A Breakdown of Intersubjective Measurement: The Case of Solar-Rotation Measurements in the Early 20th Century

✍ Scribed by Klaus Hentschel

• Publisher: Elsevier Science
• Year: 1998
• Tongue: English
• Weight: 556 KB
• Volume: 29
• Category: Article
• ISSN: 1355-2198
• DOI: 10.1016/s1355-2198(98)00025-2

✦ Synopsis

The phenomenon of solar rotation has been known since the first solar-spot observations in the early 17th century. In the course of the 19th century, Carrington, Spo¨rer, Wolf and others systematised existing data and initiated long-term observational programmes to record the rotation of various observable tracers on the Sun's surface, in order to determine the rate of rotation as precisely as possible. The pioneering work of F. Zo¨llner and H. C. Vogel in the early 1870s on the measurement of Doppler shifts in the spectra taken from the eastern and western limbs of the Sun opened another way to measure this rotational velocity, namely spectroscopically. Lines in the spectrum from the receding western limb of the Sun are slightly shifted towards the red, and those from the approaching eastern limb are slightly shifted towards the blue end of the spectrum. Lines originating from absorption in the terrestrial atmosphere, however, are not shifted at all.

Even though these spectral shifts, induced by a solar rotation of 2 km/s, were relatively small (&10\ in relative units, or 0.1 A s ), they were well within the range of contemporary technology. Projecting eastern and western limb spectra right next to each other on the visual field or the photographic plate allowed a direct comparison of both spectra, which essentially doubled the observable shifts.