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Homework Help: Local Sidereal Time, RA of a star

  1. Feb 18, 2016 #1
    1. The problem statement, all variables and given/known data
    *A star on the celestial equator has right ascension of 16h00m00s. At what time of day will this star be at it's highest point, on Febuary 29th 2016. *

    2. Relevant equations
    HA = LST - RA

    3. The attempt at a solution

    When LST=RA, it'll be at it's highest point. How do I determine LST? There are many different explanations online. As a rough estimate, solar is off from sidereal by 4 minutes. 0 hour is on March 20th. Subtracting 4 minutes per day for 20 days gives me a difference of about 80 minutes.

    So it'd be 16h - (80minutes), roughly 2pm?
  2. jcsd
  3. Feb 18, 2016 #2


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    Not correct. The sun is at 0h RA at the Vernal Equinox, roughly March 20. So when the sun is at its highest point on Mar 20, this is 0h LST. But the sun is at its highest point at 12h solar time, not 0h solar time. So there is an additional 12h offset.
  4. Feb 18, 2016 #3


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    I'd try drawing a sketch of the situation.

    Also, consider this: on March 20, the star will be highest in the sky at 4 am. A day earlier, it would have risen 4 minutes later, so it would be highest around 4:04 am, right? Your answer of 2 pm can't be right, even ignoring the am/pm difference.
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