Astronomy 100 -- Earth & Sky Coordinates

---

---

TERRESTRIAL COORDINATE SYSTEM
===================================

IMPORTANT MAP FEATURES:

A) North and South Poles.
Set by spin axis of the Earth.
B) Equator
Imaginary "great circle" midway between the poles.
C) Prime Meridian
Agreed "zero point" for measuring a position around the Earth along the Equator. (Due south of Greenwich, England)

TERRESTRIAL COORDINATES:

1) Latitude
Angle between the equator and chosen spot.

Latitude of equator = 0°
Latitude of North Pole = 90° N (North)
Latitude of South Pole = 90° S (South)

2) Longitude

Angle around the equator nearest the chosen point.
Longitude of Prime Meridian = 0°
Moving East ==> 180° E (East)
Moving West ==> 180° W (West)

Fix Figure 1-7

Latitude and Longitude identify a UNIQUE POSITION on the Earth.
For El Centro, California
Latitude = 32.8° N
Longitude = 115.5° W

EXERCISE: What are the Latitude and Longitude of the ANTIPODE of Flagstaff, the location on the opposite side of the Earth from Flagstaff?

---

---

THE CELESTIAL SPHERE
=======================

USEFUL REFERENCE POINTS ON THE SKY

• Zenith -- Point on the Celestial Sphere directly overhead
• Meridian -- Imaginary line connecting the poles passing through the zenith.
• Altitude -- Angular distance above the horizon.
  Horizon = 0°
  Zenith = 90°

Fix Figure 2-4

Fix Figure 1-14 for Altitude Illustration.

Objects are at their highest ALTITUDE
when crossing the MERIDIAN !!!

The Adjective " Due " means " Exactly"
Due North = Exactly North
Due East = Exactly East


Meridian goes from Due North to Due South.

IMPORTANT SKY MAP FEATURES:

A) North and South Celestial Poles.
Directly above the North and South poles.
B) Celestial Equator
Projection of the equator on the sky.
Midway between N.C.P. and S.C.P.
C) Vernal Equinox
Agreed "zero point" for measuring a position around the sky along the Celestial Equator. (Measured in an easterly direction))

Fix Figure 2-2

---

---

CELESTIAL (SKY MAP) COORDINATES
=================================

1) Declination (like Latitude)
Angular distance above or below the Celestial Equator.

Declination of C.E. = 0°
Declination of N.C.P. = +90°
Declination of S.C.P. = -90°
(Note here: Latitude uses N (North) and S (South)
while Declination uses + (North) and - (South))

2) Right Ascension (R.A.) (NOT DISCUSSED IN LECTURE)

Angular distance measured eastward along the C.E. from the Vernal Equinox (defined in next lecture).
As the diurnal motion means the sky rotates once each day, a period of 24 hours, for R.A., units of hours (rather than degrees) are used and the sky is divided into 24 hours.
The sky has 360° = 24 hours ==> 1 hour = 15°

Larve Version of the Celestial Sphere Unrolled

Celestial coordinates for two bright stars:

Sirius: R.A. = 6^h 45^m --- Dec = -16.75°
Deneb: R.A. = 20^h 40^m --- Dec = +45.1°

---

---

LOCATING THE CELESTIAL EQUATOR ON THE SKY

Fix Figure 2-5

1) C.E. contacts the horizon Due East and Due West.
See Fix Fig. 2-5 above.
This is true no matter where you are on the Earth!
(Except exactly on the North and South poles)

2) Determine ALTITUDE where the C.E. crosses the MERIDIAN.

Altitude of C.E. = 90° - Latitude

For El Centro, Altitude of C.E. = 90° - 32.8° = 57.2°

These THREE points identify the location of the Celestial Equator.
This determines the range of DECLINATION that can be observed!

---

---

THE CHANGING SKY
===================================

The sky we can observe is always changing!

• Diurnal or Daily Motion. Sun moves East to West.
  Due to Earth's rotation.

• Annual Motion. Sun moves West to East! (Slowly)
  Due to Earth's revolution around the Sun.

---

---

Apparent Motion of the Sun & Seasons.

Back to The Front Page.

---