Coordiante Relationships

Question 1

Latitude

Answer 1

Angle position above or below the equator. 90°N = north pole, 0° = equator, 90°S = south pole.

Question 2

Longitude

Answer 2

Angle position around the Earth, from the Prime Meridian, which is the line of longitude (north pole to south pole) that runs through Greenwich, London.

Question 3

Altitude

Answer 3

The angle of an object above the horizon for an observer. +90° = the observer’s zenith. 0° = the horizon.

Question 4

Azimuth

Answer 4

Angle away from due North of an object on the sky for an observer. Positive = clockwise (go east).

Question 5

Declination

Answer 5

The angle position of an object on the sky above or below the celestial equator. +90° = celestial north pole, 0° = celestial equator, -90° = celestial south pole. Roughly the lowest declination visible is the negative value of your co-latitude (see below).

Question 6

Right Ascension

Answer 6

The angle position around the sky, from the First Point of Aries (in the constellation Pisces these days), measured in hours. 0h = First Point of Aries. 12h = First Point of Libra. The RA in front of you due south will change as the Earth spins. The number in front of you goes up (ASCENDS), and the numbers move off to the RIGHT (due West). An object with a particular RA will rise in the east, move across the sky, culminate in the south on your meridian, then set in the west. An object with RA 3 hours will rise and set an hour AFTER an object with RA 2 hours.

Question 7

Observer latitude

Answer 7

Observer Latitude = 90 – object Altitude at culmination + object Declination
L = 90 – A + D

Question 8

LOCAL SIDEREAL TIME (LST)

Answer 8

LOCAL SIDEREAL TIME (LST) is the Right Ascension (RA) that is due South RIGHT NOW for the OBSERVER.

Question 9

LOCAL HOUR ANGLE (LHA)

Answer 9

LOCAL HOUR ANGLE (LHA) is the angle of an object on the sky FROM SOUTH, + for west, - for east.

Question 10

Local Hour Angle =

Answer 10

Local Hour Angle = Local Sidereal Time – Right Ascension
LHA = LST – RA
To convert from an angle in hours, or even a time difference in hours, to an angle in degrees, MULTIPLY BY 15 as each hour of rotation covers an angle of 15° (24 hours = 360°). OR: Each degree is worth 4 minutes: 1° = 4m.

Question 11

Local Sidereal Time =

Answer 11

Local Sidereal Time = Greenwich Sidereal Time + Longitude

LST = GST + L

Question 12

Your Colatitude =

Answer 12

Your Colatitude = 90° – Your Latitude
(for Northern Hemisphere). Then apply the rule:
If the object’s declination is greater than the colatitude, then the star is circumpolar.

Question 13

Pole Altitude =

Answer 13

Pole Altitude = Your Latitude

Question 14

Equation of Time (value) =

Answer 14

Equation of Time (value) = Apparent Solar Time – Mean Solar Time
EoT = AST – MST
MST = AST – EoT

Question 15

Observer Longitude =

Answer 15

Observer Longitude = ¼ x (Mean Solar Time – Greenwich Mean Time)
L = ¼ x (MST – GMT)
REMEMBER WHEN SUBTRACTING TIMES THAT THERE ARE ONLY 60 MINUTES IN ONE HOUR!