The Night Sky

The celestial poles and equator north celestial pole – point in the sky directly above the Earth’s north pole south celestial pole – point in sky directly above the Earth’s south pole. celestial equator – midway between the celestial poles (90 degrees away), lies directly above the Earth’s equator.

North celestial pole

South celestial pole

Star trails on the celestial equator

Discussion This there any place on Earth where all the visible stars are circumpolar, never falling below the horizon? Explain.

Discussion Is there any place on Earth where none of the visible stars are circumpolar? Explain.

Discussion If we were at the North pole, how far above the horizon would the North celestial pole be?

Discussion How high above the north horizon is the north celestial pole from Earth’s equator?

Discussion How high above the northern horizon would the north celestial pole appear in Cleveland which has a latitude of about 42 degrees?

What is the latitude of this picture?

The constellations Traditionally, a constellation is a grouping of stars in the same part of the sky.

Orion

The constellations In modern astronomy, the constellations are 88 irregular areas that completely cover the sky. Thus, every celestial object lies within the boundaries of a constellation.

Ursa Major

Discussion Why do you think ancient astronomers invented the constellations and made up stories to go with them?

Canis Major

Stellarium Free planetarium software http://www.stellarium.org/

Annual motion of the stars The same stars are not visible all year long. Any given non-circumpolar star will set 4 minutes early each day until it becomes lost in the glare of the setting Sun. This motion is caused by Earth’s yearly motion around the Sun.

In other words While the Sun takes 24 hours on average to traverse the sky, the stars take only 23 hours and 56 minutes. The Sun moves 4 minutes eastward each day relative to the stars. Thus different stars are visible at different times of the year.

Solar and sidereal days Solar day – 24 hours Sidereal day – 23 hours 56 minutes is the actual rotation period of the Earth

The ecliptic The ecliptic is the annual path through the sky that the Sun appears to take. In other words, the ecliptic is the plane of Earth’s orbit projected onto the stars.

Constellations of the Zodiac The ecliptic passes through 12 constellations (actually 13) during the year. This are know as the zodiacal constellations.