![]() It revised our near distance scale and hence the accuracy of further distances. Mankind now had a good idea of our stellar surroundings in three dimensions out to about 1000 light years away. About four times that number of stars were measured to lower accuracy but very considerably better than any previous knowledge. The parallaxes and the motion of over 100,000 stars were measured to about 2 milliarcseconds. The next big advance in astrometry needed a jump into space, which was done with ESA’s Hipparcos satellite in the early 1990s. Over decades the distance scale was extended outwards, largely using photometry. In fact, located at the bottom of the atmosphere, one can’t really do much better in terms of accuracy than was done then. In early 20th century the motion of some nearby stars was measured and the general rotation of the Milky Way around the galactic centre hinted at. Motion across the field of view is measured by tiny changes in angle. Motion towards/away is quite easily detected by the corresponding blue shift/red shift of spectral lines. Stellar motion is somewhat of a contrast to detecting position. Importantly, this was motion across the field of view. However, stellar motion also produces a change in the position of stars and the same accuracy of observation was sufficient to detect local motion over a time of no more than a few years. By then only a modest number of stars had their distance measured using parallax. Courtesy ESA.īy early 20th century there were still only a handful of observatories that could carry out astrometry and had observers with adequate skill and patience. Our galaxy, as deduced with the aid of astrometry. They had worked out that nearby stars would appear to move relative to distant stars over a time of six months, the time it took the Earth to move its position in space to the other side of its orbit. Astronomers and their instrument makers developed equipment that could do much better than the precision needed for navigation. The seafaring nations of the world encouraged astronomers to measure stellar positions accurately as a means of improving the accuracy of navigation. However, I’d date the beginnings of astrometry in the service of astronomy to the 19th century. People have been recording star positions for millennia, probably since the idea of making records was conceived. There are means of estimating distance from spectroscopy and photometry that indirectly give results. ![]() The astrometry I want to introduce here is measurement of position and changes that can be taken back directly to the metre and the second. While still on fundamentals, I should say that the basis of distance measurement in science is the metre the basis of speed measurement is the metre and the second. The universe is a dynamic, changing place but it’s necessary to focus on the right distance and time scales. They were called ‘the fixed stars’ for good reason. ![]() Stars may be moving at hundreds of km per second but no motion can be seen from day to day. One problem is that the universe is stupendously large and even the distance to the nearest star is colossal. If we had all six for everything we can see, we would get the structure of the universe at large and how everything is changing. So, star charts capture just two of six numbers that are at the base of astrometry. Motion needs another three parameters for speed: left/right, up/down, towards/away. Position in space needs three coordinates, distance away being missing in star charts. Star charts show only the position of stars in the sky. Astrometry also turns out to be a lot harder and more sophisticated than recording flowers. Astrometry is vital, too, for creating the picture of the universe at large and understanding why what we see is where it is now. Surely it can’t get less glamorous than this - a bit like botanists mapping the location of plants or birdwatchers mapping habitats of species? I suspect that naturalists would say that such activities can be mundane but when you look at the whole picture they underpin the hugely important study of ecology. Measuring position and speed sounds like pretty rock-bottom physics. Astrometry is the measurement of stellar location and movement.
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