View Full Version : Astronomic navigation
15th Jun 2004, 05:03
I have a question again:-)
Is it possible that a star has an hour angle of more than 12 hours?What is the correct definition of a hourangle?
thanks for your help!
15th Jun 2004, 08:56
The definition of an Hour Angle is: the arc of the Equator or Equinoctial intercepted between the meridian of a datum and the meridian of a celestial body, measured westward from 0º to360º.
That may be the dictionary definition but, in more simple terms, just as declination is like latitude, but projected out into space, so Hour Angle is analgous to Longitude (or change of longitude) projected out into space.
Hour angle is always measured clockwise (looking down from the North Pole) or westwards, even if it's more than 180º. It can either be measured from Greenwich, in which case it's called the Greenwich Hour Angle (or GHA), or it can be measured from from any nominated datum longitude. The most usual case other than GHA is from the longitude of the aircraft or ship taking the astronomical observation, in which case it is called the Local Hour Angle (or LHA).
15th Jun 2004, 09:36
Thanks!So is it possible to have an hour angle more than 12 hours?
15th Jun 2004, 10:33
I will be sitting these exams in a couple of weeks, I should know this. :)
You should be able to get an hour angle of more than 12 hours.
If the celestrial body is slightly east of the greenwich meridian, then the Greenwich Hour Angle (GHA) is just short of 24hrs, since angle is measured west from the Greenwich meridian for GHA.
Hope this helps :ok:
15th Jun 2004, 10:44
Hour Angle is measured in degrees, not hours. As I said, it's just like change of longitude, but projected out into space. You can convert hour angle into time difference by dividing it by 15, but it then becomes called arc/time difference, not Hour Angle.
I think that you may be confusing Hour Angle and either arc/time difference or Standard Time Difference (STD). There are some STDs greater than 12 hours. For instance, Chatham Island keeps a STD of 12 hours and 45 minutes ahead of UTC (which is, for all practical purposes, the same as GMT).
15th Jun 2004, 10:50
The angle between an observer's meridian and the hour circle on which some celestial body lies. This angle is conventionally expressed in units of time (hours, minutes, and seconds), which gives the time elapsed since the celestial body's last transit at the observer's meridian (for a positive hour angle), or the time unit the next transit (for a negative hour angle).
hope that helps :ok:
15th Jun 2004, 12:11
Thats how Bristol teaches it too (if I remember my notes correctly). :)
So Olendirk, this confirms that the maximum GHA is 24hrs, or to more specifically answer your question, greater than 12 hours. :ok:
15th Jun 2004, 14:38
Well, first thanks for your answers. The thing is, I calculate for example a sunrise on MSL with the depression and the declination of the sun on the day I want. When I use those graphs I get for example six hours and 4 minutes. Then I take the meridian passage of the sun in LMT and calculate the sunrise and sunset. Allright. Then I saw a question showing a picture like here
and the checkers ask for hourangles. They
put pathes of stars or the sun like in the link into the painting and ask for the hour angle of such a path. The thing is is there an hourangle of 16 hours?allright you say yes. so does the expression hourangle mean the whole time the star is visible to me?so a circumpolar star would have an hour angle of 24 hours?
15th Jun 2004, 15:34
Hour angle is nothing to do with how long an astronomical body is visible! It is one co-ordinate of a position on the celestial sphere (the other being declination), the equivalent of longitude on the Earth's surface although measured 000°-360° to the West instead of 000°-180° East or West.
[aside to Aspidustra : This angle may be "... conventionally expressed in units of time" but it is not for the purpose of the General Navigation syllabus! Unless we wish to go deeper into a discussion of how to actually navigate by the stars, way beyond what most pilots here need, then we should leave it as Oxford Blue and I, both Gen Nav instructors, have stated.]
The question you describe, Olendirk, does not come within the scope of JAA Navigation, it requires knowledge far beyond the learning objectives. Do you need to go deeper into astro navigation than the JAA ATPL syllabus? If so I would suggest a more specialised site for your question, a visit to the military forum where some military navs may be able to help, or a specific heading asking for those with knowledge (such as ocean yachtmasters' certificate holders!) of real astro navigation. It has not been in common use for civil aviation for many years.
Gen Nav, BCFT
15th Jun 2004, 16:33
Military navs no longer learn or practise astro navigation, unfortunately, though there are many navs still in the forces who will have done so in the past. I suggest that the best place for the question is probably in Tech Log - there'll be someone in there who knows all about astro.
I did do a 'noon fix' once, and was within 20 miles of the GPS. Good fun, but pointless!
16th Jun 2004, 08:35
point accepted Send Clowns. apologies for being ill-informed!
might I venture though that an hour angle of 16 hours (240 deg) is the same as an hour angle of -8 hours (-120 deg)(?) (I assume here that the object revolves around the sky exactly once in 24 hours, which it probably doesn't and in the case of some objects certainly doesn't!)
in other words all that it suggests is that it has been 16 hours since the celestial body last passed through the observer's meridian and will be 8 hours (or thereabouts) before it does so again?
have never studied navigation in any detail but was once a keen astronomer! apologies if i'm just showing myself up here! :O
16th Jun 2004, 10:00
I checked my Bristol notes last night, and they do not specify the unit of measurement, though common sense suggests angle. :)
My interpretation was that Hour angle was measured in time from what the notes were explaining in the same section. Now I know the facts! :ok:
16th Jun 2004, 21:39
Ahh (sigh) Noon fixes, SOP for daylight Gan-Singapore. Sextant melts in mounting. Never had the pleasure of comparing with GPS, so had to have blind faith that the system worked. Seem also to remember doing a noon fix using the sun for latitude and working out the longitude from the time difference between LMT and GMT. Only did stupid things like this when there was nothing else going on.
16th Jun 2004, 22:53
Yes, I do remember the times when Loran and Doppler were all we had, and astro was essential! Omega, and later INS and GPS, took away the need for it.
17th Jun 2004, 16:22
You're only showing yourself as being well-informed about the astronomy for real! We know about JAA ATPL navigation, Oxford Blue may know a little more. Sorry if I was a bit short wiht you, not intended. Our students don't usually like this section of the course, although it is not as difficult as some persuade themselves it is.
Hour angle of -120°/-8 hours would be the same as +240°/+16 hours, although we only ever look at positive hour angles, measured West of the Greenwich Hour Circle.