Question

A star rises at 8pm tonight. At what time will the star rise after a month? Why?

Answer 1

Hint: The sun is also a star that takes approximately 24 hours to complete a circle around the Earth. A star takes 4min less than the sun to complete the entire circle. Hence every successive day the star will take 4min less than the previous day to reach back to the same position. Hence accordingly we will determine the position of the star in the sky.

Complete step-by-step answer:
A star takes 4min less than the sun to complete the entire circle around the Earth. Let us say we saw the star today at 8 pm in the night sky. If the star had to take 24 hrs to complete the entire circle around the Earth, then we would have seen the star at the same position at 8pm the next day. But the star took 4 minutes less than 24 hrs. hence the next day we will see the star at the same position 4min before 8pm. The next day from this time it will again take 4mins less i.e. 8min to 8pm. We are asked to calculate the time after 1 month such that the star is seen at the same position. As there is a lag of 4min every single day, after 30 days the lag in time with respect to 8pm is, $30\times 4=129\min =2hrs$ . Hence, after 30 days we will see the star at 6pm i.e. 2 hours earlier before 8pm.

Note: If one observes the stars carefully in the night sky for more than a week he will see the stars changing its position After some days or so the stars are not seen as the stars come to the same position during the day. This cycle keeps on continuing and after months after months we will be able to see the stars at the same position at the same moment of time.