Celestial Coordinates Homework Q

1
Celestial Coordinates
Homework Q A
Junior Navigation Chapter 5
2
Objectives Define declination, hour circle,
Greenwich hour angle (GHA), and local hour
angle (LHA). Understand how the geographical
position (GP) of a body is located by celestial
coordinates, and the relationships among hour
angles. Calculate LHA and declination from
Nautical Almanac data
3
Practical Exercises Problem 1. Follow the
Student Manual for guidance
4
2. Greenwich hour angle (GHA) is a. the arc
distance, measured in an easterly direction, from
the observer's meridian to the hour circle of a
GP. b. the arc distance, measured in a westerly
direction, from the Greenwich meridian to the
hour circle of a GP. c. the arc distance,
measured in a westerly direction, from the
observer's meridian to the Greenwich
meridian. d. the arc distance, measured in an
easterly direction, from the Greenwich meridian
to the observer's meridian.
Ref 8, 9
5
3. The declination of the sun is 1519'N. Its GHA
is 34341'. What are the latitude and longitude
of the GP of the sun? Ans
L 1519'N Latitude of the GP of a body
is equal to the declination of the body. Dec
1519'N therefore L of GP 1519'N. Lo
16 19'E If GHA of a body is greater than 180,
longitude of the GP of the body is equal to 360
- GHA of the body, and the GP is in east
longitude. Longitude of GP 360 - 34341'
1619'E.
Ref 8 - 11
6

• 4. Fill in the blanks
• GHA is measured from ___________________ to
• _________________.
• b. Dec is measured from ___________________ to
• _________________.

Greenwich meridian hour circle of GP
the equator the GP of the body
c. LHA is measured from ___________________ to
_________________.
c. LHA is measured from ______________________
____________ ___________________.
the local meridian of the observer to hour
circle of the GP
Ref 8, 12 13
7
5. Declination is measured a. south from the
north pole and north from the south pole. b.
from the pole of the hemisphere in which the
observer is located. c. north or south from the
equator. d. north or south from the observer's
parallel of latitude.
Ref 12
8

• 6. LHA is measured
• from the upper branch of the observer's
  meridian eastward to the hour circle of the
  celestial body.
• b. from the upper branch of the observer's
  meridian westward to the hour circle of the
  celestial body.
• c. from the hour circle of the celestial body
  eastward to the Greenwich meridian.
• d. from the hour circle of the celestial body
  westward to the Greenwich meridian.

Ref 13
9
7. Fill in the blanks a. GHA sun _____ LHA
sun.b. GHA sun - _____ LHA sun
LoE
LoW
Ref 19
10
8. Given Lo 150E ZT 0600 GHA sun is 120.
Find UT and LHA of the sun. UT 2000 previous
day SolutionCompute ZD Lo divided by 15
and rounded.ZD Lo 150 / 15 10. Since Lo is
E, the ZD is negative, ZD -10Compute UT
ZT 0600 ZD -10
UT -0400
2400 - 1 day UT 2000
previous day
LHA sun 270
SolutionGHA computation Since the mean sun
will be over Greenwich at 1200 UT, GHA at 1200
will be 000. At UT 2000, it's 8 hours later and
the sun will have moved 8 hr x 15/hr making the
GHA 120.GHA Sun 120LoE 150 LHA Sun
270
Ref 19 - 21
11
9. Given UT 2300, 29 Mar Lo 11530'E GHA
sun 16350.8'.a. Find ZT and date.
0700 30 Mar
b. Find the LHA of the sun.
279 20.8'
SolutionCompute ZDZD Lo 11530.0' / 15
7.70 which rounds to 8. Lo is east so ZD is
negative. ZD -8.Compute ZTUT 2300 ZD
-8 (rev) ZT 3100 29 Mar Corr -2400
1 day ZT 0700 30 Mar
GHA Sun 16350.8' Lo E
11530.0'LHA Sun 27920.8'
Ref 19 - 21
12
10. The sun is directly overhead at your
position. You note the time, and look up in your
Nautical Almanac for this date and time that the
GHA of the sun is 75 and its declination is 21
N. What do you know about your position? Ans
Since the sun is directly overhead, you are at
the GP of the sun your position is L 21 N,
Lo 75 W
Ref 23 - 25
13
Q7
Celestial Coordinates
End Of Homework Q A
Junior Navigation Chapter 5