Monday, February 25, 2008
Get serious about the distance to Sirius
John Goss
John Goss is chairman of the Mid-East Region of the Astronomical League and a former president of the Roanoke Valley Astronomical Society.
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Blog
Even though it is now as close as it can come to the Earth, the planet Saturn is always the solar system's farthest object that a Southwest Virginia stargazer can easily see. What is the next object past Saturn's 775-million-mile distance that can be found tonight?
Until the discovery of Uranus in 1781, astronomers thought of Saturn as marking the edge of the solar system. In the first few decades following the introduction of the telescope in 1609, its perplexing elongated shape was studied, especially by the Dutch physicist Christiaan Huygens. In 1655 with the improved optics in his new telescope, he was the first to discern both Saturn's unusual ring system and its large moon Titan.
Huygens is also remembered for determining the distance to what was then thought to be the first object lying beyond Saturn: the night sky's brightest star, Sirius. Amazingly, he didn't use a telescope to do this.
He believed, as did many astronomers of his time, that Sirius was necessarily the closest star because it was the brightest. He also believed that all stars were like the sun, only much farther away, while the sun was just another star, but viewed close-up.
Astronomers in the mid-1600s loosely speculated that Sirius was at least 6,000 times farther than the sun. This inexactness prompted Huygens to devise a method for establishing a more accurate distance, one that would lead to a better understanding of interstellar dimensions.
The Dutch physicist reasoned that if he could determine the size of a section of the sun that gave the same brightness as Sirius, he could calculate the true distance of Sirius. Our sun is extremely bright, so he knew that the portion required must be very small.
Huygens employed craftsmen to drill a series of very fine holes of various diameters through a thin disk of brass. He then had it mounted in a high window, one in which sunlight shone through every clear day. When the sun was centered directly behind the disk, he stood some distance away and tried to recall which sunlight-filled pinhole closely resembled the brightness of Sirius, which he had viewed on the previous night.
By comparing the size of the pinhole's angular area with that of the sun, he used high school trigonometry to find that the distance to Sirius was almost half a light-year. Astronomers finally began to grasp the immensity of interstellar distances.
What Huygens did not know was that, just like people, stars are different from one another. Sirius is not like the sun: It is more massive, much hotter and 25 times intrinsically brighter. Taking the brightness discrepancy into account, his distance measurement jumps to about five light-years away. Not bad, considering that today's sophisticated instruments place Sirius at almost nine light-years away -- which is 540,000 times greater than the distance to the sun and 70,000 times farther than Saturn.
Tonight, Saturn rises in the east as the sun sets in the west. It is the brightest object in the early evening eastern sky. When you look at Saturn, you see sunlight reflected off its cloud tops 75 minutes ago because it is only 75 light-minutes away.
Now, look toward Sirius, which is the brightest object shining almost directly south about 8:00 p.m. The light you see left it almost nine years ago when people here on Earth were worried about "Y2K."
Although it certainly isn't devoid of matter, you don't see anything shining within the almost nine-light-year gap between the planet Saturn and the star Sirius. Go outside on a clear, dark March night, and see for yourself how vast space really is and how little there is within it.
