Celestial Navigation

Celestial Motion 1

General Organization

• Original “celestial sphere” model devised by the Greeks is still used– Greeks saw the Earth as the center of the

universe

• Normal longitude and latitude lines are projected outward on to the celestial sphere

General Organization

• Two systems of positioning exist• Altitude/Azimuth (Alt/Az)

– Known as Horizonal System– Position dependent– Position of an object will change depending on

position on Earth

• Declination/Right Ascension (Dec/RA)– Known as Equatorial System– Universal– Coordinates of object does not change with location

on Earth

Horizonal System

• The simpler to use but less scientific system

• Altitude is determined by degrees off of the horizon (0o-90o)

• Azimuth is a compass direction (0o-359o)

• Coordinates will be different depending upon vantage point (hence less scientific)– Very useful when using standard star charts

Horizonal System

Using the Altitude/Azimuth system, a star’s coordinates is determined by measuring the height in the sky along with a normal compass direction.

Equatorial System

• Latitude denotes distance above or below the equator– Declination denotes distance from north or south

celestial pole

• Longitude denotes distance from prime meridian– Right Ascension denotes distance from celestial

“meridian”

• Celestial “prime meridian” is drawn through the intersection point of the celestial equator and the ecliptic

Equatorial System

• Dec/RA system takes into account the 23.5o tilt of the planet and makes the celestial equator in line with the planet’s (as opposed to horizon)

• By setting the celestial sky in line with the planet’s, a non-changing set of coordinates can be devised

Equatorial System

• Declination (Dec.)– Measured the same as latitude– Degrees, minutes, seconds north or south of

the celestial equator– If declination is south of the celestial equator,

(-) declination is shown• Example: Polaris +89o (Ursa Minor)

– Polaris is located at the celestial North Pole

• Example: Rigel -8o15' (Orion)

Equatorial System

• Right Ascension (RA)– Celestial equivalent of longitude– Is not measured in degree, etc.

• Measured in hours, minutes, seconds• Measured eastward

– Uses 24 hour system• 24 hr = 365o

• 12 hr = 180o

– Each hour of ascension = 15o

– Examples• Polaris: 1hr, 49 min. (Ursa Minor)• Rigel: 5 hr, 12 min. (Orion)

Equatorial System

Equatorial System

Observing the Night Sky• Earth rotates from West to East (counter

clockwise in northern hemisphere)

• Apparent movement of stars are different depending upon latitude on Earth

• Entire sky rotates around Polaris– If at the equator, a viewer can see ALL stars

in the sky– If at the poles, starfield never changes

Observing the Night Sky

• Earth lies in a plane of the solar system with all of the other planets– Called the ecliptic

• Defined as the apparent path of the sun

• Since the sun, moon and planets all travel along this path, the zodiac constellations can be seen here– If a few zodiac constellations can be

identified, confirmation of planet sightings can be reached

Observing the Night SkyThe celestial sphere with Ecliptic indicated for comparison. The apparent discrepancy between the tilt of the Earth with respect to the plane of the solar system is what makes constellations rise and set.