Date: May 15, 2012
Title: Astronomy Word of the Week: Obliquity
Podcaster: Dr. Christopher Crockett
Organization: United States Naval Observatory
Links: http://christophercrockett.com
http://astrowow.wordpress.com/
Description: The tilt of our planet is about much more than the seasons. The astronomy word of the week is “obliquity”.
Bio: Dr. Christopher Crockett is an astronomer at the United States Naval Observatory in Flagstaff, Arizona. His research involves searching for planets around very young stars (“only” a few million years old). It is hoped that the results from this research will help constrain models of planet formation and lead to a better understanding of where, when, and how often planets form. Chris is also passionate about astronomy outreach and education and will talk for hours about the Universe if you let him.
Today’s Sponsor: “This episode of 365 days of Astronomy” is sponsored by iTelescope.net – Expanding your horizons in astronomy today. The premier on-demand telescope network, at dark sky sites in Spain, New Mexico and Siding Spring, Australia.
This episode has also been sponsored by Lake County Astronomical Society: Celebrating 30 years of stellar service to members and the public.
Transcript:
The planet you’re standing on is not very steady.
She bobs and wobbles, ducks and weaves, as she makes her annual trek around the Sun. Our home constantly struggles with forces both external and internal; the Sun, the Moon, and the planets tug her back and forth while shifting masses within force her to constantly find new balance.
The tilt of our planet’s axis – what astronomers call its obliquity – is one measure of this interplanetary tug-of-war. Obliquity measures how far over a planet or moon is tipped relative to its orbit; in the Earth’s case, it’s about 23 degrees.
We feel the obliquity in the changing seasons. In June, the north pole is tipped towards the Sun. The northern hemisphere experiences longer days and more direct sunlight giving us warm summer days. In December, the north pole is tipped away from the Sun and everyone above the equator experiences the bitter chill of winter as the days get shorter and the sunlight reaches us more obliquely.
The other planets in our solar system exhibit a wide range of obliquities. Mars, Saturn, and Neptune are all tipped by roughly the same amount as Earth. Mercury and Jupiter have hardly any obliquity at all – which means no seasons for them.
Venus’ obliquity is 177 degrees; it is almost completely upside down. Put another way, it rotates in the opposite direction from Earth. Were there ever to be a break in Venus’ stifling cloud layer, native Venusians would see the Sun rise in the west and set in the east!
Uranus is another strange character; his obliquity is 97 degrees. Uranus is lying on his side! This leads to some very strange days and seasons. Each pole of Uranus alternates between being in constant sunlight for 42 years, then perpetual darkness for another 42 years. Only a very thin strip along the equator experiences anything like day/night cycles that we experience – and even then the Sun is frequently sitting very low on the horizon.
Astronomers aren’t entirely certain how Venus and Uranus came to have such extreme obliquities. A leading hypothesis is that both experienced massive collisions at some point in their distant past, essentially knocking them over.
A glancing blow from a minor planet isn’t the only thing that can change an obliquity. The tilt of our planet’s axis is drifting – ever so slightly – all the time. The poles actually trace out a rather complex dance over time scales ranging from minutes to thousands of centuries.
Like a spinning top, our planet also wobbles – though it does so very slowly. Over a period of 26,000 years, the poles trace out a giant circle in the sky. Right now, the north pole points pretty close to the star Polaris. But over time that will change, and in some epochs it will be pointing at nothing at all. This wobble – or precession – doesn’t change the amount of obliquity; it simply changes where in space the poles are pointing.
The Moon, however, does tug on our planet’s axis. In addition to rotation and precession, the Earth experiences something called nutation. This is a slight rocking of the Earth’s axis caused by the changing positions of the Moon, the Sun, and all the other bodies in the solar system. With many players involved, the motion from nutation is fairly complex. The main component nods the planet by a couple hundred meters over a nearly 19 year cycle that is tied to how the Moon’s orbit drifts around the Earth.
Our planet also has to contend with forces much closer to home. Internal motions of magma, deep sea currents, changing ocean salinity, winds, melting ice sheets, and even earthquakes all conspire to constantly change the distribution of mass within the Earth. The planet responds by continuously shifting its spin axis – an effect known as polar motion. This is a bit different than nutation; it actually changes where on Earth the north and south poles are located. Hard as it may be to fathom, these are not fixed locations on the globe. The drift is very small – centimeters per year – but it does add a complex twist to understanding our planets motion in space.
The presence of a large moon is believed to help stabilize our obliquity. The tilt does change by a few degrees over many millions of years, but manages to hold steady at around 20-25 degrees. Mars, conversely, appears to be going through a chaotic evolution in its obliquity. Computer simulations of its stability suggest that, over the past few million years, the Red Planet’s tilt has oscillated wildly between zero and sixty degrees which may lead to drastic swings in the environment. Many astrobiologists – scientists who study the evolution and origin of life in a cosmic context – argue that our moon may be an essential ingredient for providing a stable environment within which life could thrive. Does this mean that the existence of complex life and advanced civilizations is dependent on the presence of a large satellite? That’s hard to say. It is certainly plausible that our closest neighbor in space has been an indispensable partner in the evolution of our species.
I’m continually amazed at how a simple concept – like the tilt of a planet – can actually lead down a labyrinthian path of subtleties. Studying Earth’s obliquity, and the obliquity of all the other planets and moons, reveals much about the dynamics of our planet, the interplay of our home and neighboring worlds, and even the often times cataclysmic evolution of the solar system.
While the subtleties are far removed from our day-to-day lives, our Earth’s position in space actually is inseparable from the human experience. The June solstice is just over a month away. Whether you’re bundling up for winter, or heading to the beach for summer, remember: the Earth’s obliquity is the reason for the seasons!
End of podcast:
365 Days of Astronomy
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