cyclically over many thousands to hundreds of thousand years. These small changes, called the Milankovitch cycles, alter how much sunlight reaches Earth and where, which can push the poles into a long-term cooling event. When a polar region is cold for a prolonged time, it can trigger an Ice Age.
This article discusses how the Milankovitch Cycles help explain the patterns found in relatively recent paleoclimate data recorded in rocks, sediment, and ice. And this article goes deeper into the effects of the Milankovitch cycles.
Suggested Time: 90 min to read and take notes.
Use the software to interactively explore how a planet's radiation budget changes when its orbit changes.
The software provides the opportunity to design and analyze controlled experiments to piece together the complex response of planets to orbital changes. This involves setting the parameters to a state with the least variability and then change one parameter at a time to see its effect on the system. Highly recommended that the Sun-Earth Connection software and activities have been completed before using this software.
Acknowledgements A special thank you to the beta testers of version 1-0. Their insightful comments, meticulous attention to detail, and creative ideas improved the software greatly: Elliot Blume-Pickle, Mason Glidden, Alan Gould, and Mattea Horne.
Mac users: Download the zipped dmg file. Uncompress and double click on the dmg file. Drag the folder that pops up to the application folder on your computer.
Windows users: Select the location you want to put the files before uncompressing everything in the zip file.
Suggested Time: 2 60-minute blocks of time to explore the many options.
The following documents support using the software to:
Change the obliquity, eccentricity, and precession to visualize how the illumination of the planet changes during its orbit.
Explore how the illumination at sunrise, local noon and sunset; hours of daylight; star angle above the horizon at local noon; and total daily solar energy hitting the ground change at different locations as orbital parameters change.
Viewing from the planet's surface, explore how the star's apparent motion across the sky create the planet's seasons, climate regimes, and diurnal heating patterns.
Compare how different latitudes are affected by the star's illumination throughout the year.
Explore the range of change of an illumination variable for latitudes on the planet.
Explore how the planet would differ from Earth - will it be hotter, colder, more extreme, milder, and will it support life the same way Earth does?
The most important calculation was to be able to locate the planet on its orbital path for any given day (or sol) of the year. This article was invaluable in illustrating how to use a reference circle centered on the center of the elliptical orbit and with a radius of the semi major axis of the ellipse.
Earth's insolation calculations can be applied to other planets by modifying key parameters.
Suggested Time: optional. If you do delve into this, it will take a good bit of time.
Screenshot of Star-Planet Connection using the Orbit View section.
The amount and type of radiation emitted from a star are two critical components that create liquid water on a planet (see the radiation simulation software on the energy page). The range of distances from the star where liquid water would be expected is called the habitable zone or the Goldilocks zone. But this is just a starting point, which is why this arc (course) is focused on Earth systems - the systems needed to allow our planet to support and sustain complex life on it for most of its 4.5 billion year existence. Earth is in the habitable zone, but so are other planets in our solar system. Yet only Eath has complex life. Why?
Suggested Time: 30 minutes
The above image is from Sonny Harman's website. He has a number of other fascinating analyses of habitable zones for our solar system and those being discovered through ever improving space and Earth-based telescopes.
Complex life takes a long time to evolve from simpler single-cell organisms. On Earth, there is evidence that simple life forms were present at least 3.5 billion years ago, and complex multicellular life forms didn't appear until 600 million years ago. The early life forms altered Earth's climate dramatically with the release of oxygen to the atmosphere and the removal of a portion of the carbon dioxide. And if the Earth's orbit were not stable, the drastic changes may have been amplified. This brief article is an excellent summary of why Earth was favorable for the long-term evolution of complex life.
Suggested Time: 60 minutes to read and take notes.