Approximately how long does the Earth take to complete one full rotation on its imaginary axis relative to distant stars (the sidereal day)?

Difficulty: Medium

Correct Answer: 23 hr 56 min 4 sec

Explanation:


Introduction / Context:
The Earth rotates on its imaginary axis and also revolves around the Sun. These motions are fundamental to understanding day and night, seasons, and timekeeping. Many textbooks distinguish between the solar day, which is based on the Sun's apparent motion, and the sidereal day, which is based on the Earth's rotation relative to distant stars. This question asks about the time taken for one full rotation relative to distant stars, that is, the sidereal day, not the approximate 24 hour solar day.


Given Data / Assumptions:

  • We are considering the Earth's rotation on its axis.
  • The rotation time is measured relative to distant stars, not to the Sun.
  • Options provide four approximate times around 24 hours.
  • We assume standard astronomical values used in school level textbooks.


Concept / Approach:
The solar day, which is the average time between two successive noons, is about 24 hours. However, the Earth also moves along its orbit while it rotates, so it has to turn a little more than 360 degrees to bring the Sun back to the same position in the sky. The time taken to complete exactly 360 degrees of rotation relative to distant stars is slightly less than 24 hours and is called the sidereal day. Astronomical measurements show that the sidereal day is about 23 hours 56 minutes 4 seconds. Therefore, we must pick the option closest to this value.


Step-by-Step Solution:
Step 1: Recall that the commonly quoted 24 hour day is based on the Sun and is a solar day. Step 2: Understand that the sidereal day measures rotation relative to distant stars and is slightly shorter. Step 3: Note that the exact sidereal day is roughly 23 hours 56 minutes and a few seconds. Step 4: Compare this with the options given and identify 23 hr 56 min 4 sec as the closest match. Step 5: Conclude that 23 hr 56 min 4 sec is the correct length of one complete rotation relative to stars.


Verification / Alternative check:
Astronomy references and standard physics textbooks provide the sidereal day value as approximately 23 hours 56 minutes 4 seconds. This can also be reasoned from the fact that the Earth completes one extra rotation each year relative to the Sun compared to the stars, leading to a difference of about four minutes per day. Multiplying four minutes by roughly 365 days gives about 24 hours, which matches one extra rotation, confirming the approximate four minute difference between sidereal and solar days.


Why Other Options Are Wrong:
24 hr 37 min 23 sec is much longer than the known sidereal day and does not correspond to any standard Earth rotation period, so option A is incorrect. 24 hr is the approximate solar day and does not represent the sidereal day, which is slightly shorter, making option B wrong for this particular question. 23 hr 52 min is too short and does not match the established sidereal day value, so option D is also incorrect. Only 23 hr 56 min 4 sec aligns with widely accepted astronomical data.


Common Pitfalls:
Many students automatically choose 24 hr whenever they see a question about Earth's rotation because that is the familiar value for one day. Another pitfall is not noticing that the question explicitly concerns rotation relative to distant stars, which signals that it is asking about the sidereal day. To avoid these mistakes, pay attention to phrases like relative to stars or sidereal and remember that this period is about four minutes shorter than 24 hours.


Final Answer:
The Earth takes approximately 23 hr 56 min 4 sec to complete one full rotation relative to distant stars, which is the sidereal day.

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