Astronomy homework, right ascension

[petha1]

Homework Statement

Will a star with right ascension of 14h,15m,39,3s be visible on October 21 from a latitude of 56 degrees 53 minutes, 0 seconds? Assume that night is between [22.00,02.00]

Relevant Equations

Right ascension measures how many hours ahead or behind the sun a star is at the vernal equinox, decreses by 2 hours/month.

March 21 - October 21 = 7 months. So the star is only 15m 39,3 seconds behind the sun. This means that the star won't be up when the sun is down.

Answer: No, the star will set only 15 minutes after the sun has set. Therefore it won't be on the sky at the specified latitude during dark hours.

Is this correct reasoning?

 

[phyzguy]

Relevant Equations:: Right ascension measures how many hours ahead or behind the sun a star is at the vernal equinox, decreses by 2 hours/month.

Your answer is correct, but this statement is misleading. The right ascension of a star does not change. It is the position of the sun which changes through the year.

 

[petha1]

Thanks for the help. I think I figured out the correct way to answer this question.
October 21 is 9 months since the vernal equinox. That means that the LST at noon is 18, leading to
the LST at midnight beeing 18+12 = 30 = 6 (mod 24) so the visible RAs are in the range [6-2,6+2] = [4,8]so the visible right ascensions are between 04h and 08h.