The celestial sphere, the coordinates system, seasons ...

[Pages:73]The celestial sphere, the coordinates system, seasons, phases of the moon and eclipses

Chapters 2 and S1

The celestial sphere and the coordinates system

Chapter S1

How to find our way in the sky? Let's start with the Earth Coordinate System

Latitude: N-S of the equator, Longitude: E-W along equator

From Earth to Space

The Celestial Sphere

? The Celestial Sphere: An

imaginary sphere of infinite

radius centered on Earth. ? The extensions of the Earth

North Celestial Pole

North and South Pole define

the North and South celestial

poles.

? The projection of Earth equator defines the Celestial equator.

? Celestial Sphere can then be divided into a grid, just like the Earth is divided into a grid of latitude and longitude.

North celestial poleNorth

Celestial Equato r

South Celestial Pole

The Celestial Sphere: Motions

? Stars, planets and Sun are "attached" to this imaginary sphere.

? As the Earth rotates, the celestial sphere (with the stars attached to it) appears to rotate in the opposite direction.

? To explain the daily motions of the sky you can imagine the sphere rotating once in 23 hours 56 minutes (using a star as reference).

Celestial Sphere: Measuring Angles

Longitude (E ? W along Equator) Latitude (N ? S of Equator)

The celestial coordinate system

RA, measured in hr, min, sec

(0 to 24 hours)

1 hour = 60 min 1 min = 60 sec (1 hour = 15 degrees of Earth rotation)

Dec, measured in degrees, arcmin, arcsec

(0 celestial equator, +90 north hemisphere, -90 south hemisphere)

1 degree = 60 arcminutes 1 arcminute = 60 arcseconds

Celestial Equator

Right Ascension (RA)

Declination (Dec)

North Celestial Pole

orth celestial poleNorth

South Celestial Pole

The use of RA and Dec to locate objects in the celestial sphere

There are two coordinates that allow to locate an object in the sky: Azimuth and Altitude. Their value depends in the location of the observer Azimuth: Use as reference the north direction (close to Polaris) and the range of values is from 0 to 360 degrees. 0 degrees is N, 90 degrees E, 180 is S and 270 is W. Altitude: Use as reference the horizon. The range of values is from 0 degrees (horizon) to 90 degrees (zenith)

Locating the star Vega and the Sun in the celestial sphere

Ecliptic: Apparent annual path of the Sun in the celestial sphere The Sun crosses the celestial equator on March 21 (Spring equinox) and on September 21 (Fall equinox) The Sun reaches a declination of +23.5 degrees on June 21 (Summer solstice) The Sun reaches a declination of ? 23.5 degrees on December 21 (Winter solstice)

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