Mapping the Celestial sphere Some definitionsLocating stars and deep space

objects Describing an objects position

The Celestial Sphere

Some Definitions

• Celestial (Heavens)– Imaginary Points in space directly in line with

the earth• Celestial sphere:

– The canopy of stars resembling a painting in the night Sky

• Celestial Poles:– Points where the earth’s axis intersects with

the celestial sphere

Celestial North Pole

• Polaris (North Star)

Celestial South Pole

• No Southern star

• Southern pole: Use the southern cross:

Some Definitions

• Celestial Equator– Extend the

earth's equator outward to the celestial sphere

• Describing an objects position in the night sky with respect to the celestial Equator

1. Declination • (Latitude)

2. Right Ascension • (Longitude)

Location of a star• Betelgeuse (470 light-years away; 20x > Sun)• RA: 5h 55m 10.2s east of vernal equinox• dec: 7°24’26” north of celestial equator

Some Definitions• Zenith

– Imaginary point on celestial sphere directly overhead (90°)

• Nadir– Exact opposite of the

Zenith (you can not see it)

• Horizons– Place where the earth

and sky appear to meet – Halfway between zenith

and nadir

Some Definitions• Meridian

– Imaginary north-south line on the celestial sphere that is directly overhead on the observer

Morning half (a.m.) Evening half (p.m.)

It cuts the celestial sphere into two halves

Constellations • Every star and object in the

celestial sphere is in a constellation

• To locate a star– Specify a constellation– Rank it by order of brightness

•Brightest star?Sirius

Found in Canis Major Also known as alpha Canis Major (or CMa)

Putting the maps together

Not a constellation

• Asterisms– A small grouping of stars, that do not make a

constellation – E.g.. The big dipper is part of Ursa Major

Location of nearest stars• Alpha Centauri,

Beta Centauri, and Proxima Centauri

• RA: 14h 29m 41s east of vernal equinox

• dec: -62°40’46” south

• Proxima Centauri (4.2 l-y but too dim to see w/o telescope)

• Alpha and Beta (4.4 l-y; apparent magnitude of -.28 combined)

Changing Constellations

• Throughout the seasons positions of constellation change do to earths revolution around the sun

• Over long periods of time constellations can change– Star positions mover relative to each other on

the celestial sphere