January 20th: The Celestial Police

By on January 20, 2010 in

Date: January 20, 2010

Title: The Celestial Police

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Podcaster: Brian Gray

Organization: Wilderness Center Astronomy Club: www.twcac.org

Description: An international association of astronomers join together to find a planet that many suspect to exist between the orbits of Mars and Jupiter.

Bio: I am an amateur astronomer living in northeast Ohio.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by — no one. We still need sponsors for many days in 2010, so please consider sponsoring a day or two. Just click on the “Donate” button on the lower left side of this webpage, or contact us at signup@365daysofastronomy.org.

Transcript:

Hello, my name is Brian Gray. I am a member of the Wilderness Center Astronomy Club located in Wilmot, Ohio. We are a family-oriented group of amateurs with a broad range of experience and interests. Our charter is to promote the knowledge and enjoyment of astronomy among our club members, our parent organization, the Wilderness Center, and the northeast Ohio community at large. If you like to listen to podcasts about nature in general, try Wild Ideas … the Podcast at http://www.wildernesscenter.org/podcasts/ .

Is there a missing planet in the gap between the orbits of Mars and Jupiter? In an early example of big science, a group of astronomers formed an international collaboration to search for this missing planet. They called themselves the Celestial Police and extended invitations to astronomers throughout Europe to become part of this effort. This effort was motivated by a Law that appeared to describe the locations of the known planets at that time. This law was known as the Titius-Bode Law or sometimes just Bode’s Law.

With the work of Nicolaus Copernicus and Johannes Kepler, astronomers had known the particular order of the planets as you move further away from the Sun and their distances in proportion to the Earth’s distance from the Sun. Even at this early stage, many had noticed a large gap between the orbits of Mars and Jupiter. While translating the work of another astronomer, Johann Titius inserted a formula that appeared to place the locations of the planets in their proper spots. First, you start with zero as the first number and then a three as the second number and then double the previous entry to obtain the next in line. You will have a list of numbers: 0, 3, 6, 12, 24, and so on. Second, add four to each, and then divide each number by ten. The final numbers are 0.4, 0.7, 1.0, 1.6, 2.8, 5.2, 10.0, and so on. These numbers roughly correspond to the distances of the known planets from the Sun in AU except for the entry for 2.8 which would describe an orbit between Mars and Jupiter.

In 1768, Johann Bode included this Law in one of his works and suggested that a planet did exist at 2.8 AU but was yet to be discovered. This idea did not have much traction until Uranus was discovered in 1781 by William Herschel. When the semi-major axis of that planet’s orbit was calculated to be close to the predicted value of 19.6 AU, many astronomers gained more confidence in the ability of the Titius-Bode Law to predict the location of additional planets, especially the missing wanderer at 2.8 AU.

Baron Franz von Zach decided to take on the task of looking for this missing planet and began to search in 1787. He made the assumption that the planet would be located near the ecliptic since the known planets have nearly coplanar orbits with the Earth. Despite this restriction on the areas of the sky to be searched, much work would be needed as each small patch of sky would have to be examined and charted over multiple nights to detect an object wandering through a particular star field.

At a meeting in 1796, Joseph-Jerome de Lalande suggested to von Zach that the search could be expedited if the project was divided among several astronomers assigned to different sections of the ecliptic. This idea fostered the formation of the Celestial Police in 1800 with Johann Schroter as president and von Zach as secretary. The other charter members included Karl Harding, Heinrich Olbers, Freiherr von Ende, and Johann Gildemeister. They decided that more astronomers would be needed and invitations would be extended to an international assortment of observers to bring the total to 24 members. Each member would be assigned a 15 degree by 15 degree section of the zodiacal band. They would be responsible for charting the telescopic stars in their section and taking note of any changes that would indicate the presence of a solar system object.

One astronomer who would receive an invitation to join the Celestial Police was Giuseppe Piazzi of the University of Palermo in Sicily. Before he would receive that invitation, he observed a moving star over several nights in early 1801 while working on a sky catalogue. The object, eventually named Ceres, was determined to have an orbit consistent with one that the Titius-Bode Law predicted would exist at 2.8 AU. Ceres was hailed as the missing planet but soon other similar objects were found during the following years. Heinrich Olbers discovered Pallas in March 1802 and Vesta in March 1807. Karl Harding found Juno in September 1804.

The similar orbital parameters suggested to Olbers that these planets were once part of a larger planet that was destroyed by collision with another object or shredded by Jupiter’s gravity. Today, the current thinking is that the gravity of Jupiter prevented the material between Mars and Jupiter from forming a large object in the first place.

The discovery of Neptune in 1846 discredited the Titius-Bode Law in the minds of most astronomers since the semi-major axis of Neptune’s orbit is much shorter than the one predicted by the Law. Ceres, Pallas, Juno, Vesta, and some others that were initially classified as planets were eventually reassigned as asteroids, a term coined by William Herschel who could only see them as star like objects in his telescopes.

Even though the Celestial Police did not find a planet, they did find the first minor planets in the asteroid belt and did so as an international collaboration that predated ones that are now commonplace in science.

End of podcast:

365 Days of Astronomy
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