Constellations

Unit 1: Astronomical Observations

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The Celestial Sphere

• Vast distances to stars prevent us from sensing their true 3-D arrangement
• Naked eye observations treat all stars at the same distance on a giant celestial sphere with the Earth at its center

Constellations

• Constellations are fixed arrangements of stars that resemble animals, objects, and mythological figures
• Stars in a constellation are not physically related

Cassiopeia – “ The Queen”

Cygnus – “The Swan”

Scorpius – “The Scorpion”

Orion – “The Hunter”

Appearance of Constellations over Time

• Positions of stars change very slowly; constellations will look the same for thousands of years

Motion of Constellations

• For a given time (say 10:00 PM), as the months proceed, constellations do not appear in the same part of the sky

Daily vs. Annual Motion

• Daily Motion
• Sun, Moon, planets, and stars rise in the east and set in the west
• Due to the Earth’s rotation: spinning on its axis

• Annual Motion
• “Yearly Motion”
• Due to the Earth’s revolution: orbit around the sun

Daily Motion

• Daily motion can be explained by the rotation of the celestial sphere about the north and south celestial poles located directly above the Earth’s north and south poles
• The celestial equator lies directly above the Earth’s equator

Annual Motion

• Annual motion is caused by the Earth’s revolution around the Sun
• A given star rises 3 minutes 56 seconds earlier each night

The Ecliptic

• The path of the Sun across the sky on the celestial sphere is called the ecliptic
• The ecliptic is the plane of the Earth’s orbit around the sun and is tipped relative to the celestial equator

The Zodiac

• The belt-shaped region of the sky surrounding the ecliptic passes primarily through twelve constellations and is called the zodiac

• A thirteenth constellation has joined the zodiac, Ophiuchus, the snake charmer

Constellations Across the World