A. Which stars will be within a certain hour angle of the observer’s meridian at a given time?
1. Find out the SHA of an imaginary star which is on the observer’s meridian. The SHA of a star on
observer’s meridian can be obtained by keeping LHA Star= 0° or 360° .
2. Add and subtract the hour angle within which stars are to be found. This will give two values of
SHA. Take these values as limits of the SHA & check the stars from the table on page 268 of the
1992 almanac.
3. The table lists the stars in increasing order of SHA. Use the star names given on the right side
page of the table, these are English names. The names given on left side page are Latin names.
4. Remember stars with magnitude 1.0 or lower fall under First magnitude & magnitude 1.1 – 2.0
under second magnitude & so on.
5. If the time is not mentioned in the question then for AM use Nautical twilight starting time &
for PM use Civil twilight ending time.
B. Which of these stars will be visible to the observer OR suitable for taking sight?
1. The stars found within a certain hour angle may or may not be visible to the observer.
2. To find which ones are visible, calculate Latitude difference Declination (L ~D) for each star.
3. If L ~D is less than or equal to 90 the star will be visible & available for sight.
4. If L~D is greater than 90 the star will be below observer’s Rational Horizon & not visible.
5. If the time is not mentioned in the question then for AM use Nautical twilight starting time &
for PM use Civil twilight ending time.
The following table can be used to list the stars
C. Which of these stars will be suitable for ex meridian sight?
1. All the visible stars may not be suitable for Ex Meridian sight.
2. To check if a star is suitable for Ex Meridian, calculate LHA and then angle ‘P’. ‘P’ will be either
equal to LHA or 360 – LHA.
3. Divide ‘P’ by 15° 02.5′. This will give time to/from Meridian Passage of the star.
4. If this time is within the ex meridian limit, this star is suitable.
5. Ex meridian limit is obtained from Norie’s Table (Ex meridian table I & IV). IF the Norie’s table is
not available, approximate limit in minutes can be obtained by L ~D also.
6. If an hour angle is not mentioned in the question, you can check the stars falling within 20°
hour angle from the observer’s meridian for ex meridian suitability.
The following table can be used to list the stars for ex meridian
D. Which stars will cross the observer’s meridian during a given period of time & will be visible?
1. In these questions, time duration is specified in the question. In case of twilight the time
duration is to be obtained from the almanac.
2. Find out the SHA of an imaginary star which is on the observer’s meridian for starting time and
the ending time.
3. All the stars having SHA within this range will be crossing the observer’s meridian during this
period.
4. Out of these stars the ones having L ~o less than or equal to 90 will be visible.
5. If the duration of time is not mentioned in the question then for AM use Start of Nautical
twilight to Start of Civil twilight & for PM use End of Civil twilight to End of Nautical twilight.
E. Which stars will cross the observer’s inferior meridian during a given period of time & will be
visible?
1. In these questions, time duration is specified in the question. In case of twilight the time
duration is to be obtained from the almanac.
2. Find out the SHA of an imaginary star which is on the observer’s INFERIOR MERIDIAN for
starting time and the ending time. The SHA of a star on Inferior meridian can be obtained by
keeping LHA Star= 180° .
3. All the stars having SHA within this range will be crossing the observer’s inferior meridian
during this period.
4. Out of these stars only those which are circumpolar will be visible to the observer. A star will
be circumpolar if L & Dare of same name & L+D is greater than or equals 90.
5. If the duration of time is not mentioned in the question then for AM use Start of Nautical
twilight to Start of Civil twilight & for PM use End of Civil twilight to End of Nautical twilight.