Celestial Coordinates
Homework
Q&A
Junior Navigation
Chapter 5
1
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
2
Practical Exercises:
Problem 1.
Follow the Student Manual for guidance
3
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
4
3. The declination of the sun is 15°19'N. Its
GHA is 343°41'. What are the latitude and
longitude of the GP of the sun?
Ans: L = 15°19'N
Latitude of the GP of a body is equal to the
declination of the body.
Dec = 15°19'N; therefore L of GP = 15°19'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° - 343°41' =16°19'E.
Ref: ¶ 8 - 11
5
4. Fill in the blanks:
Greenwich meridian to
a. GHA is measured from ___________________
hour circle of GP
_________________.
the equator
b. Dec is measured from ___________________
to
the GP of the body
_________________.
c. LHA is measured from ___________________
______________________
the local meridian to
of
_________________.
____________
the observer to ___________________.
hour circle of the GP
Ref: ¶ 8, 12 & 13
6
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
7
6. LHA is measured:
a. 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
8
7. Fill in the blanks:
LoE = LHA sun.
a. GHA sun + _____
LoW = LHA sun
b. GHA sun - _____
Ref: ¶ 19
9
8. Given: Lo 150°E; ZT 0600; GHA sun is 120°.
Find: UT and LHA of the sun.
UT = 2000 previous day
LHA sun = 270°
Solution:
Compute
ZD = Lo Since
divided
15 and
GHA
computation:
thebymean
sunrounded.
will be over Greenwich at
ZD = UT,
Lo 150°
= 10.
Lo isAtE,UT
the
ZD isit's
negative,
ZD = -10
1200
GHA /at15°
1200
will Since
be 000°.
2000,
8 hours later
and the sun will have moved 8 hr x 15°/hr making the GHA 120°.
Compute UT: ZT 0600
GHA Sun 120° ZD -10
LoE
+150° UT -0400
- 1 day
LHA Sun 270° +2400
UT 2000
previous day
Ref: ¶ 19 - 21
10
9. Given:
UT 2300, 29 Mar; Lo 115°30'E;
GHA sun 163°50.8'.
a. Find: ZT and date. 0700 30 Mar
b. Find the LHA of the sun. 279° 20.8'
Solution:
Compute ZD:
ZD = Lo 115°30.0' / 15 = 7.70 which rounds to 8.
Lo is east so ZD is negative. ZD = -8.
Compute ZT:
UT
2300
ZD
-8 (rev)
ZT
3100 29 Mar
Corr -2400 + 1 day
ZT
0700 30 Mar
Ref: ¶ 19 - 21
GHA Sun
163°50.8'
Lo E
+ 115°30.0'
LHA Sun
279°20.8'
11
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
12
Q7
Celestial Coordinates
End Of
Homework Q & A
Junior Navigation
Chapter 5
13