Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Hour angle for rises and sets

  1. Mar 10, 2012 #1
    Hi,

    When you have to calculate the rising or setting time of a celestial body, you have to handle with hour angle and sidereal time.

    Sidereal time for the rising is given by T = alpha - H and by T = alpha + H for the setting (alpha = right ascension). Why - H in one hand, and + H on the other hand ?

    Maybe because :
    1/ hour angle H is computed from east toward west, so H=0 when crossing meridian, in south direction. Before crossing meridian, H = - [itex]\left|H \right|[/itex] so it's negative and it is positive after meridian is crossed H = [itex]\left|H \right|[/itex]
    2/ we can say that rising and setting are symmetric with respect to meridian transit, which occurs when T = alpha (i.e H=0)... so -H for rising and +H for setting

    Which explanation is better ? If none of them, could you explain the right way ?

    Thnaks
     
  2. jcsd
  3. Mar 12, 2012 #2
    I think you are confusing T with LMST(Local Mean Sidreal Time). The equation goes:

    H = LMST - alpha

    where H is the Hour Angle and alpha is Right Ascension. Hour Angle does not change signs.

    LMST however is made up of time.

    LMST = GMST0 + (Hour - timezone - dst + Minute/60)*15 + longitude(East+)

    where dst is daylight savings time (1 for on and 0 for off)
     
  4. Mar 14, 2012 #3
    Hi,

    I knew T was LMST, but I don't understand why local mean sidereal time for the rising is given by LMST = alpha - H and by LMST = alpha + H for the setting (where alpha = right ascension). I have some ideas (see my precedent post) but I'm not sure...

    Thanks
     
  5. Mar 14, 2012 #4
    Look more at this equation:

    LMST = GMST0 + (Hour - timezone - dst + Minute/60)*15 + longitude(East+)

    Hour is in 24hour Time.
     
  6. Mar 14, 2012 #5
    Ok. I looked at it. So ?
     
  7. Mar 22, 2012 #6
    Anyone ?
     
  8. Mar 22, 2012 #7
    The equation is always:

    Code (Text):
    LMST = alpha - HA
    Just the time changes.

    JD = 367 * yr - Int(7 * (yr + Int((mo + 9) / 12)) / 4) + Int(275 * mo / 9) + dy + 1721013.5
    d = (JDT - 2451545)

    GMST0 = 18.697374558 + 24.0657098244191 * d; in hours
    GMST0*15; in degrees
     
  9. Mar 25, 2012 #8
    Hi Philosophaie, hi everyone

    For setting, the formula is also : T = alpha - H or, if you prefer : LMST = alpha - HA ? That's it ?
    Can you tell me what do you mean with "just time changes" ? JD = julian day ? Why giving me the formula for Greenwich Mean sidereal time ?
     
  10. Mar 27, 2012 #9
    JD stands for Julian Date. Julian Date is the interval of time in days and fractions of a day since 1-1-4713 BC at Greenwich Noon. This Julian refers to Julius Caesar, who introduced the Julian calendar in 46 BC. This calendar has a regular year of 365 days divided into 12 months. A leap day is added to February every four years. The Julian year is, therefore, on average 365.25 days long.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Hour angle for rises and sets
Loading...