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39 4 AN IMPROPER PROPER MOTION With the magnitude scale now explained in Chapter 3, we return to the triple-star solar system of Chapter 2. Proxima, also known as Alpha Centauri C, is one of the tiniest stars in nearby space, truly a 10-watt bulb among stars. It circles the pair composed of Helios and Osiris and their planets at a distance of some 13,000 astronomical units, about one-fifth of a light year. Its orbital period is not well known but must be of the order of one million years assuming , as we do here, a nearly circular orbit. This small star is a very faint red dwarf with a diameter of 0.2 times that of Helios or twice our real Jupiter; its mass is about 0.1 that of Helios. With this size and mass Proxima is far denser than Jupiter and the other major planets. The barycenter between it and our system is about one-twentieth its distance or near 650 AU, 15 times the average distance from here to the real Pluto, our farthest planet. The real Proxima Centauri lies about 2 degrees from the brighter pair in the sky, but being of the eleventh magnitude it still takes a telescope with an aperture of about 4 inches even to see it. In orbit about our system at a distance of some 13,000 AU, it would be visible to the naked eye but just barely. At magnitude 4.5 it would appear as a rather faint star in skies not spoiled by light pollution. It appears as bright as the stars in Ursa Minor (the Little Dipper ), apart from Polaris, the north star at the end of the handle, and the two other bright stars at the opposite end in the bowl. Any of the four stars in between these three appears of the same magnitude as would Proxima. It would have been noted and charted by ancient civilizations as just another star, because nothing about it in its appearance or motion could inform them that it is so very close as to be a member of the solar system, gravitationally connected to it. Since our Proxima is in orbit about us (strictly speaking, about the center of mass between itself, Helios, and Osiris), it does not share the rapid space motion that stars have relative to their neighbors in inter- stellar space to which they are not gravitationally connected. Unconnected stars move at an average of some 10 to 30 kilometers per second (km/sec) with respect to their stellar neighbors, and share little of the Sun’s 20-km/sec motion in nearby space in the general direction of the bright summer stars Vega and Deneb and away from Sirius and Canopus . The Earth orbits Helios at 30 km/sec, but way out there Proxima would creep at a pace of less than 0.3 km/sec in its orbit around the brighter pair of stars, assuming the orbit to be nearly circular. It would dawdle along at a very slow pace for such a nearby star; its apparent angular speed across the sky, called proper motion, is a little over one arc second per year, slower than the apparent proper motions of the bright stars Arcturus, Sirius, and Procyon as well as a few dozen fainter stars hundreds of times as far away. This means that astronomers would have no way of recognizing its connection to our system until the eighteenth or early nineteenth century, when stellar motions of that angular amount could first be detected and measured. Until that time it would have appeared as just another star in one of the constellations along the zodiac if its orbital plane matched the ecliptic rather closely, but for one distinction discussed below. As early as the fourth century B.C., Aristotle knew enough about the solar system to consider the possibility that the Earth moved. He rejected this hypothesis because he sought to observe the parallax, the back-and-forth motion of the stars that reflects the motion of the Earth around the Sun. He failed to do so and concluded that the Earth did not move. Many others tried to detect the parallax of any star but all failed until 1838, when telescopes were first up to the job. When Nicolaus Copernicus (1473–1543) proposed that we did indeed circle the Sun, he knew the parallactic motion had to exist for his theory to be correct...

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