 In the 1830s, there was a race to see who could find the first stellar parallax. The astronomer Frederick Bessel won. Star was 61 Cygni. Here's how it works. If you recall, the maximum baseline for parallax measurements for planets was the diameter of the Earth. For stellar parallax, we have the diameter of the Earth's orbit around the Sun. That's an increase from around 13,000 kilometers, or 8,000 miles, to 300 million kilometers, or 186 million miles. That's 23,000 times larger. So from one side of the Earth's orbit, say in July, we take a line to the star and map the positions of the more distant stars. Six months later, in January, we repeat the process. This gives us the angle theta. We define stellar parallax as one-half this angle. This would be the angle at the star with the Earth and the Sun making the other two corners of a right triangle. The math is the same trigonometry we used for finding distances to the rock in my backyard and to the planets in our solar system. Of course, this is an oversimplification. Frederick Bessel mapped 61-signy against the distant star background for 28 years, observing the star's ellipses that followed the Earth's orbit. In 1838, after thousands of measurements and calculations, he made scientific headlines by announcing that the parallax of 61-signy was 0.314 arc seconds. That gives us a distance of 98 trillion kilometers. That's 61 trillion miles. Way too far to be reflecting the Sun's light. So at this point, in the middle of the 19th century, we knew that stars burned with their own light. If we were to move in a little closer to a star that had a parallax of exactly one second of arc, we'd find it to be 31 trillion kilometers away. That's 19 trillion miles. This distance is called a parsec. It gets its name from the first syllable of parallax and the first syllable of second. Astronomers like to use it for measuring distances to stars. If you're a Star Trek fan, you'll hear parsec use a lot in their distance discussions. As we discussed in our segment on the solar system, light travels at 300,000 kilometers per second, or 186,000 miles per second. To calculate how far light travels in a year, we simply multiply this number by the seconds in a minute, the minutes in an hour, the hours in a day, and the number of days in a year. That totals 9.461 trillion kilometers, or 5.88 trillion miles. We call that one light year. So one parsec is just over three light years. I'll use light years throughout this video book, but parsecs will come up from time to time.