PHY115: Professional Skills in Physics and Astronomy

Positional Astronomy: Session 3

HA revisited
We should look at hour angle in more detail, since an incomplete understanding of hour angle often causes problems with the material in this session. Remember that hour angle is measured as the time since an object has passed the observer's meridian. Local Sidereal Time is defined as the hour angle of the first point of Aries (0h RA), so if we set the LST in the explorer below to 00:00, the HA of the first point of Aries is 0, and the 0h RA circle lies on the observer's meridian.

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One thing is very important to note. Click on the "show hour angle arc" tickbox in the explorer. Note that the hour angle is the angle between the observer's meridian and the line of constant RA which passes through the star. Try and remember this fact!

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