The figure shows the positions of the solar and neutral hydrogen regions A to D rotating in a circular orbit relative to the center of the galaxy. The sun and A to D assume a Kepler rotation.

enter image description here

What is the correct explanation for the observation of the 21cm wave emitted from A to D as selected in <Example>?

b. The wavelength of D is 21cm.

Is b true even when considering the horizontal doppler effect? And what will be the observed wavelength if b is incorrect?

  • $\begingroup$ The copyright of this problem and the picture is in the Korea Institute of Curriculum and Evaluation (KICE). $\endgroup$ – KYHSGeekCode Nov 16 '18 at 1:21

Why do you think that there should be any "horizontal Doppler effect"? The rotation curve of the Galaxy is quite flat beyond the Galactocentric radius of the Sun, so there is very little relative velocity between the Sun and point D.

  • $\begingroup$ I calculated and its effect was 63nm. Is it right..? $\endgroup$ – KYHSGeekCode Nov 17 '18 at 14:02

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