(7)   Precession  

"From Stargazers to Starships" home page and index: on disk Sintro.htm, on the web
          http://www.phy6.org/stargaze/Sintro.htm

• Understand how Hipparchus, around 130 BC, used a shift in the predicted location of a lunar eclipse to detect a slight shift in the path of the Sun around the sky.

• Understand the reason for that shift--the Earth's axis moving around a cone with a half-angle of about 23.5°, making one full circuit of the cone every 26,000 years.

• Know about ice ages and the evidence for them; also about Louis Agassiz.

• Realize that because of the ellipticity of the Earth's orbit, we are now closest to the Sun around January 3-5 each year.

• Understand the Milankovich theory, by which the coupling of the above effect with the seasons of the year can perhaps influence the occurrence of ice ages.

Terms: First point in Aries or vernal equinox point, precession of the equinoxes, "precession" in modern usage, glaciers, ice ages (glaciations).

Stories and extras: The idea of Hipparchus, of measuring the position of the eclipsed Moon to find the Sun's position. "The Dawning of the Age of Aquarius" in the musical "Hair." Precession of artificial satellites and "nutation dampers", also (optional--details below) "magnetic resonance imaging" in medicine and "proton precession. " The story of Louis Agassiz, and the fact Cape Cod and its islands are a legacy of glaciers.

   The shift is very slow, and takes many centuries to be noted. The remarkable thing is that it was discovered so early, around 135 BC, by the Greek astronomer Hipparchus.

   Then go into section 7 of "Stargazers. ". If there is time, the teacher may conclude by telling about NMR--nuclear magnetic resonance; see below.

Guiding questions and additional tidbits

He noticed that the point where the celestial equator crossed the ecliptic slowly shifted. At that time it was in the constellation of Aries, the ram. It was shifting to Pisces, the fish, and is now entering Aquarius, the water carrier.

That the Earth's axis was slowly rotating around the direction perpendicular to the ecliptic, tracing a complete cone each 26000 years or so.

The half-angle of that cone is 23.5 degrees.

The pole of the heavens is the point to which the Earth's axis is pointing. Since the Earth's axis moves around a cone every 26000 years, the position of the pole in the sky during that time traces a circle.

The pole will be on the other side of the cone, a distance of 2 x 23.5° = 47°.

A motion of the axis of a spinning object, tracing a cone.

The precession of spinning satellites is undesirable, since it confuses the observations. However, it can be eliminated by "nutation dampers", tubes partially filled with mercury. If the payload precesses, the mercury sloshes back and forth, and its friction drains energy from the precession motion.

(Optional story: Nutation dampers will reduce precession of a satellite, but only when its original spin is stable--e.g. if it spins around the shortest axis possible. That was not the case with the first US satellite--Explorer 1, launched in 1958. It had the shape of a fairly long cylinder (that included its attached last-stage rocket), spinning around its long axis. Mechanics (and intuition) suggest that a long cylinder (e.g. the round cardboard box in which rolled-up posters are mailed) does not "like" to spin around its long axis, and that any secondary effects will try to convert its rotation to one around an axis perpendicular to the cylinder in its middle.

    That is indeed what happened to Explorer 1. The agent transforming the rotation were the four flexible small "whip antennas" sticking out from its middle. The waving of these antennas gradually made the satellite precess around its spin axis, and the cone of precession grew wider, until in the end it was essentially flat and the satellite spun around an axis perpendicular to it, through its middle. That could be told from the beaming of its radio signals.)

Nuclear Magnetic Resonance.

Has anyone in the class had an "MRI test"--"Magnetic Resonance Imaging"--or watched one? (If so, let the student tell about it).

   Nuclear Magnetic Resonance is a technique of using the precession of hydrogen atoms and has nothing to do with nuclear radiation.

How does it work?

   The nucleus of hydrogen, the proton, has a spin, and is magnetized along its spin axis; a bit like the Earth, except, with Earth the two axes are slightly different. In the proton this is a quantum effect, the result of the physics of very small particles, not the kind of spin we see in a gyroscope. In a glass of water, or in the human bodies, the spin axes of protons usually point in random directions. If an electromagnet is suddenly switched on nearby, the nuclei will try to line up, like pins near a magnet. However, as is well known, spinning objects tend to keep their direction (that is how the gyro-compass operates) and resist any force which tries to shift their axis.

   The result is that the little nuclei begin to precess around the direction in which the magnetic force is trying to line them up. Coils connected to sensitive amplifiers can sense the frequency of that precession and from it, measure the strength of the magnetic field: the Vanguard 3 satellite in 1959 carried such an instrument to measure the magnetic field of Earth.

   Medical MRI (Magnetic Resonance Imaging) instruments are also based on this phenomenon, but in a much more sophisticated way, using it to tell where in the body hydrogen atoms are concentrated. MRI is a modern technique, using computers to analyze complicated data to produce pictures of internal organs. The result is a bit like X-rays, with the big difference that while X-rays, or their modern extension "CAT scans, " see the dense materials, such as bones, NMR targets hydrogen atoms, making tissues stand out if they are rich in hydrogen-containing substances such as water, proteins etc.

   All of MRI machines contain big magnets, and if you ever have an MRI picture taken, you will be asked to remove anything made of iron before entering the MRI room. You will have to lie down in a hole inside the machine, very quietly and not moving (MRI is like a time exposure), while the machine measures the precession signatures of your hydrogen nuclei from many different angles.

Ice ages were periods in the past when great ice sheets covered much of Europe, Siberia and North America. They were somewhat similar to the ice sheets now covering Greenland and Antarctica, and to the glaciers still found in high mountains. It is believed the climate was appreciably colder.

A river of ice, created by accumulated snow, which get compressed under its own weight. Ice slowly deforms under pressure, and over long periods it can flow very slowly, like a liquid. In a glacier, gravity makes the ice slowly flow to lower levels, where it either melts of falls into the sea and floats away.

The last ice age ended about 11,000 years ago.

[Optional]
Such information may be obtained by radiocarbon dating, a dating system using the decay of radioactive carbon produced all the time by cosmic rays in the atmosphere; these rays act like nuclear radiation and occasionally change atmospheric nitrogen into radiocarbon (their intensity is low--comparable to starlight--so they pose no great risk). Small amounts of radiocarbon are then absorbed by living materials such as wood. After the organism which absorbed them dies, the carbon nuclei slowly decay, and their amount decreases by 1/2 every 5700 years. By calculating the ratio of radioactive carbon left to total carbon content, the age of the material can be estimated. Similar dating methods also exist using other atomic nuclei.

He found boulders that did not belong to the local rock type, but must have been brought from great distances. He proposed that they were transported by ice, the way modern glaciers such as those of Switzerland, his home country, still move boulders.

   The orbit of the Earth is slightly elliptic, so that we are closest to the Sun on 3-5 January. That makes winters in the Northern Hemisphere a little more moderate. We are most distant around July 3-5, which moderates summer temperatures as well.

   In the Southern Hemisphere, this would make the seasons more extreme, but the oceans prevent dramatic climate changes.

   In 13,000 years, because of the precession of the Earth's axis, we will be closest to the Sun in midsummer and most distant in mid-winter. Our Northern Hemisphere has much more land, and Milankovich proposed more snow would fall in the winter. Snow, being white, would reflect sunlight and reduce the heating of the ground later in the year. This small change may perhaps be enough to trigger ice ages, though other orbital changes may also be a factor.