. Astronomy for high schools and colleges . gure shows theshifting of the F line in the spectrum of Sirius, compared with onefixed line of hydrogen. This displacement of thespectral lines is now ac-counted for by a motion ofthe star toward or from theearth. It is shown in Phy-sics that if the source ofthe light which gives thespectrum a, &, c is mov-ing away from the earth,thisgroup will be shifted towardthe red end of the spec-trum ; if toward the earth,then the whole group willbe shifted toward the blueend. The amount of thisshifting is a function of thevelocity of recession or ap-proach, an
. Astronomy for high schools and colleges . gure shows theshifting of the F line in the spectrum of Sirius, compared with onefixed line of hydrogen. This displacement of thespectral lines is now ac-counted for by a motion ofthe star toward or from theearth. It is shown in Phy-sics that if the source ofthe light which gives thespectrum a, &, c is mov-ing away from the earth,thisgroup will be shifted towardthe red end of the spec-trum ; if toward the earth,then the whole group willbe shifted toward the blueend. The amount of thisshifting is a function of thevelocity of recession or ap-proach, and this velocity inmiles per second can becalculated from the meas-ured displacement. This has been done for many stars by , Dr. VoGEL, and Mr. Ciihistie. Their results agree well,when the difficult nature of the research is considered. The ratesof motion vary from insensible amounts to 100 kilometres per sec-ond ; and in some cases agree remarkably with the velocities com-puted from the proper motions and probable Fig. 134.—f-ltne in spectrum op SIRIUS. CHAPTER VI. MOTIONS AND DISTANCES OF THE STARS.§ 1. PROPER MOTIONS. We liave already stated that, to the unaided vision, thefixed stars appear to preserve the same relative position inthe heavens through many centuries, so that if the an-cient astronomers could again see them, they could hardlydetect the shghtest change in their arrangement. Butthe refined methods of modern astronomy, in which thepower of the telescope is applied to celestial measurement,have shown that there are slow changes in the positionsof the brighter stars, consisting in a motion forward in astraight line and with uniform velocity. These motionsare, for the most part, so slow that it would require thou-sands of years for the change of position to be percepti-ble to the unaided eye. They are called proper motions. As a general rule, the fainter the stars the smaller the proper mo-tions. For the most part, the proper motions of the
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