By Tim Carr
Being a contemporary of Isaac Newton and Edmund Halley is not a recipe for fame but James Bradley was not the sort to worry about such things. His contribution to
modernizing astronomy speaks for itself.
Born in 1693, he studied theology at Oxford but developed a fascination for astronomy from his uncle, James Pound, who was a friend of Halley. Despite a steady career
in the church beckoning, he resigned his position as chaplain to the Bishop of Hertford and became professor of astronomy in Oxford in 1721 and never looked back.
What Bradley wanted to do was to prove that stars showed parallax in their positions as the Earth moved around the Sun. One of the objections to the Copernican system was that there was no observable shift in the observed positions of the stars as the Earth moved from one side of it’s orbit to the other. A shift by the observer of many millions of miles should result in the stars’ position ‘appearing’ to move very slightly. Now this had occurred to Copernicus (and to Aristarchus, who, thousands of years ago also said the Earth orbited the Sun ) but both realized that the parallax effect could not be seen as the stars were just too far away for any equipment to measure it. ‘Well you would say that wouldn’t you’, said the church. Very convenient.
Bradley realized that the accurate measurement of parallax would not only silence some of the critics but, more importantly, give astronomers an idea of just how far
away the stars were. Using a telescope with a focal length of over two hundred feet, he soon discovered the long elusive parallax. The problem was that not only was the displacement too big it was in the wrong direction!
In 1728, after a couple of years of this, he realized that the cause was the movement of his observation platform – the Earth itself. By tilting the telescope to compensate, he got more accurate results. Sadly, however, a value for stellar parallax was still beyond him. But in discovering this ‘abberation’ of starlight and measuring it (between 20 and 20.5 seconds of arc), Bradley had done two important things. The ratio between the speed of light and the speed of the Earth s orbit confirmed Romer‘s value for the speed of light. Also, the very fact that light was showing any abberation at all, proved that the Earth must be moving in space.
Although this was by far the most important work of his life, Bradley was not finished there. His meticulous measuring of stars was upset by tiny shifts of the Earth’s axis, caused by the gravitational pull of the Moon. This effect is called nutation and requires accuracy in measurement of two arc-seconds. As even this level of accuracy was insufficient for detecting stellar parallax, it showed that the stars were truly a very, very long way away. These results were only published in 1748 after nearly 20 years of measurements. That same year, he received the Copely medal for his work.
In 1733 he made the first accurate measurement of Jupiter‘s diameter, disabusing scientists of the notion that their world was anything special. In 1742 he was appointed Astronomer Royal. James Bradley died in 1762.
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Can't wait to see whats in store for next month!!!!……….I'll have to look on here, to see what is a good night for sitting in the hot tub watching the sky.
The people that said you saw a GREEN meteor, are you using a telescope, to be able to see color? All I see pretty much is white!
I have an 11 year old son and he is good with the telescope. So maybe I can help you figure out how to get your kids into astronomy. Around six years ago, I got [Celestron reflector. It was for me to play with. From suburban skies, had some success. Jupiter and Saturn looked nice, and I found the Ring Nebula and Hercules cluster. Couldn’t really see much of them, just was able to say “Oh, look! it’s a fuzzy!” This sort of viewing experience is boring for kids. Very quickly the very low quality of the mount got to be a huge problem. Spent most of my time arguing with the mount and not looking for other objects. So a few years later, I stepped it up a level and got a [6in with a study mount. This is a nice compromise between many design choices. At 6in, gathers a nice amount of light. It is f/5, making it good for finding and seeing nebulas and clusters and galaxies, oh my! The mount is up to the task to holding the telescope steady and tracks well. And here is the key point, it is light enough that by removing the telescope and counter-weight from the mount, in three trips from the study to the backyard, my son can set up for observing in our backyard with no help from me. Which is to say, he can take complete ownership of doing astronomy. And as I am sure you know, our sons are at the age where this is a big deal. (My only complaint with the scope is the finder scope. Replaced it with a 9×50 and my son started having so much more success finding targets.) But here is the thing: what to do once the telescope is set up and ready to go? Turns out, without a GoTo computer-controlled mount, finding anything other than the moon or the bright planets is no easy thing. Do you know what star-hopping is? I spend six months of occasional observing sessions teaching him how to go from seeing the target in question on a starchart to having it in view in the telescope. Turns out, this is not the easiest of skills to master. Of course, the time spend with these lessons was wonderful. But without showing him how to pick targets, find them on the starchart and then translate that into a view through the telescope, using a telescope gets boring very fast. But now he has mastered the skill. We got him a [star last spring and I don’t think it will survive the year. He spends the days pouring over it, selecting targets and clear evenings finding them. When he is not doing that, he is researching astronomy on the computer. He tells us how he is happy he is an astronomer and not like his classmates, who spend all their time playing video games. And when he volunteers this to me, I feel like maybe I am not completely fucking up this parenting thing. My point: $130 is only going to be enough to learn whether backyard astronomy is something your son is interested in. It will not be enough money for him to have the type of success that will pull him deeper into it. As others have said, plan to spend a bit more should he take to it and in the meantime, visit your local star party. And now that my son can do as much observing as he wants on his own, his 8 year sister wants in on the act. So, onto another round of basis telescope use and observing lessons. Are you sure you kids just don’t want to play video games?