What is the Hour Angle of an object, and how is it used?

On Thu, 5 Oct 2000, George wrote:

> During Visitor's Evening at Lick I noticed that the Nickel Telescope's
> display read Right Ascention, Declanation & Hour Angle. Could you explain
> hour angle. What does the astronomer understand when he or she checks the
> HA?

Hello George,

Hour Angle is the angle between an object's right ascension and the
meridian (the north-south line passing overhead). The hour angle is
calculated by subtracting an object's right ascension from the siderial
time.

Hour angle is useful because it tells an astronomer how long until an
object passes overhead, or how long ago it did pass overhead. (e.g. an
hour angle of 1 hour, 30 minutes means that the object will transit the
meridian in 1 1/2 hours) Generally, the closer an object is to the
meridian, the better quality the resulting astronomical data is.

I'm glad you enjoyed your visit, and thanks for writing!

Sincerely,
Kurtis Williams

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